Java线程池的关闭
一、线程中断
在介绍线程池关闭之前，先介绍下Thread的interrupt。
在程序中，我们是不能随便中断一个线程的，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混乱的问题。正因此，JAVA里将Thread的stop方法设置为过时，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。是将线程设置一个中断状态。
interrupt方法两个作用
1、如果此线程处于阻塞状态(比如调用了wait方法，io等待)，则会立马退出阻塞，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：
1、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。
2、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、线程池的关闭
线程池提供了两个关闭方法，shutdownNow和shuwdown方法。
shutdownNow方法的解释是：
线程池拒接收新提交的任务，同时立马关闭线程池，线程池里的任务不再执行。
shutdown方法的解释是：
线程池拒接收新提交的任务，同时等待线程池里的任务执行完毕后关闭线程池。
以上的说法虽然没错，但是还有很多的细节，比如调用shutdown方法后，正在执行任务的线程做出什么反应？正在等待任务的线程又做出什么反应？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，像线程池关闭不了，关闭线程池出现报错等情况。
再说这些关闭线程池细节之前，需要强调一点的是，调用完shutdownNow和shuwdown方法后，并不代表线程池已经完成关闭操作，它只是异步的通知线程池进行关闭处理。如果要同步等待线程池彻底关闭后才继续往下执行，需要调用awaitTermination方法进行同步等待。
有了以上介绍，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。
shutdownNow
/**
* 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.
*
* <p>This method does not wait for actively executing tasks to
* terminate. Use {@link #awaitTermination awaitTermination} to
* do that.
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. This implementation
* interrupts tasks via {@link Thread#interrupt}; any task that
* fails to respond to interrupts may never terminate.
*
* @throws SecurityException {@inheritDoc}
*/
public List<Runnable> shutdownNow() {
List<Runnable> tasks;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
checkShutdownAccess();
advanceRunState(STOP);
interruptWorkers();
tasks = drainQueue();
} finally {
mainLock.unlock();
}
tryTerminate();
return tasks;
}
advanceRunState(STOP)
将线程池状态修改为STOP。
/**
* Transitions runState to given target, or leaves it alone if
* already at least the given target.
*
* @param targetState the desired state, either SHUTDOWN or STOP
* (but not TIDYING or TERMINATED -- use tryTerminate for that)
*/
private void advanceRunState(int targetState) {
// assert targetState == SHUTDOWN || targetState == STOP;
for (;;) {
int c = ctl.get();
if (runStateAtLeast(c, targetState) ||
ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c))))
break;
}
}
interruptWorkers()
是遍历线程池里的所有工作线程，然后调用线程的interrupt方法。
/**
* Interrupts all threads, even if active. Ignores SecurityExceptions
* (in which case some threads may remain uninterrupted).
*/
private void interruptWorkers() {
// assert mainLock.isHeldByCurrentThread();
for (Worker w : workers)
w.interruptIfStarted();
}
tasks = drainQueue()
是将队列里还没有执行的任务放到列表里，返回给调用方。
/**
* Drains the task queue into a new list, normally using
* drainTo. But if the queue is a DelayQueue or any other kind of
* queue for which poll or drainTo may fail to remove some
* elements, it deletes them one by one.
*/
private List<Runnable> drainQueue() {
BlockingQueue<Runnable> q = workQueue;
ArrayList<Runnable> taskList = new ArrayList<>();
q.drainTo(taskList);
if (!q.isEmpty()) {
for (Runnable r : q.toArray(new Runnable[0])) {
if (q.remove(r))
taskList.add(r);
}
}
return taskList;
}
以上就是shutdownNow方法的执行逻辑：将线程池状态修改为STOP，然后调用线程池里的所有线程的interrupt方法。
调用shutdownNow后，线程池里的线程会做如何反应呢？那就要看，线程池里线程正在执行的代码逻辑了。
runWorker方法源码
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);
try {
task.run();
afterExecute(task, null);
} catch (Throwable ex) {
afterExecute(task, ex);
throw ex;
}
} 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，如当我们调用shutdownNow时，task.run()里面正处于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。
如果getTask()方法返回null,也会导致线程的退出。我们再来看看getTask方法的实现：
getTask方法源码
/**
* 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();
// Check if queue empty only if necessary.
if (runStateAtLeast(c, SHUTDOWN)
&& (runStateAtLeast(c, 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;
}
}
}
如果我们调用shutdownNow方法时，线程处于从队列里读取任务而阻塞中，则会导致抛出InterruptedException异常，但因为异常被捕获，线程将会继续在这个for循环里执行。
还记得shutdownNow方法里将线程修改为STOP状态吧，由于STOP状态值是大于SHUTDOWN状态，STOP也大于等于STOP，不管任务队列是否为空，都会进入if语句从而返回null,线程退出。
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
总结：
当我们调用线程池的shutdownNow时：
1、如果线程正在getTask方法中执行，则会通过for循环进入到if语句，于是getTask返回null,从而线程退出。不管线程池里是否有未完成的任务。
2、如果线程因为执行提交到线程池里的任务而处于阻塞状态，则会导致报错(如果任务里没有捕获InterruptedException异常)，否则线程会执行完当前任务，然后通过getTask方法返回为null来退出。
shutdown
/**
* 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.
*
* <p>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);
interruptIdleWorkers();
onShutdown(); // hook for ScheduledThreadPoolExecutor
} finally {
mainLock.unlock();
}
tryTerminate();
}
跟shutdownNow类似，只不过它是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。这是interruptIdleWorkers方法的实现：
interruptIdleWorkers源码
/**
* 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()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
跟shutdownNow方法调用interruptWorkers方法不同的是，interruptIdleWorkers方法在遍历线程池里的线程时，有一个w.tryLock()加锁判断，只有加锁成功的线程才会被调用interrupt方法。那什么情况下才能被加锁成功?什么情况下不能被加锁成功呢？这就需要我们继续回到线程执行的runWorker方法。
正运行在w.lock和w.unlock之间的线程将因为加锁失败，而不会被调用interrupt方法，换句话说，就是正在执行线程池里任务的线程不会被中断。
不管是被调用了interrupt的线程还是没被调用的线程，什么时候退出呢？，这就要看getTask方法的返回是否为null了。
在getTask里的if判断中，由于线程池被shutdown方法修改为SHUTDOWN状态，SHUTDOWN大于等于SHUTDOWN成立没问题，但是SHUTDOWN不在大于等于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。
总结：
1、当我们调用线程池的shuwdown方法时，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。
2、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，如果不为空会继续从队列里读取任务，为空则线程退出。
三、优雅的关闭线程池
有了上边对两个关闭线程池方法的了解，相信优雅安全关闭线程池将不再是问题。
我们知道，使用shutdownNow方法，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方法关闭线程池时，一定要对任务里进行异常捕获。
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。
总结：
shutdownNow和shuwdown调用完，线程池并不是立马就关闭了，要想等待线程池关闭，还需调用awaitTermination方法来阻塞等待。