java 中ThreadPoolExecutor原理分析

java 中ThreadPoolExecutor原理分析

线程池简介

Java线程池是开发中常用的工具，当我们有异步、并行的任务要处理时，经常会用到线程池，或者在实现一个服务器时，也需要使用线程池来接收连接处理请求。

线程池使用

JDK中提供的线程池实现位于java.util.concurrent.ThreadPoolExecutor。在使用时，通常使用ExecutorService接口，它提供了submit,invokeAll,shutdown等通用的方法。

在线程池配置方面，Executors类中提供了一些静态方法能够提供一些常用场景的线程池，如newFixedThreadPool,newCachedThreadPool,newSingleThreadExecutor等，这些方法最终都是调用到了ThreadPoolExecutor的构造函数。

ThreadPoolExecutor的包含所有参数的构造函数是

/**
* @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
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;
}

• corePoolSize设置线程池的核心线程数，当添加新任务时，如果线程池中的线程数小于corePoolSize，则不管当前是否有线程闲置，都会创建一个新的线程来执行任务。
• maximunPoolSize是线程池中允许的最大的线程数
• workQueue用于存放排队的任务
• keepAliveTime是大于corePoolSize的线程闲置的超时时间
• handler用于在任务逸出、线程池关闭时的任务处理 ，线程池的线程增长策略为，当前线程数小于corePoolSize时，新增线程，当线程数=corePoolSize且corePoolSize时，只有在workQueue不能存放新的任务时创建新线程，超出的线程在闲置keepAliveTime后销毁。
  

实现(基于JDK1.8)

ThreadPoolExecutor中保存的状态有

当前线程池状态, 包括RUNNING,SHUTDOWN,STOP,TIDYING,TERMINATED。

当前有效的运行线程的数量。

将这两个状态放到一个int变量中，前三位作为线程池状态，后29位作为线程数量。

例如0b11100000000000000000000000000001， 表示RUNNING, 一个线程。

通过HashSet来存储工作者集合，访问该HashSet前必须先获取保护状态的mainLock:ReentrantLock

submit、execute

execute的执行方式为，首先检查当前worker数量，如果小于corePoolSize，则尝试add一个core Worker。线程池在维护线程数量以及状态检查上做了大量检测。

public void execute(Runnable command) {
int c = ctl.get();
// 如果当期数量小于corePoolSize
if (workerCountOf(c) < corePoolSize) {
// 尝试增加worker
if (addWorker(command, true))
return;
c = ctl.get();
}
// 如果线程池正在运行并且成功添加到工作队列中
if (isRunning(c) && workQueue.offer(command)) {
// 再次检查状态，如果已经关闭则执行拒绝处理
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
// 如果工作线程都down了
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}

addWorker方法实现

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) {
// 如果添加成功，则启动该线程，执行Worker的run方法，Worker的run方法执行外部的runWorker(Worker)
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}

Worker类继承了AbstractQueuedSynchronizer获得了同步等待这样的功能。

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) {
}
}
}

runWorker(Worker)是Worker的轮询执行逻辑，不断地从工作队列中获取任务并执行它们。Worker每次执行任务前需要进行lock，防止在执行任务时被interrupt。

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);
}
}

ThreadPoolExecutor的submit方法中将Callable包装成FutureTask后交给execute方法。

FutureTask

FutureTask继承于Runnable和Future，FutureTask定义的几个状态为
NEW, 尚未执行
COMPLETING, 正在执行
NORMAL, 正常执行完成得到结果
EXCEPTIONAL, 执行抛出异常
CANCELLED， 执行被取消
INTERRUPTING，执行正在被中断
INTERRUPTED, 已经中断。

其中关键的get方法

public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}

先获取当前状态，如果还未执行完成并且正常，则进入等待结果流程。在awaitDone不断循环获取当前状态，如果没有结果，则将自己通过CAS的方式添加到等待链表的头部，如果设置了超时，则LockSupport.parkNanos到指定的时间。

static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}

FutureTask的run方法是执行任务并设置结果的位置,首先判断当前状态是否为NEW并且将当前线程设置为执行线程，然后调用Callable的call获取结果后设置结果修改FutureTask状态。

public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}

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