死锁 - 《Java 核心》笔记

定位死锁常见方式

必定会出现死锁的程序：

public class DeadLockSample extends Thread {
  private String first;
  private String second;
  public DeadLockSample(String name, String first, String second) {
      super(name);
      this.first = first;
      this.second = second;
  }

  public  void run() {
      synchronized (first) {
          System.out.println(this.getName() + " obtained: " + first);
          try {
              Thread.sleep(1000L);
              synchronized (second) {
                  System.out.println(this.getName() + " obtained: " + second);
              }
          } catch (InterruptedException e) {
              // Do nothing
          }
      }
  }
  public static void main(String[] args) throws InterruptedException {
      String lockA = "lockA";
      String lockB = "lockB";
      DeadLockSample t1 = new DeadLockSample("Thread1", lockA, lockB);
      DeadLockSample t2 = new DeadLockSample("Thread2", lockB, lockA);
      t1.start();
      t2.start();
      t1.join();
      t2.join();
  }
}

jstack查看

${JAVA_HOME}\bin\jstack your_pid

结果：

代码中用ThreadMXBean

public static void main(String[] args) throws InterruptedException {

  ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
  Runnable dlCheck = new Runnable() {

      @Override
      public void run() {
          long[] threadIds = mbean.findDeadlockedThreads();
          if (threadIds != null) {
                     ThreadInfo[] threadInfos = mbean.getThreadInfo(threadIds);
                     System.out.println("Detected deadlock threads:");
              for (ThreadInfo threadInfo : threadInfos) {
                  System.out.println(threadInfo.getThreadName());
              }
          }
       }
    };

       ScheduledExecutorService scheduler =Executors.newScheduledThreadPool(1);
       // 稍等 5 秒，然后每 10 秒进行一次死锁扫描
        scheduler.scheduleAtFixedRate(dlCheck, 5L, 10L, TimeUnit.SECONDS);
// 死锁样例代码…
}

重新编译执行，就能看到死锁被定位到的输出。

如何避免死锁

• 尽量避免使用多个锁，只有需要时才持有锁；
• 如果必须使用多个锁，尽量设计好锁的获取顺序；
• 使用带超时的方法，为程序带来更多可控性；
• 通过静态代码分析（如FindBugs）查找固定的模式。