concurrency(3)死锁检测与相关工具详解

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死锁

线程1等待线程2互斥持有的资源，而线程2也在等待线程1互斥持有的资源，两个线程都无法继续执行。

活锁

线程持续重试一个总是失败的操作，导致无法继续执行。

饿死

线程一直被调度延迟访问其赖以执行的资源，也许是调度器先于低优先级的线程而执行高优先级的线程，同时总是会有一个高优先级的线程可以执行，饿死也叫作无限延迟。

死锁实例

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public class MyTest6 {

    private Object lock1 = new Object();
    private Object lock2 = new Object();

    public void method1(){
        synchronized (lock1){
            synchronized (lock2){
                System.out.println("method1 invoked");
            }
        }
    }

    public void method2(){
        synchronized (lock2){
            synchronized (lock1){
                System.out.println("method2 invoked");
            }
        }
    }


    public static void main(String[] args) {
        MyTest6 myTest6 = new MyTest6();

        Runnable runnable1 = () ->{
            while(true){
                myTest6.method1();
                try{
                    Thread.sleep(100);
                }catch (Exception e){

                }
            }
        };
        Thread t1 = new Thread(runnable1,"myThread1");

        Runnable runnable2 = () ->{
            while(true){
                myTest6.method2();
                try{
                    Thread.sleep(100);
                }catch (Exception e){

                }
            }
        };
        Thread t2 = new Thread(runnable2,"myThread2");

        t1.start();
        t2.start();
    }
}

输出：
method2 invoked
method1 invoked

死锁检测

jvisualvm

这个时候我们使用jvisualvm查看死锁，在cmd里边敲入jvisualvm
deadlock.png
deadlock1.png
myThread2持有0x000000076f1a8300，等待0x000000076f1a82f0；
myThread1持有0x000000076f1a82f0，等待0x000000076f1a8300；
死锁产生。

下边也有死锁的信息打印：

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Found one Java-level deadlock:
=============================
"myThread2":
  waiting to lock monitor 0x000000001ce80d18 (object 0x000000076f1a82f0, a java.lang.Object),
  which is held by "myThread1"
"myThread1":
  waiting to lock monitor 0x000000001ce80c68 (object 0x000000076f1a8300, a java.lang.Object),
  which is held by "myThread2"

jstack命令

首先使用jstack查看当前的java进程：

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C:\Users\Administrator>jps
20340
20664 Launcher
21352 MyTest6
19660 Jps

我们死锁程序是MyTest6，接下来使用jstack查看：

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jstack 21352

deadlock2.png
显示的结果和jvisualvm打印的是一致的。