线程池ThreadPoolExecutor分析（一）

线程池，线程池是什么，说到底，线程池是处理多线程的一种形式，管理线程的创建，任务的执行，避免了无限创建新的线程带来的资源消耗，能够提高应用的性能。很多相关操作都是离不开的线程池的，比如android应用中网络请求的封装。这篇博客要解决的问题是：

1.线程池的工作原理及过程。

要分析线程池的工作原理及过程，还是要从它的源码实现入手，首先是线程是构造方法，何谓构造方法，构造方法就是对成员变量进行初始化，在这里，我们可以看到它的构造方法：

/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
*        if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
*        pool
* @param keepAliveTime when the number of threads is greater than
*        the core, this is the maximum time that excess idle threads
*        will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
*        executed.  This queue will hold only the {@code Runnable}
*        tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
*        creates a new thread
* @param handler the handler to use when execution is blocked
*        because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
*         {@code corePoolSize < 0}<br>
*         {@code keepAliveTime < 0}<br>
*         {@code maximumPoolSize <= 0}<br>
*         {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
*         or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}

第一个参数，corePoolSize核心线程的数量；maximumPoolSize最大线程数量；keepAliveTime和 TimeUnit非核心线程闲置时间，超过这个设置时间将会被终止，TimeUnit含有多种静态成员变量作为单位，如seconds；BlockingQueue任务阻塞队列，当核心线程创建数量达到最大值时，任务首先会加入到阻塞队列，等待执行;ThreadFactory 线程构造工厂，常用的有DefaultThreadFactory，我们也可以重写它的newThread方法，实现这个类；RejectedExecutionHandler,当任务被拒绝添加时，将会交给这个类处理。好了，构造的过程，就是这么简单，就是初始化一些成员变量。

分析的时候，从关键点着手，这里分析是从execute()方法入手的：

/**
* Executes the given task sometime in the future.  The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
*         {@code RejectedExecutionHandler}, if the task
*         cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
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);
}

上面execute的方法，传入的是一个Runnable，这个方法就是我们需要执行的任务。下面我们分析execute是怎么执行这个任务的，也就是execute执行的过程：

1.如果线程池中线程的数量小于核心线程数目，则启动一个新的线程处理这个任务。

2.如果核心线程处于非空闲状态，则将任务插入到阻塞队列中，当有线程空闲时，会自动取出任务执行。

3.如果阻塞队列任务已经满了，并且当前线程小于最大线程数目，则启动新的线程，执行任务，如果超过了最大线程数，则拒绝接受新的任务。

上面是整个代码的过程，现在我们队代码进行分析

if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;

workerCountOf(c)表示的是当前线程的数量，如果这个数量小于corePoolSize，则调用addWorker(command,true)方法，如果addWorker执行成功，返回true，表示任务执行完成。下面我们看addWorker方法：

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;
}

addWorker方法比较长，我们分为两块，第一块的for循环，第二块try...。第一块，for循环，主要是判断当前是否可以执行任务，如果可以，则进行下面的try,通过的条件，可以分析出来，是当前线程小于核心线程，或者当前阻塞队列已满，线程数小于最大线程数量，满足这两个中的条件，才能往下走。 try里面通过当前的参数，新建了一个Worker，这个Worker实现了Runable接口，Worker里面通过ThreadFactory构建了一个Thread来执行这个任务，代码的后面，调用了t.start()，实际上，调用的是，Worker实现Runable的run方法，run方法调用的又是runWorker(),我们看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);
}
}

首先我们要知道，整个runWorker方法是在Thread的线程中执行的。runWorker中，while循环中，加锁，第一次task不为空，执行这个firstTask,当task.run()执行完，解锁，然后调用getTask（），getTask是从阻塞队列中取出任务来执行，因此，这里，我们得出结论，当线程完成一个任务时，会从阻塞队列里取出任务来执行。 while循环的条件是，task 不为空，或者getTask不为空，如果task为空，并且getTask也为空，就跳出循环，但是，先请看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?
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;
}
}
}

这个方法的主要作用是从队列中取出一个任务执行，然而，我们仔细分析，如果allowCoreThreadTimeOut为true（也就是说允许核心线程超时），或者当前线程是非核心线程，那么timed=true,这时候，进入if语句判断，wc>maximuPoolsize,这个一般不成立，直接看timed&&timeOut,timed=true,timeOut为false,直接进入try,最后timeOut=true,当后面循环时，如果队列的任务为空，就会执行到compareAndDecrementWorkerCount(c)减少一个线程数量,然后返回null，线程也就终止执行了；但是如果allowCoreThreadTimeOut=false,就会直接调用workQueue.take(),直接取出一个Runnable，如果runnable为空，就将一直循环，线程阻塞在这里，也就是核心线程处于空闲状态。这里得出新的结论，如果核心线程没有允许超时，那么它将一直处于空闲状态，不会被回收。

我们再次回到execute方法，第二部分

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);
}

如果当前线程池处于运行状态，并且这个任务能插入到队列中（因为这里线程数目已经时等于核心线程数了，因此插入到队列中，等待执行），第二次判断，如果当前线程池不处于运行状态，那么移除这个任务，调用reject方法处理；如果当前线程池的线程数量为0，addWorker(),传入null和false，也就是说，不添加新任务，并且时启动一个非核心线程，这个核心线程会通过getTask()方法取出任务执行，这个和上面分析的过程一样。

第三部分

else if (!addWorker(command, false))
reject(command);

先启动一个非核心线程执行任务，如果非核心线程达到了最大线程数，才会拒绝执行任务。

最后的结论就是：

1.如果线程池中线程的数量小于核心线程数目，则启动一个新的线程处理这个任务。

2.如果核心线程处于非空闲状态，则将任务插入到阻塞队列中，当有线程空闲时，会自动取出任务执行。

3.如果阻塞队列任务已经满了，并且当前线程小于最大线程数目，则启动新的线程，执行任务，如果超过了最大线程数，则拒绝接受新的任务。

如果核心线程没有设置为允许超时，那么核心线程会一直存在，即时等待执行任务。