Java 简明教程

Java - Thread Deadlock

死锁描述一种情况，其中两个或更多线程永远被阻塞，互相等待。当 multiple threads 需要相同的锁但按不同的顺序获取它们时，就会发生死锁。Java 多线程程序可能会遇到死锁情况，因为 synchronized 关键字会导致执行线程在等待与指定对象关联的锁或监视器时阻塞。这是一个示例。

Deadlock describes a situation where two or more threads are blocked forever, waiting for each other. Deadlock occurs when multiple threads need the same locks but obtain them in different order. A Java multithreaded program may suffer from the deadlock condition because the synchronized keyword causes the executing thread to block while waiting for the lock, or monitor, associated with the specified object. Here is an example.

Example: Demonstrating Deadlock Situation

public class TestThread {
   public static Object Lock1 = new Object();
   public static Object Lock2 = new Object();

   public static void main(String args[]) {
      ThreadDemo1 T1 = new ThreadDemo1();
      ThreadDemo2 T2 = new ThreadDemo2();
      T1.start();
      T2.start();
   }

   private static class ThreadDemo1 extends Thread {
      public void run() {
         synchronized (Lock1) {
            System.out.println("Thread 1: Holding lock 1...");

            try { Thread.sleep(10); }
            catch (InterruptedException e) {}
            System.out.println("Thread 1: Waiting for lock 2...");

            synchronized (Lock2) {
               System.out.println("Thread 1: Holding lock 1 & 2...");
            }
         }
      }
   }
   private static class ThreadDemo2 extends Thread {
      public void run() {
         synchronized (Lock2) {
            System.out.println("Thread 2: Holding lock 2...");

            try { Thread.sleep(10); }
            catch (InterruptedException e) {}
            System.out.println("Thread 2: Waiting for lock 1...");

            synchronized (Lock1) {
               System.out.println("Thread 2: Holding lock 1 & 2...");
            }
         }
      }
   }
}

当您编译并执行上述程序时，您会发现死锁情况，以下是程序产生的输出 −

When you compile and execute the above program, you find a deadlock situation and following is the output produced by the program −

Output

Thread 1: Holding lock 1...
Thread 2: Holding lock 2...
Thread 1: Waiting for lock 2...
Thread 2: Waiting for lock 1...

上述程序将永久挂起，因为该位置的线程既无法进行，又等待彼此释放锁，所以您可以通过按 CTRL+C 退出程序。

The above program will hang forever because neither of the threads in position to proceed and waiting for each other to release the lock, so you can come out of the program by pressing CTRL+C.

Deadlock Solution Example

让我们更改锁的顺序并运行相同的程序，看看这两个线程是否仍在互相等待 −

Let’s change the order of the lock and run of the same program to see if both the threads still wait for each other −

Example

public class TestThread {
   public static Object Lock1 = new Object();
   public static Object Lock2 = new Object();

   public static void main(String args[]) {
      ThreadDemo1 T1 = new ThreadDemo1();
      ThreadDemo2 T2 = new ThreadDemo2();
      T1.start();
      T2.start();
   }

   private static class ThreadDemo1 extends Thread {
      public void run() {
         synchronized (Lock1) {
            System.out.println("Thread 1: Holding lock 1...");

            try {
               Thread.sleep(10);
            } catch (InterruptedException e) {}
            System.out.println("Thread 1: Waiting for lock 2...");

            synchronized (Lock2) {
               System.out.println("Thread 1: Holding lock 1 & 2...");
            }
         }
      }
   }
   private static class ThreadDemo2 extends Thread {
      public void run() {
         synchronized (Lock1) {
            System.out.println("Thread 2: Holding lock 1...");

            try {
               Thread.sleep(10);
            } catch (InterruptedException e) {}
            System.out.println("Thread 2: Waiting for lock 2...");

            synchronized (Lock2) {
               System.out.println("Thread 2: Holding lock 1 & 2...");
            }
         }
      }
   }
}

因此，仅仅更改锁的顺序就可以防止程序进入死锁情况，并完成以下结果 −

So just changing the order of the locks prevent the program in going into a deadlock situation and completes with the following result −

Output

Thread 1: Holding lock 1...
Thread 1: Waiting for lock 2...
Thread 1: Holding lock 1 & 2...
Thread 2: Holding lock 1...
Thread 2: Waiting for lock 2...
Thread 2: Holding lock 1 & 2...

上述示例只是为了说明这个概念，然而，这是一个复杂的概念，在您开发应用程序以处理死锁情况之前，您应该深入了解它。

The above example is to just make the concept clear, however, it is a complex concept and you should deep dive into it before you develop your applications to deal with deadlock situations.