ThreadPoolExecutor源码翻译
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2022-04-19 11:02:00
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/*
* Oracle专有/机密。使用须遵守许可条款。
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/*
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* 由Doug Lea在JCP JSR-166专家组成员的协助下编写，
* 并发布到公共领域, 解释在
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
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.*;
/**
* 一个{@link ExecutorService}执行其中一个已提交的任务,任务由池中可能的几个
* 线程之一提交。一般配置用{@link Executors}工厂方法。
*
* <p>线程池强调两个不同的问题：它们通常提供改进的性能，当执行大数量的异步任务，
* 通过减少每个任务的调用开销，且它们提供了一种方法来限定并管理资源，
* 包含线程集，进行消费当执行一个集的任务。
* 每个{@code ThreadPoolExecutor}同时维护一些基本的统计，比如完成的任务数。
*
* <p>考虑可用性通过一个大范围的上下文，该类提供了许多可调节的参数和可扩展的钩子。
* 然而，程序员迫切使用更方便的{@link Executors}工厂方法
* {@link Executors#newCachedThreadPool}（无限的线程池，自动增加线程个数），
* {@link Executors#newFixedThreadPool} (固定个数的线程池)
* {@link Executors#newSingleThreadExecutor} (单个后台线程)
* 这些预设置用于最常用的场景。 若非如此，使用下面的指导当需要手动配置和调节该类时：
*
* <dl>
*
* <dt>核心和最大线程数</dt>
*
* <dd>一 {@code ThreadPoolExecutor} 将自动的调节线程数 (see {@link #getPoolSize})
* 根据边界设定，通过核心线程数 (see {@link #getCorePoolSize}) 和
* 最大线程数 (see {@link #getMaximumPoolSize}).
*
* 当一个新的任务提交于方法 {@link #execute(Runnable)},
* 且小于核心池数的线程在运行, 一个新的线程被创建来处理请求，即使其他工作线程
* 是空闲的。如果有超过核心池数但是少于最大池数的线程在运行，一个新的线程将
* 只会在队列满时才会被创建。 通过设置核心池数和最大池数，你创建一个固定尺寸的
* 线程池。通过设置最大池数给一个本质上无边界值比如{@code Integer.MAX_VALUE}，
* 你允许池容纳一个任意数量并发任务。 最典型的，核心和最大池尺寸只能基于上面的
* 限制被设置，但是它们可能也会被动态改变使用 {@link #setCorePoolSize} 和
* {@link #setMaximumPoolSize}</dd>
*
* <dt>按需构造</dt>
*
* <dd>默认，每个核心线程被初始创建并启动，只会在一个新的任务到达的时候，
* 但是这可以被动态重写用方法{@link #prestartCoreThread}或者 {@link
* #prestartAllCoreThreads}。 你可能会想预启动线程，如果你构建的池持有一个
* 非空的队列</dd>
*
* <dt>创建新线程</dt>
*
* <dd>新线程被创建用一个{@link ThreadFactory}。如无其他特别规定，一个
* {@link Executors#defaultThreadFactory}可以用，其创建的线程会归为相同的
* {@link ThreadGroup}以及相同{@code NORM_PRIORITY}优先级及非守护状态。
* 通过提供一个不同的线程工程，你可以改变线程名，线程组，优先级，守护状态，等。
* 如果一个{@code ThreadFactory}创建线程失败当返回null自{@code newThread}，
* 执行器将继续，但可能不能执行任何任务。线程可能处理"modifyThread" {@code
* RuntimePermission}.如果工作者线程或其他线程使用池确实没有处理这个使命，
* 服务会被降级：配置改变可能不能及时的起作用，且关闭池会维护在一个状态，即关闭
* 了但是没有完成。
* </dd>
*
* <dt>保持活跃时间</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>排队</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>
*
* 三种一般排队策略:
* <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> 无限队列.</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>有限队列.</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>拒绝任务</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>钩子方法</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>队列维护</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>收尾处理</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>扩展例子</b>. 大多数该类的扩展覆写一个或多个protected钩子方法。比如，
* 下面的子类添加了一个简单的pause/resume（暂停/恢复）特性：
*
* <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 {
/**
* 主要池控制状态，ctl，是一个原子整数包装了两个概念字段
* workerCount，指有效线程数
* runState，指是否运行，关闭等
*
* 为了将他们包装到一个int里，我们限制工作线程数在(2^29)-1(大约5亿)
* 线程而不是(2^31)-1（2十亿）别的可表示的。如果将来这仍是一个问题，
* 该变量可以被改为一个AtomicLong，且移位/屏蔽常数低于调整值。但在此之前，
* 用int要快一点，简单一点
*
* workerCount是工作者的个数，工作者允许启动且不允许停止。 这个值会有
* 瞬时的不同于实际存活的线程数，比如当一个线程工厂创建线程失败，和当退出
* 线程仍在进行记录在终止之前。用户可见池尺寸报告为工作集当前尺寸。
*
* runState提供主要生命周期控制，管理着这些值：
*
* RUNNING: 接收新任务并处理入队任务
* SHUTDOWN: 不接受新任务，但处理入队任务
* STOP: 不接受新任务，不处理入队任务，并中断处理中的任务。
* TIDYING: 结束所有任务，workerCount为0，线程转为状态TIDYING
* 将执行terminated()钩子方法
* TERMINATED: terminated() 完成
*
* 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.
*
* 检测从SHUTDOWN到TIDYING的转变，比你想要的更直接。 因为队列会从非空变空
* 且反之亦然在SHUTDOWN状态下，但我们只能终结如果在看到它为空后，我们看到
* workerCount为0（这有时候必要的做个复检--如下）
*/
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState 存储于高位
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// 包装和开箱 ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
/*
* Bit字段访问器无需开箱ctl
