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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.*;
/**
* <pre>
* 线程池状态转换
* • RUNNING -> SHUTDOWN：显式调用{@link ThreadPoolExecutor#shutdown()}或者隐式调用了{@link ThreadPoolExecutor#finalize()}方法里面的{@link ThreadPoolExecutor#shutdown()}方法。
* • RUNNING 或 SHUTDOWN -> STOP：显式调用{@link ThreadPoolExecutor#shutdownNow()}方法时。
* • SHUTDOWN -> TIDYING：当线程池和任务队列都为空时。
* • STOP -> TIDYING：当线程池为空时
* • TIDYING -> TERMINATED：当{@link ThreadPoolExecutor#terminated()}hook方法执行完成时。
* </pre>
*
* <pre>
* 线程场类型如下
* • newFixedThreadPool：创建一个核心线程个数和最大线程个数都为nThreads的线程池，并且阻塞队列长度为Integer.MAX_VALUE。
* keepAliveTime=0说明只要线程个数比核心线程个数多并且当前空闲则回收。
* • newSingleThreadExecutor：创建一个核心线程个数和最大线程个数都为1的线程池，并且阻塞队列长度为Integer.MAXVALUE。
* keepAliveTime=0说明只要线程个数比核心线程个数多并且当前空闲则回收。
* • newCachedThreadPooI：创建线程个数为Integer.MAX_VALUE，并阻塞队列为同步队列。keepAliveTime=60说明只要
* 当前线程在60s内空闲则回收。这个类型的特殊之处在于，加入同步队列的任务会被马上执行，同步队列里面最多只有一个任务。
* </pre>
*
* <pre>
* 线程池运行逻辑
* 本次只分析{@link #execute(Runnable)},submit其实差不多。
* 线程池默认逻辑是：
* 1. 先执行addWorker(),此时是核心线程 见(3)。
* 2. 添加失败，执行添加队列。见(4)
* 3. 队列已满，添加最大线程数创建工作线程执行。
* 上述的关键逻辑其实就是{@link #addWorker(Runnable, boolean)}方法。
* <a href='https://blog.csdn.net/ctwctw/article/details/117407152'>runWorker方法的各个细节</a>
* </pre>
*
*
* An {@link ExecutorService} that executes each submitted task using
* one of possibly several pooled threads, normally configured
* using {@link Executors} factory methods.
*
* <p>Thread pools address two different problems: they usually
* provide improved performance when executing large numbers of
* asynchronous tasks, due to reduced per-task invocation overhead,
* and they provide a means of bounding and managing the resources,
* including threads, consumed when executing a collection of tasks.
* Each {@code ThreadPoolExecutor} also maintains some basic
* statistics, such as the number of completed tasks.
*
* <p>To be useful across a wide range of contexts, this class
* provides many adjustable parameters and extensibility
* hooks. However, programmers are urged to use the more convenient
* {@link Executors} factory methods {@link
* Executors#newCachedThreadPool} (unbounded thread pool, with
* automatic thread reclamation), {@link Executors#newFixedThreadPool}
* (fixed size thread pool) and {@link
* Executors#newSingleThreadExecutor} (single background thread), that
* preconfigure settings for the most common usage
* scenarios. Otherwise, use the following guide when manually
* configuring and tuning this class:
*
* <dl>
*
* <dt>Core and maximum pool sizes</dt>
*
* <dd>A {@code ThreadPoolExecutor} will automatically adjust the
* pool size (see {@link #getPoolSize})
* according to the bounds set by
* corePoolSize (see {@link #getCorePoolSize}) and
* maximumPoolSize (see {@link #getMaximumPoolSize}).
*
* When a new task is submitted in method {@link #execute(Runnable)},
* and fewer than corePoolSize threads are running, a new thread is
* created to handle the request, even if other worker threads are
* idle. If there are more than corePoolSize but less than
* maximumPoolSize threads running, a new thread will be created only
* if the queue is full. By setting corePoolSize and maximumPoolSize
* the same, you create a fixed-size thread pool. By setting
* maximumPoolSize to an essentially unbounded value such as {@code
* Integer.MAX_VALUE}, you allow the pool to accommodate an arbitrary
* number of concurrent tasks. Most typically, core and maximum pool
* sizes are set only upon construction, but they may also be changed
* dynamically using {@link #setCorePoolSize} and {@link
* #setMaximumPoolSize}. </dd>
*
* <dt>On-demand construction</dt>
*
* <dd>By default, even core threads are initially created and
* started only when new tasks arrive, but this can be overridden
* dynamically using method {@link #prestartCoreThread} or {@link
* #prestartAllCoreThreads}. You probably want to prestart threads if
* you construct the pool with a non-empty queue. </dd>
*
* <dt>Creating new threads</dt>
*
* <dd>New threads are created using a {@link ThreadFactory}. If not
* otherwise specified, a {@link Executors#defaultThreadFactory} is
* used, that creates threads to all be in the same {@link
* ThreadGroup} and with the same {@code NORM_PRIORITY} priority and
* non-daemon status. By supplying a different ThreadFactory, you can
* alter the thread's name, thread group, priority, daemon status,
* etc. If a {@code ThreadFactory} fails to create a thread when asked
* by returning null from {@code newThread}, the executor will
* continue, but might not be able to execute any tasks. Threads
* should possess the "modifyThread" {@code RuntimePermission}. If
* worker threads or other threads using the pool do not possess this
* permission, service may be degraded: configuration changes may not
* take effect in a timely manner, and a shutdown pool may remain in a
* state in which termination is possible but not completed.</dd>
*
* <dt>Keep-alive times</dt>
*
* <dd>If the pool currently has more than corePoolSize threads,
* excess threads will be terminated if they have been idle for more
* than the keepAliveTime (see {@link #getKeepAliveTime(TimeUnit)}).
* This provides a means of reducing resource consumption when the
* pool is not being actively used. If the pool becomes more active
* later, new threads will be constructed. This parameter can also be
* changed dynamically using method {@link #setKeepAliveTime(long,
* TimeUnit)}. Using a value of {@code Long.MAX_VALUE} {@link
* TimeUnit#NANOSECONDS} effectively disables idle threads from ever
* terminating prior to shut down. By default, the keep-alive policy
* applies only when there are more than corePoolSize threads. But
* method {@link #allowCoreThreadTimeOut(boolean)} can be used to
* apply this time-out policy to core threads as well, so long as the
* keepAliveTime value is non-zero. </dd>
*
* <dt>Queuing</dt>
*
* <dd>Any {@link BlockingQueue} may be used to transfer and hold
* submitted tasks. The use of this queue interacts with pool sizing:
*
* <ul>
*
* <li> If fewer than corePoolSize threads are running, the Executor
* always prefers adding a new thread
* rather than queuing.</li>
*
* <li> If corePoolSize or more threads are running, the Executor
* always prefers queuing a request rather than adding a new
* thread.</li>
*
* <li> If a request cannot be queued, a new thread is created unless
* this would exceed maximumPoolSize, in which case, the task will be
* rejected.</li>
*
* </ul>
*
* There are three general strategies for queuing:
* <ol>
*
* <li> <em> Direct handoffs.</em> A good default choice for a work
* queue is a {@link SynchronousQueue} that hands off tasks to threads
* without otherwise holding them. Here, an attempt to queue a task
* will fail if no threads are immediately available to run it, so a
* new thread will be constructed. This policy avoids lockups when
* handling sets of requests that might have internal dependencies.
