既然可以上锁，那么假如有2个线程，一个线程想先锁对象1，再锁对象2，恰好另外有一个线程先锁对象2，再锁对象1。在这个过程中，当线程1把对象1锁好以后，就想去锁对象2，但是不巧，线程2已经把对象2锁上了，也正在尝试去锁对象1。什么时候结束呢，只有线程1把2个对象都锁上并把方法执行完，并且线程2把2个对象也都锁上并且把方法执行完毕，那么就结束了，但是，谁都不肯放掉已经锁上的对象，所以就没有结果，这种情况就叫做线程死锁。

死锁实例

public class DeadThread implements Runnable{
    private  static Object object1 = new Object();
    private  static Object object2 = new Object();
    private boolean flag;
    public DeadThread(boolean flag){
        this.flag=flag;
    }
    @Override
    public void run() {
        if(flag){
            synchronized(object1){
                System.out.println(Thread.currentThread().getName()+"--1");
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized(object2){
                    System.out.println(Thread.currentThread().getName()+"--2");
                }
            }
        }else{
            synchronized(object2){
                System.out.println(Thread.currentThread().getName()+"--2");
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized(object1){
                    System.out.println(Thread.currentThread().getName()+"--1");
                }
            }
        }
    }
    public static void main(String[] args) {
        DeadThread thread1=new DeadThread(true);
        DeadThread thread2=new DeadThread(false);
        new Thread(thread1).start();
        new Thread(thread2).start();
    }
}

程序死锁定位

1 获取java进程
bogon:effectivejava mazh$ jps
9521 Launcher
2466
9523 DeadThread —你的java类名
95269 Launcher
9543 Jps
67211 Launcher
4398 RemoteMavenServer

此处找到java进程pid为9523

2 使用jstack指令获取锁详细栈信息

bogon:effectivejava mazh$ jstack 9523
2019-10-11 18:34:08
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.181-b13 mixed mode):

“Attach Listener” #13 daemon prio=9 os_prio=31 tid=0x00007fb951880800 nid=0xc07 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

“DestroyJavaVM” #12 prio=5 os_prio=31 tid=0x00007fb952856000 nid=0x2603 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

“Thread-1” #11 prio=5 os_prio=31 tid=0x00007fb95182c800 nid=0x3f03 waiting for monitor entry [0x0000700003b8b000]
java.lang.Thread.State: BLOCKED (on object monitor)
at concurrency.deathlock.Deathlock.run(Deathlock.java:42)

    - waiting to lock <0x000000076ab0b958> (a java.lang.String)  
    - locked <0x000000076ab0bde0> (a java.lang.String)  
    at java.lang.Thread.run(Thread.java:748)

“Thread-0” #10 prio=5 os_prio=31 tid=0x00007fb95182b800 nid=0x3d03 waiting for monitor entry [0x0000700003a88000]
java.lang.Thread.State: BLOCKED (on object monitor)
at concurrency.deathlock.Deathlock.run(Deathlock.java:30)

    - waiting to lock <0x000000076ab0bde0> (a java.lang.String)  
    - locked <0x000000076ab0b958> (a java.lang.String)  
    at java.lang.Thread.run(Thread.java:748)

………………..
………………..(省略无效内容)

Found ones Java-level deadlock:

“Thread-1”:
waiting to lock monitor 0x00007fb95200c158 (object 0x000000076ab0b958, a java.lang.String),
which is held by “Thread-0”
“Thread-0”:
waiting to lock monitor 0x00007fb952009818 (object 0x000000076ab0bde0, a java.lang.String),
which is held by “Thread-1”

Java stack information for the threads listed above:

“Thread-1”:
at concurrency.deathlock.Deathlock.run(Deathlock.java:42)

    - waiting to lock <0x000000076ab0b958> (a java.lang.String)  
    - locked <0x000000076ab0bde0> (a java.lang.String)  
    at java.lang.Thread.run(Thread.java:748)

“Thread-0”:
at concurrency.deathlock.Deathlock.run(Deathlock.java:30)

    - waiting to lock <0x000000076ab0bde0> (a java.lang.String)  
    - locked <0x000000076ab0b958> (a java.lang.String)  
    at java.lang.Thread.run(Thread.java:748)

Found 1 deadlock.

重点部分如下：

“Thread-1”:
at concurrency.deathlock.Deathlock.run(Deathlock.java:42)

    - waiting to lock <0x000000076ab0b958> (a java.lang.String)  
    - locked <0x000000076ab0bde0> (a java.lang.String)  
    at java.lang.Thread.run(Thread.java:748)

“Thread-0”:
at concurrency.deathlock.Deathlock.run(Deathlock.java:30)

    - waiting to lock <0x000000076ab0bde0> (a java.lang.String)  
    - locked <0x000000076ab0b958> (a java.lang.String)  
    at java.lang.Thread.run(Thread.java:748)