1、现象

main方法执行线程池后 发现线程创建完毕 但是mian 方法一直卡在那里无法退出

2、源码

2.1、执行execute后代码如下

注意看下面的addworker

class: java.util.concurrent.ThreadPoolExecutor
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
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);
}

2.2、执行start会回调run方法

private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
**
**
..
..
**
/** Delegates main run loop to outer runWorker
*/
public void run() {
runWorker(this);
}

2.3、runworker下有个死循环

final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//这块会一直进行拉取任务
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 {

2.4、继续查看getTask方法

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;
}
}
}

如果无超时时间，则不会结束

3,结束办法：

但是实际测试时候发现这里并不是死循环  只是一直去取  如果没任务就没了

实际测试  如果设置线程池为守护进程 就会退出（默认是非守护进程） 这个跟jvm有关

setDaemo(false);

参考:https://www.yuque.com/u1300481/kb/gr403g

参考:https://www.jianshu.com/p/7c7eb33a4424

最后

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