前言
通常我们在java项目中使用线程时不会单独地去开一个线程，如果并发的线程数量很多，并且每个线程都是执行一个时间很短的任务就结束了，这样频繁创建线程就会大大降低系统的效率，因为频繁创建线程和销毁线程需要时间。那么有没有一种办法使得线程可以复用，就是执行完一个任务，并不被销毁，于是java提供了ThreadPoolExecutor这个类来让我们可以快速创建一个线程池，有了线程池上述问题都可以得到解决。偶尔机会发现这个类的实现不是很复杂，但是却设计得很优雅，于是拿出来分析一下源码。
使用简介
在分析前我们先看一下这个类是如何使用的，这里介绍最简单的使用方法：
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 5, 1, TimeUnit.HOURS, new ArrayBlockingQueue<Runnable>(1));
for (int i = 0; i < 20; i++) {
threadPoolExecutor.execute(new ThreadPrint());//ThreadPrint是实现了Runnable接口的类
}
threadPoolExecutor.shutdown();
new一个threadPoolExecutor对象，并依次传入参数：核心线程池大小，最大线程池大小，保持线程存活时间，存活时间单位，任务队列；当然其他的构造方法也类似，这里举最重要的几个参数。
这里面我觉得最有意思的是任务队列这个参数，这个参数通常是这三个队列：SynchronousQueue,LinkedBlockingDeque,ArrayBlockingQueue。
- SynchronousQueue：无容量队列
这个队列接收到任务的时候，会直接提交给线程处理，而不保留它，如果所有线程都在工作怎么办？那就新建一个线程来处理这个任务！所以为了保证不出现<线程数达到了maximumPoolSize而不能新建线程>的错误，使用这个类型队列的时候，maximumPoolSize一般指定成Integer.MAX_VALUE，即无限大
- LinkedBlockingDeque：无界队列
这个队列接收到任务的时候，如果当前线程数小于核心线程数，则新建线程(核心线程)处理任务；如果当前线程数等于核心线程数，则进入队列等待。由于这个队列没有最大值限制，即所有超过核心线程数的任务都将被添加到队列中，这也就导致了maximumPoolSize的设定失效，因为总线程数永远不会超过corePoolSize
- ArrayBlockingQueue：有界队列
可以限定队列的长度，接收到任务的时候，如果没有达到corePoolSize的值，则新建线程(核心线程)执行任务，如果达到了，则入队等候，如果队列已满，则新建线程(非核心线程)执行任务，又如果总线程数到了maximumPoolSize，并且队列也满了，则发生错误
源码摘要
打开ThreadPoolExecutor的源码发现了一些有意思的东西：
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
看起来都是一些位运算，那么我们来把他们都换成2进制看一下：
CAPACITY 00011 111111111111111111111111111
RUNNING 11100 000000000000000000000000000
SHUTDOWN 00000 000000000000000000000000000
STOP 00100 000000000000000000000000000
TIDYING 01000 000000000000000000000000000
TERMINATED 01100 000000000000000000000000000
其实可以很容易看出来，一个32位的int被分成了两部分，高5位表示线程池的状态，低27位表示线程数量。
从execute看起：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
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);
}
一开始通过全局的ctl拿到线程数，与设定的核心线程数比较，如果小于，就将当前的线程当做核心线程加入。否则如果线程池还在运行且队列能塞下线程就将该线程当做非核心线程塞入，如果此时线程数是0，那么塞入一个null，当然，如果塞入线程时出错，则都报个错。
我们再来看一下addWorker这个方法的实现：
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
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
}
}
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;
}
首先进入该方法时会有一个无限循环，该循环先检测线程池的状态和线程池的大小（核心或非核心）是否已经超了过了预设值，如果这两个检测有一个通过不了就直接返回false，上一层会报错。如果检测都通过，则先将线程数计数加1，也就是在ctl的低27位加1。然后将传进来的线程firstTask包装为一个Worker，并且加入到workers中，更新largestPoolSize并执行该Worker.thread的start方法，start方法会调用run，run调用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 {
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);
}
}
如果传入的task不是null（怎么才会为null？这里是循环，执行完了会被设为null）就执行task的run方法，相当于启动了线程，如果是null，那么执行一次getTask方法拿到一个task去执行，执行完了会将completedTasks加1，且将task置为null。
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;
}
}
}
首先也是检查线程池的状态和大小，如果有异常则退出，接着是在循环中取workQueue中的一个task，直到取出正确的task，则返回，上层拿去运行。这里取runnable时可能会被阻塞。
最后调用shutdown来关闭线程池：
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(SHUTDOWN);
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
主要是将线程池状态设为关闭，且将线程都杀死，实现在interruptIdleWorkers中。至此差不多流程都已走通，更多的细节在代码中，也不好面面俱到。
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