* 因为bit结构，及workerCount为非负数
*/
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;
}
/**
* 尝试比较并设置增加ctl的workerCount字段
*/
private boolean compareAndIncrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect + 1);
}
/**
* 尝试比较并设置减小ctl的workerCount字段
*/
private boolean compareAndDecrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect - 1);
}
/**
* 减小ctl的workerCount字段。这只在突然终止一个线程时（见processWorkerExit）被调用
* 其他减小运行于getTask中
*/
private void decrementWorkerCount() {
do {} while (! compareAndDecrementWorkerCount(ctl.get()));
}
/**
* 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.
* 该队列用于持有任务并传递给工作者线程集。 我们要求workQueue.poll()不返回null
* 必然需要通过workQueue.isEmpty()判断，所以单靠isEmpty看是否队列为
* 空（我们必须这样，比如当决定是否从SHUTDOWN转变到TIDYING）。这兼容特定目的
* 的队列，比如DelayQueues之于poll()允许返回null，即使当延迟到期它会晚一点
* 返回非null
*/
private final BlockingQueue<Runnable> workQueue;
/**
* 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.
*/
/**
* 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;
/**
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* 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;
/**
* 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;
/**
* 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");
/**
* 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).
*/
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) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** 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
*/
/**
* 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;
}
}
/**
* 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 {
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
try {
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
/*
* Methods for controlling interrupts to worker threads.
*/
/**
* 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();
}
}
}
/**
* 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;
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
/**
* 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
*/
/**
* 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);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
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();
}
}
/**
* 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();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
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);
}
}
/**
* 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;
}
}
}
/**
* 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;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
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 {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
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.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) {
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.
*/
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
/**
* 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 {
checkShutdownAccess();
advanceRunState(SHUTDOWN);
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
/**
* 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();
advanceRunState(STOP);
interruptWorkers();
tasks = drainQueue();
} 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);
}
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();
}
}
/**
* Invokes {@code shutdown} when this executor is no longer
* referenced and it has no threads.
*/
protected void finalize() {
shutdown();
}
/**
* 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);
}
/**
* Same as prestartCoreThread except arranges that at least one
* thread is started even if corePoolSize is 0.
*/
void ensurePrestart() {
int wc = workerCountOf(ctl.get());
if (wc < corePoolSize)
addWorker(null, true);
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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