Java中常见死锁的实例

• 顺序死锁：过度加锁，导致由于执行顺序的原因，互相持有对方正在等待的锁
• 资源死锁：多个线程在相同的资源上发生等待

由于调用顺序而产生的死锁

public class Test {
    Object leftLock = new Object();
    Object rightLock = new Object();
    public static void main(String[] args) {
        final Test test = new Test();
        Thread a = new Thread(new Runnable() {
            @Override            public void run() {
               int i=0;
                while (i<10)
                {
                    test.leftRight();
                    i++;
                }
            }
        },"aThread");
        Thread b = new Thread(new Runnable() {
            @Override            public void run() {
                int i=0;
                while (i<10)
                {
                    test.rightleft();
                    i++;
                }
            }
        },"bThread");
        a.start();
        b.start();
    }

    public void leftRight(){
        synchronized (leftLock){
            System.out.println(Thread.currentThread().getName()+":leftRight:get left");
            synchronized (rightLock){
                System.out.println(Thread.currentThread().getName()+":leftRight:get right");
            }
        }
    }

    public void  rightleft(){
        synchronized (rightLock){
            System.out.println(Thread.currentThread().getName()+":rightleft: get right");
            synchronized (leftLock){
                System.out.println(Thread.currentThread().getName()+":rightleft: get left");
            }
        }
    }

}
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运行后输出如下

aThread:leftRight:get left
bThread:rightleft: get right
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可以通过jstack发现死锁的痕迹

"bThread" prio=5 tid=0x00007fabb2001000 nid=0x5503 waiting for monitor entry [0x000000011d54b000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at main.lockTest.Test.rightleft(Test.java:52)
    - waiting to lock <0x00000007aaee5748> (a java.lang.Object)
    - locked <0x00000007aaee5758> (a java.lang.Object)
    at main.lockTest.Test$2.run(Test.java:30)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

"aThread" prio=5 tid=0x00007fabb2801000 nid=0x5303 waiting for monitor entry [0x000000011d448000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at main.lockTest.Test.leftRight(Test.java:43)
    - waiting to lock <0x00000007aaee5758> (a java.lang.Object)
    - locked <0x00000007aaee5748> (a java.lang.Object)
    at main.lockTest.Test$1.run(Test.java:19)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None
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可以看到 bThread 持有锁 0x00000007aaee5758 ,同时等待 0x00000007aaee5748 ,然而恰好 aThread 持有锁 0x00000007aaee5748 并等待 0x00000007aaee5758 ,从而形成了死锁

线程饥饿死锁

public class ExecutorLock {
    private static ExecutorService single=Executors.newSingleThreadExecutor();
    public static class AnotherCallable implements Callable<String>{

        @Override        public String call() throws Exception {
            System.out.println("in AnotherCallable");
            return "annother success";
        }
    }


    public static class MyCallable implements Callable<String>{

        @Override        public String call() throws Exception {
            System.out.println("in MyCallable");
            Future<String> submit = single.submit(new AnotherCallable());
            return "success:"+submit.get();
        }
    }
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        MyCallable task = new MyCallable();
        Future<String> submit = single.submit(task);
        System.out.println(submit.get());
        System.out.println("over");
        single.shutdown();
    }
}
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执行的输出只有一行

in MyCallable
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通过jstack观察可以看到如下

"main" prio=5 tid=0x00007fab3f000000 nid=0x1303 waiting on condition [0x0000000107d63000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00000007aaeed1d8> (a java.util.concurrent.FutureTask)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
    at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:425)
    at java.util.concurrent.FutureTask.get(FutureTask.java:187)
    at main.lockTest.ExecutorLock.main(ExecutorLock.java:32)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

   Locked ownable synchronizers:
    - None
..
"pool-1-thread-1" prio=5 tid=0x00007fab3f835800 nid=0x5303 waiting on condition [0x00000001199ee000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00000007ab0f8698> (a java.util.concurrent.FutureTask)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
    at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:425)
    at java.util.concurrent.FutureTask.get(FutureTask.java:187)
    at main.lockTest.ExecutorLock$MyCallable.call(ExecutorLock.java:26)
    at main.lockTest.ExecutorLock$MyCallable.call(ExecutorLock.java:20)
    at java.util.concurrent.FutureTask.run(FutureTask.java:262)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - <0x00000007aaeed258> (a java.util.concurrent.ThreadPoolExecutor$Worker)
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主线程在等待一个FutureTask完成，而线程池中一个线程也在等待一个FutureTask完成。

从代码实现可以看到，主线程往线程池中扔了一个任务A，任务A又往同一个线程池中扔了一个任务B，并等待B的完成，由于线程池中只有一个线程，这将导致B会被停留在阻塞队列中，而A还得等待B的完成，这也就是互相等待导致了死锁的反生

这种由于正在执行的任务线程都在等待其它工作队列中的任务而阻塞的现象称为 线程饥饿死锁