Java - 线程死锁

死锁描述了一种情况，其中两个或多个线程永久阻塞，互相等待。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。这是一个例子。

示例：演示死锁情况

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...");
            }
         }
      }
   } 
}

编译并执行上述程序时，你会发现死锁情况，程序产生的输出如下：

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，因为任何线程都无法继续执行，都在等待对方释放锁，你可以按 CTRL+C 结束程序。

死锁解决方案示例

让我们更改锁的顺序并运行相同的程序，看看两个线程是否仍然相互等待：

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...");
            }
         }
      }
   } 
}

只需更改锁的顺序即可防止程序陷入死锁，并以以下结果完成：

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...

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，在开发应用程序以处理死锁情况之前，你应该深入研究它。