* Direct handoffs generally require unbounded maximumPoolSizes to
* avoid rejection of new submitted tasks. This in turn admits the
* possibility of unbounded thread growth when commands continue to
* arrive on average faster than they can be processed. </li>
*
* <li><em> Unbounded queues.</em> Using an unbounded queue (for
* example a {@link LinkedBlockingQueue} without a predefined
* capacity) will cause new tasks to wait in the queue when all
* corePoolSize threads are busy. Thus, no more than corePoolSize
* threads will ever be created. (And the value of the maximumPoolSize
* therefore doesn't have any effect.) This may be appropriate when
* each task is completely independent of others, so tasks cannot
* affect each others execution; for example, in a web page server.
* While this style of queuing can be useful in smoothing out
* transient bursts of requests, it admits the possibility of
* unbounded work queue growth when commands continue to arrive on
* average faster than they can be processed. </li>
*
* <li><em>Bounded queues.</em> A bounded queue (for example, an
* {@link ArrayBlockingQueue}) helps prevent resource exhaustion when
* used with finite maximumPoolSizes, but can be more difficult to
* tune and control. Queue sizes and maximum pool sizes may be traded
* off for each other: Using large queues and small pools minimizes
* CPU usage, OS resources, and context-switching overhead, but can
* lead to artificially low throughput. If tasks frequently block (for
* example if they are I/O bound), a system may be able to schedule
* time for more threads than you otherwise allow. Use of small queues
* generally requires larger pool sizes, which keeps CPUs busier but
* may encounter unacceptable scheduling overhead, which also
* decreases throughput. </li>
*
* </ol>
*
* </dd>
*
* <dt>Rejected tasks</dt>
*
* <dd>New tasks submitted in method {@link #execute(Runnable)} will be
* <em>rejected</em> when the Executor has been shut down, and also when
* the Executor uses finite bounds for both maximum threads and work queue
* capacity, and is saturated. In either case, the {@code execute} method
* invokes the {@link
* RejectedExecutionHandler#rejectedExecution(Runnable, ThreadPoolExecutor)}
* method of its {@link RejectedExecutionHandler}. Four predefined handler
* policies are provided:
*
* <ol>
*
* <li> In the default {@link ThreadPoolExecutor.AbortPolicy}, the
* handler throws a runtime {@link RejectedExecutionException} upon
* rejection. </li>
*
* <li> In {@link ThreadPoolExecutor.CallerRunsPolicy}, the thread
* that invokes {@code execute} itself runs the task. This provides a
* simple feedback control mechanism that will slow down the rate that
* new tasks are submitted. </li>
*
* <li> In {@link ThreadPoolExecutor.DiscardPolicy}, a task that
* cannot be executed is simply dropped. </li>
*
* <li>In {@link ThreadPoolExecutor.DiscardOldestPolicy}, if the
* executor is not shut down, the task at the head of the work queue
* is dropped, and then execution is retried (which can fail again,
* causing this to be repeated.) </li>
*
* </ol>
*
* It is possible to define and use other kinds of {@link
* RejectedExecutionHandler} classes. Doing so requires some care
* especially when policies are designed to work only under particular
* capacity or queuing policies. </dd>
*
* <dt>Hook methods</dt>
*
* <dd>This class provides {@code protected} overridable
* {@link #beforeExecute(Thread, Runnable)} and
* {@link #afterExecute(Runnable, Throwable)} methods that are called
* before and after execution of each task. These can be used to
* manipulate the execution environment; for example, reinitializing
* ThreadLocals, gathering statistics, or adding log entries.
* Additionally, method {@link #terminated} can be overridden to perform
* any special processing that needs to be done once the Executor has
* fully terminated.
*
* <p>If hook or callback methods throw exceptions, internal worker
* threads may in turn fail and abruptly terminate.</dd>
*
* <dt>Queue maintenance</dt>
*
* <dd>Method {@link #getQueue()} allows access to the work queue
* for purposes of monitoring and debugging. Use of this method for
* any other purpose is strongly discouraged. Two supplied methods,
* {@link #remove(Runnable)} and {@link #purge} are available to
* assist in storage reclamation when large numbers of queued tasks
* become cancelled.</dd>
*
* <dt>Finalization</dt>
*
* <dd>A pool that is no longer referenced in a program <em>AND</em>
* has no remaining threads will be {@code shutdown} automatically. If
* you would like to ensure that unreferenced pools are reclaimed even
* if users forget to call {@link #shutdown}, then you must arrange
* that unused threads eventually die, by setting appropriate
* keep-alive times, using a lower bound of zero core threads and/or
* setting {@link #allowCoreThreadTimeOut(boolean)}. </dd>
*
* </dl>
*
* <p><b>Extension example</b>. Most extensions of this class
* override one or more of the protected hook methods. For example,
* here is a subclass that adds a simple pause/resume feature:
*
* <pre> {@code
* class PausableThreadPoolExecutor extends ThreadPoolExecutor {
* private boolean isPaused;
* private ReentrantLock pauseLock = new ReentrantLock();
* private Condition unpaused = pauseLock.newCondition();
*
* public PausableThreadPoolExecutor(...) { super(...); }
*
* protected void beforeExecute(Thread t, Runnable r) {
* super.beforeExecute(t, r);
* pauseLock.lock();
* try {
* while (isPaused) unpaused.await();
* } catch (InterruptedException ie) {
* t.interrupt();
* } finally {
* pauseLock.unlock();
* }
* }
*
* public void pause() {
* pauseLock.lock();
* try {
* isPaused = true;
* } finally {
* pauseLock.unlock();
* }
* }
*
* public void resume() {
* pauseLock.lock();
* try {
* isPaused = false;
* unpaused.signalAll();
* } finally {
* pauseLock.unlock();
* }
* }
* }}</pre>
*
* @since 1.5
* @author Doug Lea
*/
public class ThreadPoolExecutor extends AbstractExecutorService {
/**
* The main pool control state, ctl, is an atomic integer packing
* two conceptual fields
* workerCount, indicating the effective number of threads
* runState, indicating whether running, shutting down etc
*
* In order to pack them into one int, we limit workerCount to
* (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
* billion) otherwise representable. If this is ever an issue in
* the future, the variable can be changed to be an AtomicLong,
* and the shift/mask constants below adjusted. But until the need
* arises, this code is a bit faster and simpler using an int.
*
* The workerCount is the number of workers that have been
* permitted to start and not permitted to stop. The value may be
* transiently different from the actual number of live threads,
* for example when a ThreadFactory fails to create a thread when
* asked, and when exiting threads are still performing
* bookkeeping before terminating. The user-visible pool size is
* reported as the current size of the workers set.
*
* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
*
* Threads waiting in awaitTermination() will return when the
* state reaches TERMINATED.
*
* Detecting the transition from SHUTDOWN to TIDYING is less
* straightforward than you'd like because the queue may become
* empty after non-empty and vice versa during SHUTDOWN state, but
* we can only terminate if, after seeing that it is empty, we see
* that workerCount is 0 (which sometimes entails a recheck -- see
* below).
