这是因为 stop() 方法会释放此线程中的所有锁,导致程序执行紊乱,破坏了程序的原子操作逻辑。
以上的这些问题,导致了 JDK 废弃了 stop() 的方法,它的废弃源码如下:
/** * Forces the thread to stop executing. * <p> * If there is a security manager installed, its <code>checkAccess</code> * method is called with <code>this</code> * as its argument. This may result in a * <code>SecurityException</code> being raised (in the current thread). * <p> * If this thread is different from the current thread (that is, the current * thread is trying to stop a thread other than itself), the * security manager's <code>checkPermission</code> method (with a * <code>RuntimePermission("stopThread")</code> argument) is called in * addition. * Again, this may result in throwing a * <code>SecurityException</code> (in the current thread). * <p> * The thread represented by this thread is forced to stop whatever * it is doing abnormally and to throw a newly created * <code>ThreadDeath</code> object as an exception. * <p> * It is permitted to stop a thread that has not yet been started. * If the thread is eventually started, it immediately terminates. * <p> * An application should not normally try to catch * <code>ThreadDeath</code> unless it must do some extraordinary * cleanup operation (note that the throwing of * <code>ThreadDeath</code> causes <code>finally</code> clauses of * <code>try</code> statements to be executed before the thread * officially dies). If a <code>catch</code> clause catches a * <code>ThreadDeath</code> object, it is important to rethrow the * object so that the thread actually dies. * <p> * The top-level error handler that reacts to otherwise uncaught * exceptions does not print out a message or otherwise notify the * application if the uncaught exception is an instance of * <code>ThreadDeath</code>. * * @exception SecurityException if the current thread cannot * modify this thread. * @see #interrupt() * @see #checkAccess() * @see #run() * @see #start() * @see ThreadDeath * @see ThreadGroup#uncaughtException(Thread,Throwable) * @see SecurityManager#checkAccess(Thread) * @see SecurityManager#checkPermission * @deprecated This method is inherently unsafe. Stopping a thread with * Thread.stop causes it to unlock all of the monitors that it * has locked (as a natural consequence of the unchecked * <code>ThreadDeath</code> exception propagating up the stack). If * any of the objects previously protected by these monitors were in * an inconsistent state, the damaged objects become visible to * other threads, potentially resulting in arbitrary behavior. Many * uses of <code>stop</code> should be replaced by code that simply * modifies some variable to indicate that the target thread should * stop running. The target thread should check this variable * regularly, and return from its run method in an orderly fashion * if the variable indicates that it is to stop running. If the * target thread waits for long periods (on a condition variable, * for example), the <code>interrupt</code> method should be used to * interrupt the wait. * For more information, see * <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why * are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>. */ @Deprecated public final void stop() { SecurityManager security = System.getSecurityManager(); if (security != null) { checkAccess(); if (this != Thread.currentThread()) { security.checkPermission(SecurityConstants.STOP_THREAD_PERMISSION); } } // A zero status value corresponds to "NEW", it can't change to // not-NEW because we hold the lock. if (threadStatus != 0) { resume(); // Wake up thread if it was suspended; no-op otherwise } // The VM can handle all thread states stop0(new ThreadDeath()); }
可以看出 stop() 方法被 @Deprecated 注释修饰了,而被此注解修饰的代码表示为过时方法,不建议被使用。从 stop() 的备注信息可以看出,官方也不建议使用 stop() ,说它是一个非安全的方法。
正确终止线程
那如何终止线程呢?这里提供 2 个正确的方法:
- 设置退出标识退出线程;
- 使用 interrupt() 方法终止线程。
1.自定义退出标识
我们可以自定义一个布尔变量来标识是否需要退出线程,实现代码如下:
// 自定义退出标识退出线程 static class FlagThread extends Thread { public volatile boolean exit = false; public void run() { while (!exit) { // 执行正常的业务逻辑 } } }
可以看出我们使用了关键字 volatile 对线程进行了修饰,这样就可以保证多线程的执行安全了,在我们需要让线程退出时,只需要把变量 exit 赋值为 true 就可以了。
2.interrupt 终止线程
当我们使用 interrupt() 方法时,以上两个示例的执行结果就正常了,执行代码如下:
public class ThreadStopExample { public static void main(String[] args) throws InterruptedException { // 问题一:破坏了程序的完整性 Thread t1 = new Thread(() -> { try { System.out.println("子线程开始执行"); // 模拟业务处理 Thread.sleep(1000); } catch (Exception e) { } // 伪代码:重要业务方法 System.out.println("子线程的重要业务方法"); }); t1.start(); // 让子线程先运行一点业务 Thread.sleep(100); // 终止子线程 t1.interrupt(); // 等待一段时间,确保子线程“执行完” Thread.sleep(3000); System.out.println("主线程执行完成"); // 问题二:破坏了原子逻辑 MyThread myThread = new MyThread(); Thread t2 = new Thread(myThread); // 开启线程 t2.start(); for (int i = 0; i < 10; i++) { Thread t = new Thread(myThread); t.start(); } // 结束线程 t2.interrupt(); } /** * 自定义原子测试线程 */ static class MyThread implements Runnable { // 计数器 int num = 0; @Override public void run() { // 同步代码块,保证原子操作 synchronized (MyThread.class) { // 自增 num++; try { // 线程休眠 0.1 秒 Thread.sleep(100); } catch (InterruptedException e) { System.out.println(e.getMessage()); } // 自减 num--; System.out.println(Thread.currentThread().getName() + " | num=" + num); } } } }
以上程序的执行结果为:
子线程开始执行
子线程的重要业务方法
主线程执行完成
sleep interrupted
Thread-1 | num=0
Thread-9 | num=0
Thread-10 | num=0
Thread-7 | num=0
Thread-6 | num=0
Thread-5 | num=0
Thread-4 | num=0
Thread-2 | num=0
Thread-3 | num=0
Thread-11 | num=0
Thread-8 | num=0
可以看出以上的执行都符合我们的预期,这才是正确的终止线程的方式。
总结
本文我们讲了线程的三种终止方式,自定义退出标识的方式、使用 stop() 的方式或 interrupt() 的方式。其中 stop() 的方式会导致程序的完整性和原子性被破坏的问题,并且此方法被 JDK 标识为过期方法,不建议使用,而 interrupt() 方法无疑是最适合我们的终止线程的方式。