[第三篇]深入学习线程池之优雅的关闭线程池
通过 《深入学习线程池之线程池简介及工作原理》、《深入学习线程池之通过ThreadPoolExecutor创建线程池及工作原理》 两篇文章，相信大家已经了解怎么去创建一个线程池，并对线程池的工作原理有了认识，但你知道如何去关闭线程池么？直接调用shutdown()方法为什么关不掉线程池呢？shutdownNow()和shutdown()有什么区别？下面我们以ThreadPoolExecutor为例，来介绍下如何优雅的关闭线程池。在介绍线程池关闭之前，先介绍下线程中断。
一、线程中断
在程序中，我们不能随便中断一个线程，因为这是极其不安全的操作，我们无法知道这个线程正运行在什么状态，它可能持有某把锁，强行中断可能导致锁不能释放的问题；或者线程可能在操作数据库，强行中断导致数据不一致，从而混乱的问题。正因此，Java里将Thread的stop法?设置为过时，以禁止大家使用。
一个线程什么时候可以退出呢？当然只有线程自己才能知道。
所以我们这里要说的Thread的interrrupt方法，本质不是用来中断一个线程，而是将线程设置一个中断状态。当我们调用线程的interrupt方法，它有两个作用：
1、如果此线程处于阻塞状态(比如调用了wait方法，io等待)，则会立刻退出阻塞，并抛出InterruptedException异常，线程就可以通过捕获InterruptedException来做一定的处理，然后让线程退出。
2、如果此线程正处于运行之中，则线程不受任何影响，继续运行，仅仅是线程的中断标记被设置为true。所以线程要在适当的位置通过调用isInterrupted方法来查看自己是否被中断，并做退出操作。
注：如果线程的interrupt方法先被调用，然后线程调用阻塞方法进入阻塞状态，InterruptedException异常依旧会抛出。如果线程捕获InterruptedException异常后，继续调用阻塞方法， 将不再触发InterruptedException异常。
二、线程池的两种关闭方式
线程池提供了两个关闭方法：shuwdown() 和 shutdownNow() 方法。我们都知道这两个方法的处理逻辑，如下：
shutdown()方法处理逻辑是： 线程池不再接收新提交的任务，同时等待线程池?的任务执行完毕后关闭线程池。
【[第三篇]深入学习线程池之优雅的关闭线程池】shutdownNow()方法处理逻辑是： 线程池不再接收新提交的任务，同时立刻关闭线程池，线程池里的任务不再执行，并返回待所有未处理的线程list列表。
但是，调用shutdown()方法后，为什么正在执?任务的线程会继续执行完任务而不是立即停止？调用完shutdown() 或者 shutdownNow()方法后，线程池会立即关闭么？线程在什么情况下才会彻底退出?
如果不了解这些细节，在关闭线程池时就难免遇到，“线程池关闭不了”，“关闭线程池出现报错” 等情况。下面就结合线程池源码，分别说说这两个线程池关闭方法的一些实现细节。
1. 线程池中执行任务的方法runWorker()
/**
* Main worker run loop.Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
*
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters.Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and then we
* ensure that unless pool is stopping, this thread does not have
* its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to afterExecute.
* We separately handle RuntimeException, Error (both of which the
* specs guarantee that we trap) and arbitrary Throwables.
* Because we cannot rethrow Throwables within Runnable.run, we
* wrap them within Errors on the way out (to the thread's
* UncaughtExceptionHandler).Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
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);
}
}
正常情况下，线程池里的线程，就是在这个while循环里不停地执行。其中代码task.run()就是在执行我们提交给线程池的任务，如下:
threadpool.execute(new Runnable() {
@Override
public void run() {
// todo 具体的业务逻辑
}
});
从runWorker()方法看得出来，如果getTask()方法返回null，会导致线程的退出。我们再来看看getTask()方法的实现:
/**
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of:
* 1. There are more than maximumPoolSize workers (due to
*a call to setMaximumPoolSize).
* 2. The pool is stopped.
* 3. The pool is shutdown and the queue is empty.
* 4. This worker timed out waiting for a task, and timed-out
*workers are subject to termination (that is,
*{@code allowCoreThreadTimeOut || workerCount > corePoolSize})
*both before and after the timed wait, and if the queue is
*non-empty, this worker is not the last thread in the pool.