*
* <p>
* (高3位)表示 线程状态，（低29位）表示线程个数。
* 默认位RUNNING状态，线程个数为0
* </p>
*/
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
/** 线程个数掩码位数，并不是所有平台的int类型都是32位的，所以准确地说，是具体平台下Integer二进制位数-3后的剩余位数所表示的数才是线程的个数 **/
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
/** (高3位) : 11100000000000000000000000000000 <br> 接受新任务并且处理阻塞队列里的任务 {@value}**/
private static final int RUNNING = -1 << COUNT_BITS;
/** (高3位) : 00000000000000000000000000000000 <br> 拒绝新任务但是处理阻塞队列里的任务 {@value}**/
private static final int SHUTDOWN = 0 << COUNT_BITS;
/** (高3位) : 00100000000000000000000000000000 <br> 拒绝新任务并且抛弃阻塞队列的任务,同时会中断正在处理的任务 {@value}**/
private static final int STOP = 1 << COUNT_BITS;
/** (高3位) : 01000000000000000000000000000000 <br> 所有任务都执行完(包含阻塞队列里面的任务)后当前线程池活动线程数为0,将要调用terminated方法 {@value}**/
private static final int TIDYING = 2 << COUNT_BITS;
/** (高3位) : 01100000000000000000000000000000 <br> 终止状态。terminated方法调用完成以后的状态 {@value}**/
private static final int TERMINATED = 3 << COUNT_BITS;
static {
System.out.println("RUNNING =====" + RUNNING); // -536870912
System.out.println("SHUTDOWN =====" + SHUTDOWN); // 0
System.out.println("STOP =====" + STOP); // 536870912
System.out.println("TIDYING =====" + TIDYING); // 1073741824
System.out.println("TERMINATED =====" + TERMINATED); // 1610612736
}
// Packing and unpacking ctl
/** 获取高3位(运行状态) **/
private static int runStateOf(int c) { return c & ~CAPACITY; }
/** 获取低29位(线程数量) **/
private static int workerCountOf(int c) { return c & CAPACITY; }
/** 计算ctl新值(线程状态与线程个数) **/
private static int ctlOf(int rs, int wc) { return rs | wc; }
/*
* Bit field accessors that don't require unpacking ctl.
* These depend on the bit layout and on workerCount being never negative.
*/
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
private static boolean isRunning(int c) {
return c < SHUTDOWN;
}
/**
* Attempts to CAS-increment the workerCount field of ctl.
*/
private boolean compareAndIncrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect + 1);
}
/**
* Attempts to CAS-decrement the workerCount field of ctl.
*/
private boolean compareAndDecrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect - 1);
}
/**
* Decrements the workerCount field of ctl. This is called only on
* abrupt termination of a thread (see processWorkerExit). Other
* decrements are performed within getTask.
*/
private void decrementWorkerCount() {
do {} while (! compareAndDecrementWorkerCount(ctl.get()));
}
/**
* 用于保存等待执行的任务的阻塞队列，比如基于数组的有界ArrayBlockingQueue、基于链表的无界LinkedBlockingQueue、
* 最多只有一个元素的 同步队列SynchronousQueue及优先级队列PriorityBlockingQueue等。 <br>
*
* The queue used for holding tasks and handing off to worker
* threads. We do not require that workQueue.poll() returning
* null necessarily means that workQueue.isEmpty(), so rely
* solely on isEmpty to see if the queue is empty (which we must
* do for example when deciding whether to transition from
* SHUTDOWN to TIDYING). This accommodates special-purpose
* queues such as DelayQueues for which poll() is allowed to
* return null even if it may later return non-null when delays
* expire.
*/
private final BlockingQueue<Runnable> workQueue;
/**
* mainLock控制新增 Worker线程操作的原子性。
* termination是该锁对应条件队列，线程调用{@link ThreadPoolExecutor#awaitTermination(long, TimeUnit)}时来存放阻塞的线程
*
* <br>
*
* Lock held on access to workers set and related bookkeeping.
* While we could use a concurrent set of some sort, it turns out
* to be generally preferable to use a lock. Among the reasons is
* that this serializes interruptIdleWorkers, which avoids
* unnecessary interrupt storms, especially during shutdown.
* Otherwise exiting threads would concurrently interrupt those
* that have not yet interrupted. It also simplifies some of the
* associated statistics bookkeeping of largestPoolSize etc. We
* also hold mainLock on shutdown and shutdownNow, for the sake of
* ensuring workers set is stable while separately checking
* permission to interrupt and actually interrupting.
*/
private final ReentrantLock mainLock = new ReentrantLock();
/**
* Set containing all worker threads in pool. Accessed only when
* holding mainLock.
*/
private final HashSet<Worker> workers = new HashSet<Worker>();
/**
* Wait condition to support awaitTermination
*/
private final Condition termination = mainLock.newCondition();
/**
* Tracks largest attained pool size. Accessed only under
* mainLock.
*/
private int largestPoolSize;
/**
* Counter for completed tasks. Updated only on termination of
* worker threads. Accessed only under mainLock.
*/
private long completedTaskCount;
/*
* All user control parameters are declared as volatiles so that
* ongoing actions are based on freshest values, but without need
* for locking, since no internal invariants depend on them
* changing synchronously with respect to other actions.
*/
/**
* 创建线程的工厂。<br>
*
* Factory for new threads. All threads are created using this
* factory (via method addWorker). All callers must be prepared
* for addWorker to fail, which may reflect a system or user's
* policy limiting the number of threads. Even though it is not
* treated as an error, failure to create threads may result in
* new tasks being rejected or existing ones remaining stuck in
* the queue.
*
* We go further and preserve pool invariants even in the face of
* errors such as OutOfMemoryError, that might be thrown while
* trying to create threads. Such errors are rather common due to
* the need to allocate a native stack in Thread.start, and users
* will want to perform clean pool shutdown to clean up. There
* will likely be enough memory available for the cleanup code to
* complete without encountering yet another OutOfMemoryError.
*/
private volatile ThreadFactory threadFactory;
/**
* 饱和策略，当队列满并且线程个数达到maximumPoolSize 后采取的策略，
* 比如AbortPolicy（抛出异常）、CallerRunsPolicy（使用调用者所在 线程来运行任务）、
* DiscardOldies1Policy（调用poll丢弃一个任务，执行当前任务） 及DiscardPolicy（默默丢弃，不抛出异常） <br>
*
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* 存活时间。如果当前线程池中的线程数量比核心线程数量多，并且是闲置状态，则这些闲置的线程能存活的最大时间。<br>
*
* Timeout in nanoseconds for idle threads waiting for work.
* Threads use this timeout when there are more than corePoolSize
* present or if allowCoreThreadTimeOut. Otherwise they wait
* forever for new work.
*/
private volatile long keepAliveTime;
/**
* If false (default), core threads stay alive even when idle.
* If true, core threads use keepAliveTime to time out waiting
* for work.
*/
private volatile boolean allowCoreThreadTimeOut;
/**
* 线程池核心线程个数。<br>
*
* Core pool size is the minimum number of workers to keep alive
* (and not allow to time out etc) unless allowCoreThreadTimeOut
* is set, in which case the minimum is zero.
*/
private volatile int corePoolSize;
/**
* 线程池最大线程数量 <br>
*
* Maximum pool size. Note that the actual maximum is internally
* bounded by CAPACITY.
*/
private volatile int maximumPoolSize;
/**
* The default rejected execution handler
*/
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
/**
* Permission required for callers of shutdown and shutdownNow.
