线程池的基本结构

线程池的基本结构

就以ThreadPoolExecutor为例。当我们把一个Runnable交给线程池去执行的时候，这个线程池处理的流程是这样的：

1. 先判断线程池中的核心线程们是否空闲，如果空闲，就把这个新的任务指派给某一个空闲线程去执行。如果没有空闲，并且当前线程池中的核心线程数还小于 corePoolSize，那就再创建一个核心线程。
2. 如果线程池的线程数已经达到核心线程数，并且这些线程都繁忙，就把这个新来的任务放到等待队列中去。如果等待队列又满了，那么
3. 查看一下当前线程数是否到达maximumPoolSize，如果还未到达，就继续创建线程。如果已经到达了，就交给RejectedExecutionHandler来决定怎么处理这个任务。

看一下代码:

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);
}
}

工作线程和等待队列

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);
}
}

这段代码里有几个地方要注意的，第一，我们可以使用beforeExecute和afterExecute这两个方法去监控任务的执行情况，这些方法在ThreadPoolExecutor里都是空方法，我们可以重写这些方法来实现线程池的监控。第二，就是线程的逻辑是不断地执行一个循环，去调用 getTask 方法来获得任务

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;
}
}
}

RejectedExecutionHandler

当队列和线程池都满了的时候，再有新的任务到达，就必须要有一种办法来处理新来的任务。Java线程池中提供了以下四种策略：

1. AbortPolicy: 直接抛异常
2. CallerRunsPolicy：让调用者帮着跑这个任务
3. DiscardOldestPolicy：丢弃队列里最老的那个任务，执行当前任务
4. DiscardPolicy：不处理，直接扔掉