Java併發死鎖

死鎖描述了兩個或多個線程等待彼此而被永久阻塞的情況。 當多個線程需要相同的鎖定但以不同的順序獲取時，會發生死鎖。 Java多線程程序可能會遇到死鎖狀況，因爲synchronized關鍵字會導致執行線程在等待與指定對象相關聯的鎖定或監視時出現阻止情況。 看看下面一個例子。

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...");
            }
         }
      }
   } 
}

當您編譯並執行上述程序時，會出現死鎖情況，以下是程序生成的輸出 -

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

上述程序將永久掛起，因爲兩個線程都不能繼續進行，等待彼此釋放鎖定，所以您可以按CTRL + C退出程序。

死鎖解決方案示例

下面我們修改鎖的順序並運行相同的程序，看看這兩個線程是否仍然相互等待 -

實例

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...");
            }
         }
      }
   } 
}

所以只是改變鎖的順序防止程序進入死鎖情況並完成以下結果 -

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...

上面的例子只是爲了更容易理解這個解決死鎖的概念，然而，這是一個複雜的概念，應該深入瞭解它，然後才能再開發應用程序來處理死鎖情況。