* We additionally require (see checkShutdownAccess) that callers
* have permission to actually interrupt threads in the worker set
* (as governed by Thread.interrupt, which relies on
* ThreadGroup.checkAccess, which in turn relies on
* SecurityManager.checkAccess). Shutdowns are attempted only if
* these checks pass.
*
* All actual invocations of Thread.interrupt (see
* interruptIdleWorkers and interruptWorkers) ignore
* SecurityExceptions, meaning that the attempted interrupts
* silently fail. In the case of shutdown, they should not fail
* unless the SecurityManager has inconsistent policies, sometimes
* allowing access to a thread and sometimes not. In such cases,
* failure to actually interrupt threads may disable or delay full
* termination. Other uses of interruptIdleWorkers are advisory,
* and failure to actually interrupt will merely delay response to
* configuration changes so is not handled exceptionally.
*/
private static final RuntimePermission shutdownPerm =
new RuntimePermission("modifyThread");
/* The context to be used when executing the finalizer, or null. */
private final AccessControlContext acc;
/**
* worker继承AQS和Runnable接囗，是具体承载任务的对象。Worker继承了AQS，
* 自己实现了简单不可重入独占锁，其中state=0表示锁未被获取状态，state=1表示锁已经
* 被获取的状态，state=1是创建worker时默认的状态，创建时状态设置为一1是为了避免
* 该线程在运行runWorker()方法前被中断，下面会具体讲解。其中变量firstTask记录该工
* 作线程执行的第一个任务，thread是具体执行任务的线程。<br>
*
* Class Worker mainly maintains interrupt control state for
* threads running tasks, along with other minor bookkeeping.
* This class opportunistically extends AbstractQueuedSynchronizer
* to simplify acquiring and releasing a lock surrounding each
* task execution. This protects against interrupts that are
* intended to wake up a worker thread waiting for a task from
* instead interrupting a task being run. We implement a simple
* non-reentrant mutual exclusion lock rather than use
* ReentrantLock because we do not want worker tasks to be able to
* reacquire the lock when they invoke pool control methods like
* setCorePoolSize. Additionally, to suppress interrupts until
* the thread actually starts running tasks, we initialize lock
* state to a negative value, and clear it upon start (in
* runWorker).
*
* @see ThreadPoolExecutor#mainLock
*/
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
/**
* 在调用runWorker之前禁止中断。
*
* 在构造函数内首先设置worker的状态为-1，这是为了避免当前worker在调用 runworker()前被中断
* (当其他线程调用了线程池的shutdownNow时，如果worker状态>=0则会中断该线程)。这里设置了线程的状态为-1，所以该线程就不会被中断了。
* 在如下runworker()代码中，运行代码（9）时会调用unlock方法，该方法把status设置为了0，
* 所以这时候调用shutdownNow会中断worker线程。
*/
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this); // 创建一个新的线程
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
/*
* Methods for setting control state
*/
/**
* <pre>
* 如果当前线程池状态＞＝targetState 直接返回，否则设置为targetState状态
* </pre>
*
* Transitions runState to given target, or leaves it alone if
* already at least the given target.
*
* @param targetState the desired state, either SHUTDOWN or STOP
* (but not TIDYING or TERMINATED -- use tryTerminate for that)
*/
private void advanceRunState(int targetState) {
for (;;) {
int c = ctl.get();
if (runStateAtLeast(c, targetState) ||
ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c))))
break;
}
}
/**
* <pre> 尝试设置线程池状态为TERMINATED </pre>
* <p>
* 判断如果当前线程池状态是SHUTDOWN并且工作队列为空，或者当前线程池状态是STOP并且当前线程池里面没有活动线程，
* 设置线程池状态为TERMINATED。如果设置为了TERMINATED状态，则还需要调用条件变量termination
* signalAll()激活所有因为调用线程池的awaitTermination方法而被阻塞的线程。
* </p>
* <br>
*
* Transitions to TERMINATED state if either (SHUTDOWN and pool
* and queue empty) or (STOP and pool empty). If otherwise
* eligible to terminate but workerCount is nonzero, interrupts an
* idle worker to ensure that shutdown signals propagate. This
* method must be called following any action that might make
* termination possible -- reducing worker count or removing tasks
* from the queue during shutdown. The method is non-private to
* allow access from ScheduledThreadPoolExecutor.
*/
final void tryTerminate() {
for (;;) {
int c = ctl.get();
if (isRunning(c) ||
runStateAtLeast(c, TIDYING) ||
(runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
return;
if (workerCountOf(c) != 0) { // Eligible to terminate
interruptIdleWorkers(ONLY_ONE);
return;
}
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try { // 设置当前线程池状态为TIDYING
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
try {
terminated();
} finally {
// 设置当前线程池状态TERMINATED
ctl.set(ctlOf(TERMINATED, 0));
// 激活因调用条件交量termination的await()方法而被阻塞的所有线程
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
/*
* Methods for controlling interrupts to worker threads.
*/
/**
* <pre>
* 检查看是否设置了安全管理器,是则看当前调用shutdown命令的线程是否有关闭线程的权限，
* 如果有权限则还要看调用线程是否有中断工作线程的权限，
* 如果没有权限则抛出SecurityException或者NullPointerException异常。
* </pre>
*
* If there is a security manager, makes sure caller has
* permission to shut down threads in general (see shutdownPerm).
* If this passes, additionally makes sure the caller is allowed
* to interrupt each worker thread. This might not be true even if
* first check passed, if the SecurityManager treats some threads
* specially.
*/
private void checkShutdownAccess() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkPermission(shutdownPerm);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
security.checkAccess(w.thread);
} finally {
mainLock.unlock();
}
}
}
/**
* <p>
* 中断所有线程(中断的所有线程包含空闲线程和正在执行任务线程。)
* </p><br>
*
* Interrupts all threads, even if active. Ignores SecurityExceptions
* (in which case some threads may remain uninterrupted).
*/
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
}
/**
* Interrupts threads that might be waiting for tasks (as
* indicated by not being locked) so they can check for
* termination or configuration changes. Ignores
* SecurityExceptions (in which case some threads may remain
* uninterrupted).
*
* @param onlyOne If true, interrupt at most one worker. This is
* called only from tryTerminate when termination is otherwise
* enabled but there are still other workers. In this case, at
* most one waiting worker is interrupted to propagate shutdown
* signals in case all threads are currently waiting.
* Interrupting any arbitrary thread ensures that newly arriving
* workers since shutdown began will also eventually exit.
* To guarantee eventual termination, it suffices to always
* interrupt only one idle worker, but shutdown() interrupts all
* idle workers so that redundant workers exit promptly, not
* waiting for a straggler task to finish.
*/
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
// 如果工作线程没有被中断，并且没有正在运行则设置中断标志
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
/**
* <pre>
* 其设置所有空闲线程的中断标志。这里首先加了全局锁，同时只有一个线程可以调用shutdown方法设置中断标志。
* 然后尝试获取Worker自己的锁， 获取成功则设置中断标志。
* 由于正在执行的任务己经获取了锁，所以正在执行的任务没有被中断。
* 这里中断的是阻塞到getTask()方法并企图从队列里面获取任务的线程，也就是空闲线程。
* </pre>
*
* Common form of interruptIdleWorkers, to avoid having to
* remember what the boolean argument means.
