构造器
目录
corePoolSize：核心线程数
maximumPoolSize：最大线程数
keepAliveTime：当总线程数大于核心线程数时，多余的空闲线程的最长停留时间
unit：对应上面的时间单位
workQueue：任务等待队列，超出核心线程数时，新任务会加入这个队列，常用的有ArrayBlockingQueue、LinkedBlockingQueue和SynchronousQueue（无界队列）等
handler：饱和策略，超出最大线程数和队列都无法存放时的饱和策略，可以实现RejectedExecutionHandler来自定义饱和策略，也可以使用jdk预制的常量，后面会解释
threadFactory:线程工厂，指定在创建线程时从此工厂中获取线程，ThreadFactory包含接口类方法 Thread newThread(Runnable r);一般用于指定线程名称
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}饱和策略
CallerRunsPolicy：直接同步运行该任务
AbortPolicy：直接抛出RejectedExecutionException异常
DiscardPolicy：任务丢弃，不执行任何操作，也不抛异常
DiscardOldestPolicy：直接丢弃队列中一个最早加入的任务，然后再次execute提交当前任务
核心
任务执行流程
execute提交任务
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//如果当前总工作线程数小于核心线程数
if (workerCountOf(c) < corePoolSize) {
//新建worker工作线程
if (addWorker(command, true))
return;
c = ctl.get();
}
//超出核心线程数后，把任务加入队列workQueue
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//如果线程池已停止，则执行饱和策略
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
//如果任务队列加入失败，并且addWorker也失败，则执行饱和策略
reject(command);
}addWoker方法
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);
//如果超过最大线程数，则返回false，不能新建Worker
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 {
//新建Worker工作线程
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) {
//线程启动
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}Worker中线程run方法如下
关键点：一个worker一旦开始执行，就一直处理while循环，不断的从队列中获取task执行，一直到没有task执行时，while才停止循环，当前worker才有可能被销毁
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//注意这个地方是while循环，一个task执行完成后，会通过getTask继续获取新的任务去执行
while (task != null || (task = getTask()) != null) {
//worker锁定
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 {
//线程执行前钩子
beforeExecute(wt, task);
Throwable thrown = null;
try {
//任务执行
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
//如果执行失败，抛出异常，当前worker线程会直接停止
thrown = x; throw new Error(x);
} finally {
//线程执行后钩子
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
//当没有任务处理时，处理worker的销毁
processWorkerExit(w, completedAbruptly);
}
}getTask方法如下
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?
//allowCoreThreadTimeOut如果为false（默认值），则即使处于空闲状态，核心线程也会保持活动状态。如果为true，则核心线程使用keepAliveTime超时等待
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//从队列中poll拉取任务，此处用到了keepAliveTime，也就是等待指定时间后，依然没有新任务，那么结合上面的runWorker，worker才会去停止
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
Woker使用到AQS分析
问题：
Worker为啥不直接使用ReentrantLock，而要自己实现AQS锁？
1、Worker不需要可重入，因为Worker没有需要重入的需求
2、可重入会带来新的问题，大概意思就是可能工作线程自己调用setCorePoolSize方法时，获取到锁之后，再Thread.interrupt()会出现自己中断自己的情况
翻译过来：这是因为可能正在等待任务的线程是通过未锁定来表示的，即tryLock为它们返回true。如果在这里使用ReentrantLock，在这种情况下会发生什么：那么可以按以下顺序进行操作：
setCorePoolSize-> interruptIdleWorkers-> tryLock（）（在这里成功！）-> Thread.interrupt（此工作程序的线程）
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
......
protected boolean isHeldExclusively() {
return getState() != 0;
}
//尝试获取
protected boolean tryAcquire(int unused) {
//把state设置为1，表示占有锁
if (compareAndSetState(0, 1)) {
//设置排他线程
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
//解锁，state改为0
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) {
}
}
}
}总结
1、提交任务给ThreadPoolExecutor，当工作线程数<核心线程数时，此时会新建工作线程。当工作线程数>核心线程数时，此时会把任务交给队列。当队列中无法放入任务时，此时会执行饱和策略
2、工作线程是通过while循环的方式不断从队列中获取任务来执行的
3、Worker使用了AQS，没有使用ReentrantLock
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