在Android开发中，不可避免的会用到线程来执行耗时任务，那如果我们想在中途停止/中断任务的执行，该怎么办呢？先来看看一个简单的线程。
private Thread threadOne = new Thread(new Runnable() {
@Override
public void run() {
try {
while(true) {
Log.d(TAG, "thread isAlive:" + threadOne.isAlive());
Thread.sleep(1000);
}
} catch (Exception e) {
Log.d(TAG, e.getClass().toString());
Log.d(TAG, "thread isAlive in catch:" + threadOne.isAlive());
}
}
});
正常运行打印结果：
image.png
当使用interrupt()方法中断该线程时，打印如下：
image.png
可以看出，调用interrupt()后，会捕获名为“InterruptedException”的异常，但是接下来的发现线程还存活，这是怎么回事呢？既然线程能够被中断，那么是否提供查询中断状态的方法呢？通过查看api我们发现，thread.isInterrupted()可以查看线程的中断状态，因此我们再加一个打印：
Log.d(TAG, e.getClass().toString());
Log.d(TAG, "thread isInterrupted::" + threadOne.isInterrupted());
Log.d(TAG, "thread isAlive in catch:" + threadOne.isAlive());
然而中断状态位依然是“未被中断”。这与我们想象的不太一样，因此回想一下是哪个方法抛出了异常，发现是sleep方法。
// BEGIN Android-changed: Implement sleep() methods using a shared native implementation.
public static void sleep(long millis) throws InterruptedException {
sleep(millis, 0);
}
我们先把sleep()方法注释掉，再运行
private Thread threadOne = new Thread(new Runnable() {
@Override
public void run() {
try {
while (!threadOne.isInterrupted()) {
Log.d(TAG, "thread isAlive:" + threadOne.isAlive());
}
Log.d(TAG, "break while thread isAlive:" + threadOne.isAlive());
Log.d(TAG, "break while thread isInterrupted::" + threadOne.isInterrupted());
} catch (Exception e) {
Log.d(TAG, e.getClass().toString());
Log.d(TAG, "thread isInterrupted::" + threadOne.isInterrupted());
Log.d(TAG, "thread isAlive in catch:" + threadOne.isAlive());
}
}
});
image.png
这次的结果比较符合我们的“直观想象”, 线程还是存活，但中断状态位标记位为true。
从上面两个两个例子可知：
1.中断正在阻塞(sleep)的线程，会抛出InterruptedException异常，中断标记位为false。
2.中断未被阻塞的线程，中断标记位会被置为true。
那么针对正在阻塞的线程，我们只需要捕获到InterruptedException异常就退出线程执行，对于未被阻塞的线程，判断中断标记是否为true，若是则退出线程执行。当然我们线程里如果调用了其它方法，不确定其它方法阻塞与否，因此可以将这两种判断结合起来，如下：
private Thread threadOne = new Thread(new Runnable() {
@Override
public void run() {
try {
while (!threadOne.isInterrupted()) {
doSomething();
}
} catch (Exception e) {
if (e instanceof InterruptedException) {
isInterrupted = true;
}
}
}
});
也许你会疑惑说你上面的线程都是在while里跑，如果线程只走一次，怎么中断呢？只能尽可能在每个关键之处停止其执行。
private Thread threadOne = new Thread(new Runnable() {
@Override
public void run() {
try {
if (!threadOne.isInterrupted())
doSomething1();
else
return;
if (!threadOne.isInterrupted())
doSomething2();
else
return;
if (!threadOne.isInterrupted())
doSomething3();
else
return;
} catch (Exception e) {
}
}
});
除了sleep方法之外，还有其它方法会抛出异常不？实际上，在Thread源码里对此都有解释，我们来看看源码怎么说的。
/**
* Interrupts this thread.
*
* <p> Unless the current thread is interrupting itself, which is
* always permitted, the {@link #checkAccess() checkAccess} method
* of this thread is invoked, which may cause a {@link
* SecurityException} to be thrown.
*
* <p> If this thread is blocked in an invocation of the {@link
* Object#wait() wait()}, {@link Object#wait(long) wait(long)}, or {@link
* Object#wait(long, int) wait(long, int)} methods of the {@link Object}
* class, or of the {@link #join()}, {@link #join(long)}, {@link
* #join(long, int)}, {@link #sleep(long)}, or {@link #sleep(long, int)},
* methods of this class, then its interrupt status will be cleared and it
* will receive an {@link InterruptedException}.
*
* <p> If this thread is blocked in an I/O operation upon an {@link
* java.nio.channels.InterruptibleChannel InterruptibleChannel}
* then the channel will be closed, the thread's interrupt
* status will be set, and the thread will receive a {@link
* java.nio.channels.ClosedByInterruptException}.
*
* <p> If this thread is blocked in a {@link java.nio.channels.Selector}
* then the thread's interrupt status will be set and it will return
* immediately from the selection operation, possibly with a non-zero
* value, just as if the selector's {@link
* java.nio.channels.Selector#wakeup wakeup} method were invoked.
*
* <p> If none of the previous conditions hold then this thread's interrupt
* status will be set. </p>
总结来说：
- 在线程里使用sleep、wait、join等方法，当线程被中断时，中断状态位会被重置为false，并且抛出InterruptedException异常(这也是为什么我们第一个例子里thread.isInterrupted()为false的原因）
- 在线程里使用nio InterruptibleChannel接口时，当线程被中断时，中断状态位会被重置为true，并且抛出ClosedByInterruptException异常
- 在线程里使用nio Selector时，当线程被中断时，中断状态位会被重置为true
- 如不属于上述条件，则中断状态位会被重置为true（对应我们上面说的没有阻塞的情况）
thread.isInterrupted() 和 Thread.interrupted区别
thread.isInterrupted()是对象方法，表示thread的中断状态。Thread.interrupted()是静态方法，表示当前线程的中断状态，举个例子：
Log.d(TAG, " curThread is:" + Thread.currentThread().getName());
Log.d(TAG, " Thread.currentThread().isInterrupted() before :" + Thread.currentThread().isInterrupted());
Log.d(TAG, " Thread.interrupted() before :" + Thread.interrupted());
Log.d(TAG, " threadOne.isInterrupted() before :" + threadOne.isInterrupted());
Thread.currentThread().interrupt();
Log.d(TAG, " Thread.currentThread().isInterrupted() after:" + Thread.currentThread().isInterrupted());
Log.d(TAG, " Thread.interrupted() after :" + Thread.interrupted());
Log.d(TAG, " Thread.currentThread().isInterrupted() after2:" + Thread.currentThread().isInterrupted());
Log.d(TAG, " threadOne.isInterrupted() after :" + threadOne.isInterrupted());
image.png
从上面可以看出来，Thread.interrupted()调用后会重置中断状态为false，而thread.isInterrupted()却不会。
总结
1、线程正在执行sleep、join、wait等方法，此时线程处在WAITING/TIMED_WAITING状态，当执行thread.interrupt()，那么会抛出InterruptedException异常，线程中断标记位为false，线程停止运行；
2、线程处在RUNNABLE状态，当执行thread.interrupt()，不会抛出异常，线程中断标记位为true，线程未停止运行；
3、如果线程处在BLOCKED(Synchronized争抢锁）状态，当执行thread.interrupt()，不会抛出异常，线程中断标记位为true，线程未停止运行（这点也说明了Synchronized不可打断）
更多关于线程状态的问题请移步：Java 线程状态
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