*/
private void interruptIdleWorkers() {
interruptIdleWorkers(false);
}
private static final boolean ONLY_ONE = true;
/*
* Misc utilities, most of which are also exported to
* ScheduledThreadPoolExecutor
*/
/**
* Invokes the rejected execution handler for the given command.
* Package-protected for use by ScheduledThreadPoolExecutor.
*/
final void reject(Runnable command) {
handler.rejectedExecution(command, this);
}
/**
* Performs any further cleanup following run state transition on
* invocation of shutdown. A no-op here, but used by
* ScheduledThreadPoolExecutor to cancel delayed tasks.
*/
void onShutdown() {
}
/**
* State check needed by ScheduledThreadPoolExecutor to
* enable running tasks during shutdown.
*
* @param shutdownOK true if should return true if SHUTDOWN
*/
final boolean isRunningOrShutdown(boolean shutdownOK) {
int rs = runStateOf(ctl.get());
return rs == RUNNING || (rs == SHUTDOWN && shutdownOK);
}
/**
* Drains the task queue into a new list, normally using
* drainTo. But if the queue is a DelayQueue or any other kind of
* queue for which poll or drainTo may fail to remove some
* elements, it deletes them one by one.
*/
private List<Runnable> drainQueue() {
BlockingQueue<Runnable> q = workQueue;
ArrayList<Runnable> taskList = new ArrayList<Runnable>();
q.drainTo(taskList);
if (!q.isEmpty()) {
for (Runnable r : q.toArray(new Runnable[0])) {
if (q.remove(r))
taskList.add(r);
}
}
return taskList;
}
/*
* Methods for creating, running and cleaning up after workers
*/
/**
* 新增线程的方法。<br>
* <pre>
* 第一部分双重循环的目的是通过CAS操作增加线程数；(1~7)
* 第二部分主要是把并发安全的任务添加到workers里面，并且启动任务执行。 (8->)
* </pre>
*
*
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started, running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
/**
* (6)等价于
* if (rs >= SHUTDOWN &&
* (rs == SHUTDOWN || ①
* firstTask == null || ②
* !workQueue.isEmpty())) ③
* ① 当前线程池状态为 STOP、TIDYING、TERMINATED。
* ② 当前线程池状态为 SHUTDOWN 并且己经有了第一个任务。
* ③ 当前线程池状态为 SHUTDOWN 并且任务队列为空。
*/
// (6) 检查队列是否只在必要时为空( Check if queue empty only if necessary.)
if (rs >= SHUTDOWN && // 状态不为RUNNING
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
// (7) 循环CAS增加线程个数
for (;;) {
int wc = workerCountOf(c);
// (7.1) 如果线程个数超限则返回 false
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// (7.2) cas增加线程个数，同时只有一个线程成功
if (compareAndIncrementWorkerCount(c))
break retry;
// (7.3) CAS失败了，则看线程池状态是否变化了，变化则跳到外层循环重新尝试获取线程池状态，否则内层循环重新CAS
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
// (8) 这里说明 cas ctl 增加线程个数成功
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
// (8.1) 创建worker
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
// (8.2) 加独占锁，为了实现worker的同步
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
// (8.3) 重写检查线程池状态，以避免在获取锁前调用了shutdown接口
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN || // 为 RUNNING
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
// (8.4) 添加任务
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
// (8.5) 如果成功添加了 Worker，就可以启动
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
/**
* Rolls back the worker thread creation.
* - removes worker from workers, if present
* - decrements worker count
* - rechecks for termination, in case the existence of this
* worker was holding up termination
*/
private void addWorkerFailed(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (w != null)
workers.remove(w);
decrementWorkerCount();
tryTerminate();
} finally {
mainLock.unlock();
}
}
/**
* 执行清理任务.<br>
*
* Performs cleanup and bookkeeping for a dying worker. Called
* only from worker threads. Unless completedAbruptly is set,
* assumes that workerCount has already been adjusted to account
* for exit. This method removes thread from worker set, and
* possibly terminates the pool or replaces the worker if either
* it exited due to user task exception or if fewer than
* corePoolSize workers are running or queue is non-empty but
* there are no workers.
*
* @param w the worker
* @param completedAbruptly if the worker died due to user exception
*/
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
// (11.1) 统计整个线程池完成的任务个数(在当前工作线程中完成的任务累加到全局计数器),并从工作集里面删除当前worker
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
// (11.2) 尝试设置线程池状态为TERMINATED
tryTerminate();
/*
* (11.3) 下面这段代码的含义是如果线程池是RUNNING或者SHUTDOWN状态的话，
* 且任务顺利完成（completedAbruptly=false）的话，那么判断是否设置了允许核心线程超时
* 如果允许核心线程超时，且任务队列不等于空的话，那么开启一个线程来执行任务。
*
* 一言以蔽之：如果线程池是RUNNING或者SHUTDOWN状态的话，且任务队列不是空，那么至少保证线程池中有一个线程在执行任务
*/
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
/**
*
* <pre>
* 从队列中获取任务，如果timed是true的话，则调用阻塞队列的poll方法阻塞一段时间获取任务，
* 这段时间没任务的话，则超时设置timeOut=true，结束生命周期。
* 否则调用take()方法一直阻塞等待任务到来，也就是核心线程为什么能一直存活的原因。
* </pre>
*
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of:
* 1. There are more than maximumPoolSize workers (due to
* a call to setMaximumPoolSize).
* 2. The pool is stopped.
* 3. The pool is shutdown and the queue is empty.
* 4. This worker timed out waiting for a task, and timed-out
* workers are subject to termination (that is,
* {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
* both before and after the timed wait, and if the queue is
* non-empty, this worker is not the last thread in the pool.
*
* @return task, or null if the worker must exit, in which case
* workerCount is decremented
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
/**
* <pre>
* 调用Worker.thread.start()执行的方法
* </pre>
*
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
*
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters. Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and then we
* ensure that unless pool is stopping, this thread does not have
* its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to afterExecute.
* We separately handle RuntimeException, Error (both of which the
* specs guarantee that we trap) and arbitrary Throwables.
* Because we cannot rethrow Throwables within Runnable.run, we
* wrap them within Errors on the way out (to the thread's
* UncaughtExceptionHandler). Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
/**
* (9) 将state设置为0，允许中断
* 因为{@link Worker#state} 初始时为 -1，为了避免当前 Worker在调用runWorker前被中断
* (当其他线程调用了线程池的shutdownNow时，如果Worker状态>=0则会中断该线程)。
*/
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
/**
* (10) 如果当前或者调用getTask从任务队列获取的任务返回null，则跳转到代码（11）执行。
* 如果task不为null则执行代码(10.1)获取工作线程内部持有的独占锁，
* 然后执行扩展接口代码(10.2)在具体任务执行前做一些事情。
* 代码(10.3)具体执行任务，
* 代码(10.4)在任务执行完毕后做一些事情，
* 代码(10.5)统计当前Worker完成了多少个任务，并释放锁。
*/
while (task != null || (task = getTask()) != null) {
/**
* (10.1) 这里在执行具体任务期间加锁，是为了避免在任务运行期间，其他线程调用了
* shutdown后正在执行的任务被中断(shutdown只会中断当前被阻塞挂起的线程）
*/
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
// (10.2) 执行任务前干一些事情
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run(); // (10.3) 执行任务
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
// (10.4) 执行任务完毕后干一些事情
afterExecute(task, thrown);
}
} finally {
task = null;
// (10.5) 统计当前worker完成了多少个任务
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
// (11) 执行清理工作
// 如果任务正常执行，没有任何异常，此处为true,否则为false
processWorkerExit(w, completedAbruptly);
}
}
// Public constructors and methods
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default thread factory and rejected execution handler.
