线程池ThreadPoolExecutor分析
线程池。线程池是什么，说究竟，线程池是处理多线程的一种形式，管理线程的创建，任务的运行，避免了无限创建新的线程带来的资源消耗，可以提高应用的性能。非常多相关操作都是离不开的线程池的，比方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，还是要从它的源代码实现入手，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，我们能够看到它的构造方法：
/**
* 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;
}
第一个參数，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任务堵塞队列，当核心线程创建数量达到最大值时，任务首先会加入到堵塞队列。等待运行;ThreadFactory 线程构造工厂。经常使用的有DefaultThreadFactory，我们也能够重写它的newThread方法，实现这个类；RejectedExecutionHandler,当任务被拒绝加入时。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，这种方法就是我们须要运行的任务。以下我们分析execute是怎么运行这个任务的，也就是execute运行的过程：
1.假设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.假设核心线程处于非空暇状态，则将任务插入到堵塞队列中，当有线程空暇时，会自己主动取出任务运行。
3.假设堵塞队列任务已经满了。而且当前线程小于最大线程数目，则启动新的线程。运行任务。假设超过了最大线程数，则拒绝接受新的任务。
上面是整个代码的过程，如今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。假设这个数量小于corePoolSize，则调用addWorker(command,true)方法，假设addWorker运行成功，返回true，表示任务运行完毕。以下我们看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方法比較长，我们分为两块，第一块的for循环，第二块try...。第一块，for循环。主要是推断当前能否够运行任务，假设能够，则进行以下的try,通过的条件。能够分析出来。是当前线程小于核心线程。或者当前堵塞队列已满，线程数小于最大线程数量。满足这两个中的条件，才干往下走。 try里面通过当前的參数，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来运行这个任务，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的线程中运行的。runWorker中，while循环中，加锁，第一次task不为空，运行这个firstTask,当task.run()运行完，解锁，然后调用getTask（）。getTask是从堵塞队列中取出任务来运行，因此，这里，我们得出结论。当线程完毕一个任务时。会从堵塞队列里取出任务来运行。 while循环的条件是，task 不为空。或者getTask不为空，假设task为空。而且getTask也为空，就跳出循环，可是，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;
}
}
}
这种方法的主要作用是从队列中取出一个任务运行，然而。我们细致分析。假设allowCoreThreadTimeOut为true（也就是说同意核心线程超时），或者当前线程是非核心线程，那么timed=true,这时候。进入if语句推断，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，假设队列的任务为空，就会运行到compareAndDecrementWorkerCount(c)降低一个线程数量,然后返回null，线程也就终止运行了；可是假设allowCoreThreadTimeOut=false,就会直接调用workQueue.take(),直接取出一个Runnable。假设runnable为空，就将一直循环，线程堵塞在这里，也就是核心线程处于空暇状态。
这里得出新的结论。假设核心线程没有同意超时。那么它将一直处于空暇状态，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);
}假设当前线程池处于执行状态，而且这个任务能插入到队列中（由于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）。第二次推断。假设当前线程池不处于执行状态。那么移除这个任务，调用reject方法处理；假设当前线程池的线程数量为0，addWorker(),传入null和false，也就是说，不加入新任务，而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运行任务。假设非核心线程达到了最大线程数，才会拒绝运行任务。
最后的结论就是：
1.假设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.假设核心线程处于非空暇状态，则将任务插入到堵塞队列中。当有线程空暇时，会自己主动取出任务运行。
3.假设堵塞队列任务已经满了，而且当前线程小于最大线程数目，则启动新的线程。运行任务，假设超过了最大线程数，则拒绝接受新的任务。
假设核心线程没有设置为同意超时。那么核心线程会一直存在，即时等待运行任务。
