Java-线程池实现线程复用和工作线程的关闭
2021-02-03 本文已影响0人
zzq_nene
1.ThreadPoolExecutor.execute
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);
}
这里主要做两件事：
- 小于corePoolSize的时候创建核心线程
- 当前核心线程都正在执行则入队，判断是否需要创建工作线程
2.ThreadPoolExecutor.addWorker
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
// 判断是否可以创建worker
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
}
}
// 创建Worker并启动
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) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
这里采用显示锁的方式实现同步创建Worker
3.Worker的run()
Worker其实也是一个Runnable的子类，那么从执行到添加Worker的过程都没有线程复用的逻辑，那么该逻辑就应该是在Worker的run方法中。
在这里可以看到，Worker的runWorker会调用ThreadPoolExecutor的runWorker方法。
而在runWorker方法中，会采用一个while循环从task任务队列中取出待执行的任务，如果任务是没有被中断的，则会调用task.run()方法执行任务。
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);
}
}
getTask()的内部是采用一个无限for循环的方式进行循环遍历
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);
// 判断是否允许核心线程超时或者当前工作线程是否大于核心线程数
// 如果为true
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
// 如果存在的线程数大于核心线程数，则从队列中取任务时没有任务返回null
// 如果存在的线程数小于核心线程数，则从队列中取任务时没有任务将阻塞队列
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
而工作线程的关闭，其实可以从上面两种情况知道，在线程通过while无限循环的时候，如果是非核心线程（即工作线程）调用BlockingQueue的poll方法，则会在队列为空的时候返回null而不会阻塞队列，当返回的task是null的时候，那么runWorker中的while循环就会退出，最终就会执行processWorkerExit(w, completedAbruptly)而completedAbruptly=false进行线程的删除；如果是核心线程的话，则会调用BlockingQueue的take()方法，那么在队列为空的时候就会通过take()方法调用而阻塞等待队列结果的返回，而不会返回一个task=null，这样runWorker中的while循环就不会退出，就会一直等待结果。
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
所以工作线程会被关闭，而核心线程不会被关闭，就是因为工作线程的run()方法会通过task=null退出循环而执行完成run()的方法体内容，从而自动关闭；而核心线程因为阻塞等待队列返回的原因而不会导致while循环的结束，所以不会关闭。
BlockingQueue的方法都是成对出现：
add和remove：这两个是非阻塞的，当队列满的时候，add会抛出异常，当队列为空的时候，remove会抛出异常。
offer和poll：使用offer往满的队列里放入元素，会返回false；poll方法往空的队列里拿元素，会返回一个null
put和take：这是真正的阻塞方法，使用put往满的队列里放元素，会被阻塞；使用take往空的队列里拿方法，会被阻塞。