* It may be more convenient to use one of the {@link Executors} factory
* methods instead of this general purpose constructor.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default rejected execution handler.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default thread factory.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* {@code RejectedExecutionHandler}, if the task
* cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
public void execute(Runnable command) {
//(1) 若任务为空，则抛 NPE
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
// (2) 获取当前线程池的状态＋线程个数变量的组合值
int c = ctl.get();
// (3) 当前线程池中的线程个数是否小于corePoolSize, 小于则开启新的核心线程运行
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
/**
* 至此，有以下两种情况：
* 1.当前工作线程数大于等于核心线程数
* 2.新建线程失败
* 此时会尝试将任务添加到阻塞队列 workQueue
*/
// (4) 若线程池处于 RUNNING 状态，将任务添加到阻塞队列 workQueue 中
if (isRunning(c) && workQueue.offer(command)) {
// (4.1) 二次检查 （执行添加队列前，ctl状态可能已经发生了改变）
int recheck = ctl.get();
// (4.2) 如果线程池已不处于 RUNNING 状态，移除已入队的任务，并且执行拒绝策略
if (!isRunning(recheck) && remove(command))
reject(command);
// (4.3) 如果线程池是 RUNNING 的，并且线程池为空，开启新的线程
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
/**
* 至此，有以下两种情况：
* 1.线程池处于非运行状态，线程池不再接受新的线程
* 2.线程处于运行状态，但是阻塞队列已满，无法加入到阻塞队列
* 此时会尝试以最大线程数为限制创建新的工作线程
*/
else if (!addWorker(command, false))
reject(command);
}
/**
* <pre>
* 调用 shutdown方法，线程池就不会再接受新的任务了，但是工作队列里面的任还是要执行的。
* 该方法会立刻返回，并不等待队列任务完成再返回
* </pre>
*
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
* <p>This method does not wait for previously submitted tasks to
* complete execution. Use {@link #awaitTermination awaitTermination}
* to do that.
*
* @throws SecurityException {@inheritDoc}
*/
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// (12) 权限检查
checkShutdownAccess();
// (13) 设置当前线程池状态为SHUTDOWN，如果已经是SHUTDOWN 则直接返回
advanceRunState(SHUTDOWN);
// (14) 设置中断标志
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
// (15) 尝试将状态变为TERMINATE
tryTerminate();
/**
* 在如上代码中，首先使用CAS设置当前线程池状态为TIDYING，
* 如果设置成功则执行扩展接口terminated在线程池状态变为TERMINATED前做一些事情，
* 然后设置当前线程池状态为TERMINATED。
* 最后调用termination.signalAll()激活因调用条件变量termination的await系列方法而被阻塞的所有线程，
* 关于这一点{@link ThreadPoolExecutor#awaitTermination(long, TimeUnit)}方法去时具体讲解。
*/
}
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution. These tasks are drained (removed)
* from the task queue upon return from this method.
*
* <p>This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. This implementation
* cancels tasks via {@link Thread#interrupt}, so any task that
* fails to respond to interrupts may never terminate.
*
* @throws SecurityException {@inheritDoc}
*/
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess(); // (16) 权限检查
advanceRunState(STOP); // (17) 设置线程池状态为STOP
interruptWorkers(); // (18) 中断所有线程(中断的所有线程包含空闲线程和正在执行任务线程。)
tasks = drainQueue(); // (19) 将队列任务移动到tasks中
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
public boolean isShutdown() {
return ! isRunning(ctl.get());
}
/**
* Returns true if this executor is in the process of terminating
* after {@link #shutdown} or {@link #shutdownNow} but has not
* completely terminated. This method may be useful for
* debugging. A return of {@code true} reported a sufficient
* period after shutdown may indicate that submitted tasks have
* ignored or suppressed interruption, causing this executor not
* to properly terminate.
*
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
int c = ctl.get();
return ! isRunning(c) && runStateLessThan(c, TERMINATED);
}
public boolean isTerminated() {
return runStateAtLeast(ctl.get(), TERMINATED);
}
/**
* <p>
* 当线程调awaitTermination方法后，当前线程会被阻塞，直到线程池状态变为TERMINATED才返回或者等待时间才返回。
* </p><br>
*
* Blocks until all tasks have completed execution after a shutdown
* request, or the timeout occurs, or the current thread is
* interrupted, whichever happens first.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if this executor terminated and
* {@code false} if the timeout elapsed before termination
* @throws InterruptedException if interrupted while waiting
*/
public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (;;) {
if (runStateAtLeast(ctl.get(), TERMINATED))
return true;
if (nanos <= 0)
return false;
nanos = termination.awaitNanos(nanos);
}
} finally {
mainLock.unlock();
}
/**
* 如上代码首先获取独占锁，然后在无限循环内部判断当前线程池状态是否至少是TERMINATED状态，
* 如果是则直接返回，否则说明当前线程池里面还有线程在执行，则看设置的超时时间nanos是否小于0，
* 小于0则说明不需要等待，那就直接返回，如果大于0则调用条件变量termination的awaitNanos方法等待nanos时间，
* 期望在这段时间内线程池状态变为TERMINATED。
*
* 在讲解shutdown方法时提到过，当线程池状态变为TERMINATED时，
* 会调用termination.signalAll()用来激活调用条件变量termination的await系列方法被阻塞的所有线程，
* 所以如果在调用awaitTermination之后又调用了shutdown方法，
* 并且在shutdown内部将线程池状态设置为TERMINATED，则termination.awaitNanos方法会返回。
*
* 另外在工作线程worker的runworker方法内，当工作线程运行结束后，会调用processWorkerExit方法，
* 在processWorkerExit方法内部也会调用tryTerminate方法测试当前是否应该把线程池状态设置为TERMINATED，
* 如果是，则也会调用termination.signalAll()用来激活调用线程池的awaitTermination方法而被阻塞的线程。
*
* 而且当等待时间超时后，termination.awaitNanos也会返回，这时候会重新检查当前线
* 程池状态是否为TERMINATED，如果是则直接返回，否则继续阻塞挂起自己。
*/
}
/**
* Invokes {@code shutdown} when this executor is no longer
* referenced and it has no threads.
*/
protected void finalize() {
SecurityManager sm = System.getSecurityManager();
if (sm == null || acc == null) {
shutdown();
} else {
PrivilegedAction<Void> pa = () -> { shutdown(); return null; };
AccessController.doPrivileged(pa, acc);
}
}
/**
* Sets the thread factory used to create new threads.
*
* @param threadFactory the new thread factory
* @throws NullPointerException if threadFactory is null
* @see #getThreadFactory
*/
public void setThreadFactory(ThreadFactory threadFactory) {
if (threadFactory == null)
throw new NullPointerException();
this.threadFactory = threadFactory;
}
/**
* Returns the thread factory used to create new threads.
*
* @return the current thread factory
* @see #setThreadFactory(ThreadFactory)
*/
public ThreadFactory getThreadFactory() {
return threadFactory;
}
/**
* Sets a new handler for unexecutable tasks.
