今天面试被问到线程池如何复用线程的？当场就懵掉了...于是面试完毕就赶紧打开源码看了看，在此记录下:
我们都知道线程池的用法，一般就是先new一个ThreadPoolExecutor对象，再调用execute(Runnable runnable)传入我们的Runnable，剩下的交给线程池处理就行了，于是这次我就从ThreadPoolExecutor的execute方法看起：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//1.如果workerCountOf(c)即正在运行的线程数小于核心线程数，就执行addWork
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2.如果线程池还在运行状态并且把任务添加到任务队列成功
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//3.如果线程池不在运行状态并且从任务队列移除任务成功，执行线程池饱和策略（默认直接抛出异常）
if (! isRunning(recheck) && remove(command))
reject(command);
//4.否则如果此时运行线程数==0，就直接调用addWork方法
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//5.如果2条件不成立，继续判断如果addWork返回false，执行线程池饱和策略
else if (!addWorker(command, false))
reject(command);
}大致过程就是如果核心线程未满，则直接addWorker（该方法下面会再分析）；如果核心线程已满，则尝试将任务加进消息队列中，并再判断如果此时运行线程数==0则调addWorker方法，否则不做任何处理（因为运行的线程处理完自己的任务后会去消息队列中取任务来执行，下面会分析）；如果任务队列添加任务失败，那么直接addWorker()，如果addWorker返回false，执行饱和策略，下面我们就来看看addWorker里面做了什么
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);
//1.如果正在运行的线程数大于corePoolSize 或 maximumPoolSize（core代表以核心线程数还是最大线程数为边界），return false，表示addWorker失败
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//2.否则将运行线程数+1，并跳出这个for循环
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 {
//3.创建一个Worker对象，传入我们的runnable
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) {
//4.开始启动线程
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//1.当firstTask不为空或getTask不为空时一直循环
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 {
//2.执行任务
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);
}
}可以看到addWorker方法主要就是先判断正在运行线程数是否超过了最大线程数（具体根据边界取），如果未超过则创建一个worker对象，其中firstTask是我们传入的Runnable，当然根据上面的execute方法可知当4条件满足时，传入的firstTask是null，Thread是用ThreadFactory创建的线程，传入的Runnable是Worker自己，最后开启线程，于是执行Worker这里的run、runWorker方法，在runWorker方法里，开启一个while循环，当firstTask不为空或getTask不为空时，执行task，下面我们接着看看getTask里面做了什么：
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?
//1.会不会淘汰空闲线程
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//2.return null意味着回收一个Worker即淘汰一个线程
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//3.等待指定时间
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}可以看1、2注释，allowCoreThreadTimeOut代表存活一定时间是否对核心线程有效（默认为false），先看它为ture的情况，此时不管是核心线程还是非核心线程在3处都会等待一定时间（就是我们传入的线程保活时间），等待时间内如果从任务队列取到任务，则返回执行，否则timeout为true，继续走到2，由于(timed && timedOut)和workQueue.isEmpty()均为true，返回null，代表回收一个线程；如果allowCoreThreadTimeOut为false，代表不回收核心线程，此时如果在3处没有取到任务，继续执行到2处，只有当wc > corePoolSize或wc > maximumPoolSize时才会执行return null，否则一直循环，相当于该线程一直处于运行状态，直到从任务队列拿到新的任务
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