问题:
1.线程池是怎么保证他的核心线程不释放 而一直等待任务的执行的呢?
2.我们一直理解的线程run方法执行完毕线程就销毁是不正确的?
3.还有我们为何通过设置allowCoreThreadTimeOut(true) 就能使核心线程销毁的呢?
答案:
1.线程阻塞
2.和那个关系不大
3.可以的
有兴趣的话,可以看看下面的源码分析:
从 exexute 方法开始:
public void execute(Runnable command) {
//如果提交的任务为null 抛出异常
if (command == null)
throw new NullPointerException();
// 线程池是通过Integer类型的 高3位 表述当前线程池的状态RUNNING,SHUTDOWN,STOP,TIDYING,TERMINATED 。
// 低29位表示当前线程的运行任务数量。然后通过位运算来计算运行状态和任务的数量。
int c = ctl.get();
//1.如果当前的任务数小于等于设置的核心线程大小,那么调用addWorker直接执行该任务
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2.如果当前的任务数大于设置的核心线程大小,而且当前的线程池状态时运行状态,那么向阻塞队列中添加任务
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);
}
//3.如果向队列中添加失败,那么就新开启一个线程来执行该任务
else if (!addWorker(command, false))
reject(command);
}
下面看看 addWorker 方法:
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
//一直尝试,
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
// 如果当前线程池的状态时SHUTDOWN,STOP,TIDYING,TERMINATED
// 并且为SHUTDOWN状态时任务队列为空,那么就返回false
// 原因:如果调用了shutdown方法,此时的线程池还会继续工作并且会在任务队列中的所有任务执行完成后才会结束线程池。
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
//
int wc = workerCountOf(c);
//core是在execute方法中传的参数:true表示 核心线程,false表示最大线程
//CAPACITY 可以理解为Integer的最大值 1左移29位再-1
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//如果增加任务数量成功那么退出这个循环执行下面的代码,否则继续
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 {
//再判断一次当前线程池的状态 避免在执行过程中线程时被使用者关闭
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
//向正在执行的任务队列(workers)中添加work
// 区别一下:workqueue 是等待执行的阻塞队列
workers.add(w);
int s = workers.size();
//记录曾经并发执行的最大任务个数
if (s > largestPoolSize)
largestPoolSize = s;
//添加任务成功
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//如果任务添加成功那么开始执行任务
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
下面看看 Worker 类的run 方法,实际是调用的ThreadPoolExecutor#runWorker方法:
final void runWorker(ThreadPoolExecutor.Worker w) {
Thread wt = Thread.currentThread();
//得到worker对象中我们提交的任务
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//如果当前任务为空 那么就从getTask中获得任务
while (task != null || (task = getTask()) != null) {
w.lock();
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
//任务执行前调用的方法
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 {
//任务结束后调用的方法
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
下面看看 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?
//从阻塞任务队列中取任务,如果设置了allowCoreThreadTimeOut(true)
// 或者
// 当前运行的任务数大于设置的核心线程数,那么timed =true
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//根据 timed true 或 false 来判断从哪里取任务,是否超时取任务
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
// take()会一直阻塞,等待任务的添加。
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
到这里,所有的谜团都解开了。