Java 多线程中的死锁

java programming java8object oriented programming

死锁描述了两个或多个线程被永远阻塞,相互等待的情况。当多个线程需要相同的锁但以不同的顺序获取它们时,就会发生死锁。Java 多线程程序可能会遭遇死锁情况,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。下面是一个例子。

示例

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...");
            }
         }
      }
   }
}

当你编译并执行上述程序时,你会发现一个死锁的情况,以下是程序产生的输出 −

输出

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

上述程序将永远挂起,因为两个线程都无法继续执行,并且都在等待对方释放锁,因此您可以按 CTRL+C 退出程序。

死锁解决方案示例

让我们改变同一程序的锁定和运行顺序,看看两个线程是否仍在等待对方 −

示例

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,只需改变锁的顺序即可防止程序陷入死锁情况,并完成以下结果 −

输出

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...

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