*
* @return task, or null if the worker must exit, in which case
*workerCount is decremented
*/
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;
}
}
}
2. shutdown()方法 当我们调用shutdown()方法时，源码如下，我们看到，它先将线程池的状态修改为SHUTDOWN状态，然后调用interruptIdleWorkers()方法，来中断空闲的线程，为什么是空闲线程呢？
在上边runWorker方法的代码中，我们看到获取任务之后第一步是进行加锁操作，即，
w.lock()。而，shutdown()方法调用的interruptIdleWorkers方法，会尝试进行w.tryLock()加锁操作，换言之，在runWorker方法中w.lock和w.unlock之间的线程将因为加锁成功，就会导致interruptIdleWorkers方法的w.tryLock() 加锁失败，进而不会被调用interrupt方法，也就是说正在执行线程池里任务的线程不会被中断。/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
* This method does not wait for previously submitted tasks to
* complete execution.Use {@link #awaitTermination awaitTermination}
* to do that.
*
* @throws SecurityException {@inheritDoc}
*/
public void shutdown() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(SHUTDOWN);
// ① 将线程池状态置为SHUTDOWN
interruptIdleWorkers();
// ② 停用线程池中的线程
onShutdown();
// hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
/**
* Interrupts threads that might be waiting for tasks (as
* indicated by not being locked) so they can check for
* termination or configuration changes. Ignores
* SecurityExceptions (in which case some threads may remain
* uninterrupted).
*
* @param onlyOne If true, interrupt at most one worker. This is
* called only from tryTerminate when termination is otherwise
* enabled but there are still other workers.In this case, at
* most one waiting worker is interrupted to propagate shutdown
* signals in case all threads are currently waiting.
* Interrupting any arbitrary thread ensures that newly arriving
* workers since shutdown began will also eventually exit.
* To guarantee eventual termination, it suffices to always
* interrupt only one idle worker, but shutdown() interrupts all
* idle workers so that redundant workers exit promptly, not
* waiting for a straggler task to finish.
*/
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
if (!t.isInterrupted() && w.tryLock()) {// 线程没有被中断且worker获取到锁的时候才处理
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}/**
* Common form of interruptIdleWorkers, to avoid having to
* remember what the boolean argument means.
*/
private void interruptIdleWorkers() {
interruptIdleWorkers(false);
}
3. shutdownNow()方法 当我们调用shutdownNow()方法时，源码如下，我们看到，它先将线程池的状态修改为STOP状态，然后调用interruptWorkers()方法，遍历中断线程，最后返回未执行的任务的线程list。
在runWorker方法中，代码task.run()就是在执行我们提交给线程池的任务，当我们调用shutdownNow方法时，task.run()里面正处于IO阻塞，即，我们提交任务的逻辑，涉及到IO阻塞，则会导致报错，如果task.run()里正在正常执行，则不受影响，继续执行完这个任务。
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution. These tasks are drained (removed)
* from the task queue upon return from this method.
*
* This method does not wait for actively executing tasks to
* terminate.Use {@link #awaitTermination awaitTermination} to
* do that.
*
* There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks.This implementation
* cancels tasks via {@link Thread#interrupt}, so any task that
* fails to respond to interrupts may never terminate.
*
* @throws SecurityException {@inheritDoc}
*/
public List shutdownNow() {
List tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP);
interruptWorkers();
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
/**
* Interrupts all threads, even if active. Ignores SecurityExceptions
* (in which case some threads may remain uninterrupted).
*/
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
}
总结
一、当我们调用线程池的shutdownNow方法时，会将线程池状态修改为STOP，当执行runWorker方法中
while (task != null || (task = getTask()) != null)时，在getTask方法中，由于STOP状态值是大于SHUTDOWN状态，STOP也大于等于STOP，所以不管任务队列是否为空，都会进入if语句，即， if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
从而返回null，导致
(task = getTask()) != null条件不成立，进而执行线程退出。二、当我们调用线程池的shuwdown方法时，会将线程池状态修改为SHUTDOWN，当执行runWorker方法中
while (task != null || (task = getTask()) != null)时，在getTask方法中，SHUTDOWN大于等于SHUTDOWN成立没问题，但是SHUTDOWN不大于等于STOP状态，所以只有队列为空，getTask方法才会返回null，导致线程退出。如果线程正在执行线程池里的任务，即便任务处于阻塞状态，线程也不会被中断，而是继续执行。如果线程池阻塞等待从队列里读取任务，则会被唤醒，但是会继续判断队列是否为空，若不为空，则会继续从队列里读取任务，若为空则线程退出。优雅的关闭线程池
使用shutdownNow?法，可能会引起报错，使用shutdown方法可能会导致线程关闭不了。
所以当我们使用shutdownNow?法关闭线程池时，一定要对任务里进行异常捕获。即，在我们提交的任务里有try{}catch{}处理
当我们使用shuwdown方法关闭线程池时，一定要确保任务里不会有永久阻塞等待的逻辑，否则线程池就关闭不了。
最后，一定要记得shutdownNow和shuwdown调用完，线程池并不是立刻就关闭了，要想等待线程池关闭，还需调用awaitTermination方法来阻塞等待。
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