*
* @param handler the new handler
* @throws NullPointerException if handler is null
* @see #getRejectedExecutionHandler
*/
public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
if (handler == null)
throw new NullPointerException();
this.handler = handler;
}
/**
* Returns the current handler for unexecutable tasks.
*
* @return the current handler
* @see #setRejectedExecutionHandler(RejectedExecutionHandler)
*/
public RejectedExecutionHandler getRejectedExecutionHandler() {
return handler;
}
/**
* Sets the core number of threads. This overrides any value set
* in the constructor. If the new value is smaller than the
* current value, excess existing threads will be terminated when
* they next become idle. If larger, new threads will, if needed,
* be started to execute any queued tasks.
*
* @param corePoolSize the new core size
* @throws IllegalArgumentException if {@code corePoolSize < 0}
* @see #getCorePoolSize
*/
public void setCorePoolSize(int corePoolSize) {
if (corePoolSize < 0)
throw new IllegalArgumentException();
int delta = corePoolSize - this.corePoolSize;
this.corePoolSize = corePoolSize;
if (workerCountOf(ctl.get()) > corePoolSize)
interruptIdleWorkers();
else if (delta > 0) {
// We don't really know how many new threads are "needed".
// As a heuristic, prestart enough new workers (up to new
// core size) to handle the current number of tasks in
// queue, but stop if queue becomes empty while doing so.
int k = Math.min(delta, workQueue.size());
while (k-- > 0 && addWorker(null, true)) {
if (workQueue.isEmpty())
break;
}
}
}
/**
* Returns the core number of threads.
*
* @return the core number of threads
* @see #setCorePoolSize
*/
public int getCorePoolSize() {
return corePoolSize;
}
/**
* Starts a core thread, causing it to idly wait for work. This
* overrides the default policy of starting core threads only when
* new tasks are executed. This method will return {@code false}
* if all core threads have already been started.
*
* @return {@code true} if a thread was started
*/
public boolean prestartCoreThread() {
return workerCountOf(ctl.get()) < corePoolSize &&
addWorker(null, true);
}
/**
* <pre>
* 至少保证一个线程在线程池运行
* </pre>
*
* Same as prestartCoreThread except arranges that at least one
* thread is started even if corePoolSize is 0.
*/
void ensurePrestart() {
int wc = workerCountOf(ctl.get());
// 增加核心线程数
if (wc < corePoolSize)
addWorker(null, true);
// 如果初始化corePoolSize==0,则也添加一个线程
else if (wc == 0)
addWorker(null, false);
}
/**
* Starts all core threads, causing them to idly wait for work. This
* overrides the default policy of starting core threads only when
* new tasks are executed.
*
* @return the number of threads started
*/
public int prestartAllCoreThreads() {
int n = 0;
while (addWorker(null, true))
++n;
return n;
}
/**
* Returns true if this pool allows core threads to time out and
* terminate if no tasks arrive within the keepAlive time, being
* replaced if needed when new tasks arrive. When true, the same
* keep-alive policy applying to non-core threads applies also to
* core threads. When false (the default), core threads are never
* terminated due to lack of incoming tasks.
*
* @return {@code true} if core threads are allowed to time out,
* else {@code false}
*
* @since 1.6
*/
public boolean allowsCoreThreadTimeOut() {
return allowCoreThreadTimeOut;
}
/**
* Sets the policy governing whether core threads may time out and
* terminate if no tasks arrive within the keep-alive time, being
* replaced if needed when new tasks arrive. When false, core
* threads are never terminated due to lack of incoming
* tasks. When true, the same keep-alive policy applying to
* non-core threads applies also to core threads. To avoid
* continual thread replacement, the keep-alive time must be
* greater than zero when setting {@code true}. This method
* should in general be called before the pool is actively used.
*
* @param value {@code true} if should time out, else {@code false}
* @throws IllegalArgumentException if value is {@code true}
* and the current keep-alive time is not greater than zero
*
* @since 1.6
*/
public void allowCoreThreadTimeOut(boolean value) {
if (value && keepAliveTime <= 0)
throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
if (value != allowCoreThreadTimeOut) {
allowCoreThreadTimeOut = value;
if (value)
interruptIdleWorkers();
}
}
/**
* Sets the maximum allowed number of threads. This overrides any
* value set in the constructor. If the new value is smaller than
* the current value, excess existing threads will be
* terminated when they next become idle.
*
* @param maximumPoolSize the new maximum
* @throws IllegalArgumentException if the new maximum is
* less than or equal to zero, or
* less than the {@linkplain #getCorePoolSize core pool size}
* @see #getMaximumPoolSize
*/
public void setMaximumPoolSize(int maximumPoolSize) {
if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize)
throw new IllegalArgumentException();
this.maximumPoolSize = maximumPoolSize;
if (workerCountOf(ctl.get()) > maximumPoolSize)
interruptIdleWorkers();
}
/**
* Returns the maximum allowed number of threads.
*
* @return the maximum allowed number of threads
* @see #setMaximumPoolSize
*/
public int getMaximumPoolSize() {
return maximumPoolSize;
}
/**
* Sets the time limit for which threads may remain idle before
* being terminated. If there are more than the core number of
* threads currently in the pool, after waiting this amount of
* time without processing a task, excess threads will be
* terminated. This overrides any value set in the constructor.
*
* @param time the time to wait. A time value of zero will cause
* excess threads to terminate immediately after executing tasks.
* @param unit the time unit of the {@code time} argument
* @throws IllegalArgumentException if {@code time} less than zero or
* if {@code time} is zero and {@code allowsCoreThreadTimeOut}
* @see #getKeepAliveTime(TimeUnit)
*/
public void setKeepAliveTime(long time, TimeUnit unit) {
if (time < 0)
throw new IllegalArgumentException();
if (time == 0 && allowsCoreThreadTimeOut())
throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
long keepAliveTime = unit.toNanos(time);
long delta = keepAliveTime - this.keepAliveTime;
this.keepAliveTime = keepAliveTime;
if (delta < 0)
interruptIdleWorkers();
}
/**
* Returns the thread keep-alive time, which is the amount of time
* that threads in excess of the core pool size may remain
* idle before being terminated.
*
* @param unit the desired time unit of the result
* @return the time limit
* @see #setKeepAliveTime(long, TimeUnit)
*/
public long getKeepAliveTime(TimeUnit unit) {
return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
}
/* User-level queue utilities */
/**
* Returns the task queue used by this executor. Access to the
* task queue is intended primarily for debugging and monitoring.
* This queue may be in active use. Retrieving the task queue
* does not prevent queued tasks from executing.
*
* @return the task queue
*/
public BlockingQueue<Runnable> getQueue() {
return workQueue;
}
/**
* Removes this task from the executor's internal queue if it is
* present, thus causing it not to be run if it has not already
* started.
*
* <p>This method may be useful as one part of a cancellation
* scheme. It may fail to remove tasks that have been converted
* into other forms before being placed on the internal queue. For
* example, a task entered using {@code submit} might be
* converted into a form that maintains {@code Future} status.
* However, in such cases, method {@link #purge} may be used to
* remove those Futures that have been cancelled.
*
* @param task the task to remove
* @return {@code true} if the task was removed
*/
public boolean remove(Runnable task) {
boolean removed = workQueue.remove(task);
tryTerminate(); // In case SHUTDOWN and now empty
return removed;
}
/**
* Tries to remove from the work queue all {@link Future}
* tasks that have been cancelled. This method can be useful as a
* storage reclamation operation, that has no other impact on
* functionality. Cancelled tasks are never executed, but may
* accumulate in work queues until worker threads can actively
* remove them. Invoking this method instead tries to remove them now.
