Java高并发专题之1、必须知道的几个概念

public class Demo4 { public static void main(String[] args) { Obj1 obj1 = new Obj1(); Obj2 obj2 = new Obj2(); Thread thread1 = new Thread(new SynAddRunalbe(obj1, obj2, 1, 2, true)); thread1.setName("thread1"); thread1.start(); Thread thread2 = new Thread(new SynAddRunalbe(obj1, obj2, 2, 1, false)); thread2.setName("thread2"); thread2.start(); } /** * 线程死锁等待演示 */ public static class SynAddRunalbe implements Runnable { Obj1 obj1; Obj2 obj2; int a, b; boolean flag; public SynAddRunalbe(Obj1 obj1, Obj2 obj2, int a, int b, boolean flag) { this.obj1 = obj1; this.obj2 = obj2; this.a = a; this.b = b; this.flag = flag; } @Override public void run() { try { if (flag) { synchronized (obj1) { Thread.sleep(100); synchronized (obj2) { System.out.println(a + b); } } } else { synchronized (obj2) { Thread.sleep(100); synchronized (obj1) { System.out.println(a + b); } } } } catch (InterruptedException e) { e.printStackTrace(); } } } public static class Obj1 { } public static class Obj2 { } }

运行上面代码，可以通过jstack查看到死锁信息:

"thread2" #13 prio=5 os_prio=0 tid=0x0000000029225000 nid=0x3c94 waiting for monitor entry [0x0000000029c9f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:50)
    - waiting to lock <0x00000007173d40f0> (a com.jvm.visualvm.Demo4$Obj1)
    - locked <0x00000007173d6310> (a com.jvm.visualvm.Demo4$Obj2)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

"thread1" #12 prio=5 os_prio=0 tid=0x0000000029224800 nid=0x6874 waiting for monitor entry [0x0000000029b9f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:43)
    - waiting to lock <0x00000007173d6310> (a com.jvm.visualvm.Demo4$Obj2)
    - locked <0x00000007173d40f0> (a com.jvm.visualvm.Demo4$Obj1)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

thread1持有com.jvm.visualvm.Demo4$Obj1的锁，等待获取com.jvm.visualvm.Demo4$Obj2的锁
thread2持有com.jvm.visualvm.Demo4$Obj2的锁，等待获取com.jvm.visualvm.Demo4$Obj1的锁，两个线程相互等待获取对方持有的锁，出现死锁。

饥饿死锁的例子

package com.jvm.jconsole;

import java.util.concurrent.*;

/**
 * Java干货铺子,只生产干货，公众号：javacode2018
 */
public class ExecutorLock {
    private static ExecutorService single = Executors.newSingleThreadExecutor();

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


    public static class MyCallable implements Callable {
        @Override
        public String call() throws Exception {
            System.out.println("in MyCallable");
            Future submit = single.submit(new AnotherCallable());
            return "success:" + submit.get();
        }
    }

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        MyCallable task = new MyCallable();
        Future submit = single.submit(task);
        System.out.println(submit.get());
        System.out.println("over");
        single.shutdown();
    }
}

执行代码，输出：

1. in MyCallable

使用jstack命令查看线程堆栈信息：

"pool-1-thread-1" #12 prio=5 os_prio=0 tid=0x0000000028e3d000 nid=0x58a4 waiting on condition [0x00000000297ff000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x0000000717921bf0> (a java.util.concurrent.FutureTask)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:429)
    at java.util.concurrent.FutureTask.get(FutureTask.java:191)
    at com.jvm.jconsole.ExecutorLock$MyCallable.call(ExecutorLock.java:25)
    at com.jvm.jconsole.ExecutorLock$MyCallable.call(ExecutorLock.java:20)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - <0x00000007173f2690> (a java.util.concurrent.ThreadPoolExecutor$Worker)

"main" #1 prio=5 os_prio=0 tid=0x00000000033e4000 nid=0x5f94 waiting on condition [0x00000000031fe000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00000007173f1d48> (a java.util.concurrent.FutureTask)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:429)
    at java.util.concurrent.FutureTask.get(FutureTask.java:191)
    at com.jvm.jconsole.ExecutorLock.main(ExecutorLock.java:32)

   Locked ownable synchronizers:
    - None

堆栈信息结合图中的代码，可以看出主线程在32行处于等待中，线程池中的工作线程在25行处于等待中，等待获取结果。由于线程池是一个线程，AnotherCallable得不到执行，而被饿死，最终导致了程序死锁的现象。