线程池的状态
shutdown状态下不接受新的任务，但是已经提交到线程池的任务会被正常执行
stop状态下不接受新的任务且已经提交的任务不再执行，正在执行的任务会被中断
类中是用AtomicInteger(ctl)的高3位表示池的状态的， 后29位计数池中的线程数
image.png
worker的设计
worker继承AQS，实现了Runnable接口
run方法调用外部类中的runWorker方法
而runWorker方法中涉及到任务队列中任务的调度与执行
runWorker方法
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//task从当前worker或者任务队列获取， 如果不空会一直执行下去
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);
}
}
任务的提交--execute
源码
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//1. 核心池未满， 添加核心线程执行
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//2.尝试加到非阻塞的加到任务队列
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. new一个非核心线程执行
else if (!addWorker(command, false))
reject(command);
}
调用流图
image.png
新增一个工作线程
主要分两步：一是线程数+1， 二是新建一个工作线程，并且开始执行
调用流图
image.png
线程池状态变量的线程数加一
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
// 我的理解： 对于 （rs == shutdown && 任务为空 && 任务队列不空）的情况仍然会创建线程， 用来执行任务队列的任务， 这里不与shutdown状态的语义矛盾（不接受新的任务，但已提交的任务会处理）
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;
//CAS增加线程数
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
//如果CAS失败是因为线程池的状态变了， 跳到外层循环，否则内层循环重试。
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
创建一个线程
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());
//如果通过了recheck线程池的状态， 就把worker加到集合中
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();
}
//如果worker被加到集合中，运行线程
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
//如果worker没有被加到集合中， 线程销毁等操作
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
线程池的keepAlive是如何实现的
主要体现在getTask方法中
当前线程状态判断
是否满足线程超时回收的条件--如果是的话当前线程就被回收。
获取任务（poll OR take 根据是否超时获取）。
源码
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;
}
}
}