* However, this method may fail to remove tasks in
* the presence of interference by other threads.
*/
public void purge() {
final BlockingQueue<Runnable> q = workQueue;
try {
Iterator<Runnable> it = q.iterator();
while (it.hasNext()) {
Runnable r = it.next();
if (r instanceof Future<?> && ((Future<?>)r).isCancelled())
it.remove();
}
} catch (ConcurrentModificationException fallThrough) {
// Take slow path if we encounter interference during traversal.
// Make copy for traversal and call remove for cancelled entries.
// The slow path is more likely to be O(N*N).
for (Object r : q.toArray())
if (r instanceof Future<?> && ((Future<?>)r).isCancelled())
q.remove(r);
}
tryTerminate(); // In case SHUTDOWN and now empty
}
/* Statistics */
/**
* Returns the current number of threads in the pool.
*
* @return the number of threads
*/
public int getPoolSize() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Remove rare and surprising possibility of
// isTerminated() && getPoolSize() > 0
return runStateAtLeast(ctl.get(), TIDYING) ? 0
: workers.size();
} finally {
mainLock.unlock();
}
}
/**
* Returns the approximate number of threads that are actively
* executing tasks.
*
* @return the number of threads
*/
public int getActiveCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int n = 0;
for (Worker w : workers)
if (w.isLocked())
++n;
return n;
} finally {
mainLock.unlock();
}
}
/**
* Returns the largest number of threads that have ever
* simultaneously been in the pool.
*
* @return the number of threads
*/
public int getLargestPoolSize() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
return largestPoolSize;
} finally {
mainLock.unlock();
}
}
/**
* Returns the approximate total number of tasks that have ever been
* scheduled for execution. Because the states of tasks and
* threads may change dynamically during computation, the returned
* value is only an approximation.
*
* @return the number of tasks
*/
public long getTaskCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
long n = completedTaskCount;
for (Worker w : workers) {
n += w.completedTasks;
if (w.isLocked())
++n;
}
return n + workQueue.size();
} finally {
mainLock.unlock();
}
}
/**
* Returns the approximate total number of tasks that have
* completed execution. Because the states of tasks and threads
* may change dynamically during computation, the returned value
* is only an approximation, but one that does not ever decrease
* across successive calls.
*
* @return the number of tasks
*/
public long getCompletedTaskCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
long n = completedTaskCount;
for (Worker w : workers)
n += w.completedTasks;
return n;
} finally {
mainLock.unlock();
}
}
/**
* Returns a string identifying this pool, as well as its state,
* including indications of run state and estimated worker and
* task counts.
*
* @return a string identifying this pool, as well as its state
*/
public String toString() {
long ncompleted;
int nworkers, nactive;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
ncompleted = completedTaskCount;
nactive = 0;
nworkers = workers.size();
for (Worker w : workers) {
ncompleted += w.completedTasks;
if (w.isLocked())
++nactive;
}
} finally {
mainLock.unlock();
}
int c = ctl.get();
String rs = (runStateLessThan(c, SHUTDOWN) ? "Running" :
(runStateAtLeast(c, TERMINATED) ? "Terminated" :
"Shutting down"));
return super.toString() +
"[" + rs +
", pool size = " + nworkers +
", active threads = " + nactive +
", queued tasks = " + workQueue.size() +
", completed tasks = " + ncompleted +
"]";
}
/* Extension hooks */
/**
* Method invoked prior to executing the given Runnable in the
* given thread. This method is invoked by thread {@code t} that
* will execute task {@code r}, and may be used to re-initialize
* ThreadLocals, or to perform logging.
*
* <p>This implementation does nothing, but may be customized in
* subclasses. Note: To properly nest multiple overridings, subclasses
* should generally invoke {@code super.beforeExecute} at the end of
* this method.
*
* @param t the thread that will run task {@code r}
* @param r the task that will be executed
*/
protected void beforeExecute(Thread t, Runnable r) { }
/**
* Method invoked upon completion of execution of the given Runnable.
* This method is invoked by the thread that executed the task. If
* non-null, the Throwable is the uncaught {@code RuntimeException}
* or {@code Error} that caused execution to terminate abruptly.
*
* <p>This implementation does nothing, but may be customized in
* subclasses. Note: To properly nest multiple overridings, subclasses
* should generally invoke {@code super.afterExecute} at the
* beginning of this method.
*
* <p><b>Note:</b> When actions are enclosed in tasks (such as
* {@link FutureTask}) either explicitly or via methods such as
* {@code submit}, these task objects catch and maintain
* computational exceptions, and so they do not cause abrupt
* termination, and the internal exceptions are <em>not</em>
* passed to this method. If you would like to trap both kinds of
* failures in this method, you can further probe for such cases,
* as in this sample subclass that prints either the direct cause
* or the underlying exception if a task has been aborted:
*
* <pre> {@code
* class ExtendedExecutor extends ThreadPoolExecutor {
* // ...
* protected void afterExecute(Runnable r, Throwable t) {
* super.afterExecute(r, t);
* if (t == null && r instanceof Future<?>) {
* try {
* Object result = ((Future<?>) r).get();
* } catch (CancellationException ce) {
* t = ce;
* } catch (ExecutionException ee) {
* t = ee.getCause();
* } catch (InterruptedException ie) {
* Thread.currentThread().interrupt(); // ignore/reset
* }
* }
* if (t != null)
* System.out.println(t);
* }
* }}</pre>
*
* @param r the runnable that has completed
* @param t the exception that caused termination, or null if
* execution completed normally
*/
protected void afterExecute(Runnable r, Throwable t) { }
/**
* Method invoked when the Executor has terminated. Default
* implementation does nothing. Note: To properly nest multiple
* overridings, subclasses should generally invoke
* {@code super.terminated} within this method.
*/
protected void terminated() { }
/* Predefined RejectedExecutionHandlers */
/**
* A handler for rejected tasks that runs the rejected task
* directly in the calling thread of the {@code execute} method,
* unless the executor has been shut down, in which case the task
* is discarded.
*/
public static class CallerRunsPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code CallerRunsPolicy}.
*/
public CallerRunsPolicy() { }
/**
* Executes task r in the caller's thread, unless the executor
* has been shut down, in which case the task is discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
r.run();
}
}
}
/**
* A handler for rejected tasks that throws a
* {@code RejectedExecutionException}.
*/
public static class AbortPolicy implements RejectedExecutionHandler {
/**
* Creates an {@code AbortPolicy}.
*/
public AbortPolicy() { }
/**
* Always throws RejectedExecutionException.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}
/**
* A handler for rejected tasks that silently discards the
* rejected task.
*/
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardPolicy}.
*/
public DiscardPolicy() { }
/**
* Does nothing, which has the effect of discarding task r.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
}
}
/**
* A handler for rejected tasks that discards the oldest unhandled
* request and then retries {@code execute}, unless the executor
* is shut down, in which case the task is discarded.
*/
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardOldestPolicy} for the given executor.
*/
public DiscardOldestPolicy() { }
/**
* Obtains and ignores the next task that the executor
* would otherwise execute, if one is immediately available,
* and then retries execution of task r, unless the executor
* is shut down, in which case task r is instead discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
}
}
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