之前已经学习了关于同步锁的知识，但是在实际编写多线程程序时可能会存在死锁的情况，所以这次来模拟一下死锁，并且学会用一个命令来确认是否程序已经出现死锁了，下面开始：
首先新建两个类：
此时当然得到OtherService类中增加这个s1()方法喽，如下：
目前是DeadLock调用了OtherService的方法，这时反过了，让OtherService去调用DeadLock类的方法，所以代码如下：
所以此时在DeadLock中新建一个m2()方法，如下：
此时编写测试代码来测试一下，首先构造相关依赖的关系，如下：
接下来新建两个线程然后发起方法调用，具体如下：
接下来编译运行：
执行了一段之后程序卡住了，而且进程一直没有结束，其实这就是典型死锁原因造成，用什么方式来以查看到这个死锁的状态呢？其实可以通过一个命令行可以查询，具体如下：
先用"jps"命令查看当前程序的进程号，如下：
然后用"jstack"命令查看该进程的线程情况，如下：
xiongweideMacBook-Pro:wechat_jump_game xiongwei$ jstack 99374 2018-02-04 15:13:00 Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.60-b23 mixed mode): "Attach Listener" #12 daemon prio=9 os_prio=31 tid=0x00007fe88404b000 nid=0x1107 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "DestroyJavaVM" #11 prio=5 os_prio=31 tid=0x00007fe88380e000 nid=0x1a03 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Thread-1" #10 prio=5 os_prio=31 tid=0x00007fe883087000 nid=0x4d03 waiting for monitor entry [0x00007000061e1000] java.lang.Thread.State: BLOCKED (on object monitor) at com.javaconcurrency.synchronized4.DeadLock.m2(DeadLock.java:20) - waiting to lock <0x0000000795780bb0> (a java.lang.Object) at com.javaconcurrency.synchronized4.OtherService.s2(OtherService.java:21) - locked <0x000000079577c6d8> (a java.lang.Object) at com.javaconcurrency.synchronized4.DeadLockTest$2.run(DeadLockTest.java:22) "Thread-0" #9 prio=5 os_prio=31 tid=0x00007fe88381a000 nid=0x4b03 waiting for monitor entry [0x00007000060de000] java.lang.Thread.State: BLOCKED (on object monitor) at com.javaconcurrency.synchronized4.OtherService.s1(OtherService.java:14) - waiting to lock <0x000000079577c6d8> (a java.lang.Object) at com.javaconcurrency.synchronized4.DeadLock.m1(DeadLock.java:14) - locked <0x0000000795780bb0> (a java.lang.Object) at com.javaconcurrency.synchronized4.DeadLockTest$1.run(DeadLockTest.java:13) "Service Thread" #8 daemon prio=9 os_prio=31 tid=0x00007fe884054000 nid=0x4703 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C1 CompilerThread2" #7 daemon prio=9 os_prio=31 tid=0x00007fe884053000 nid=0x4503 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread1" #6 daemon prio=9 os_prio=31 tid=0x00007fe884010000 nid=0x4303 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread0" #5 daemon prio=9 os_prio=31 tid=0x00007fe88380d000 nid=0x4103 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007fe88380c800 nid=0x3f0b runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007fe883807000 nid=0x3203 in Object.wait() [0x00007000059c9000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x00000007955870b8> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143) - locked <0x00000007955870b8> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209) "Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007fe88281e000 nid=0x3003 in Object.wait() [0x00007000058c6000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000795586af8> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:502) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:157) - locked <0x0000000795586af8> (a java.lang.ref.Reference$Lock) "VM Thread" os_prio=31 tid=0x00007fe884006800 nid=0x2e03 runnable "GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007fe884003800 nid=0x2607 runnable "GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007fe884004000 nid=0x2803 runnable "GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007fe884004800 nid=0x2a03 runnable "GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007fe884005000 nid=0x2c03 runnable "VM Periodic Task Thread" os_prio=31 tid=0x00007fe883809000 nid=0x4903 waiting on condition JNI global references: 6 Found one Java-level deadlock: ============================= "Thread-1": waiting to lock monitor 0x00007fe882822a18 (object 0x0000000795780bb0, a java.lang.Object), which is held by "Thread-0" "Thread-0": waiting to lock monitor 0x00007fe882823d58 (object 0x000000079577c6d8, a java.lang.Object), which is held by "Thread-1" Java stack information for the threads listed above: =================================================== "Thread-1": at com.javaconcurrency.synchronized4.DeadLock.m2(DeadLock.java:20) - waiting to lock <0x0000000795780bb0> (a java.lang.Object) at com.javaconcurrency.synchronized4.OtherService.s2(OtherService.java:21) - locked <0x000000079577c6d8> (a java.lang.Object) at com.javaconcurrency.synchronized4.DeadLockTest$2.run(DeadLockTest.java:22) "Thread-0": at com.javaconcurrency.synchronized4.OtherService.s1(OtherService.java:14) - waiting to lock <0x000000079577c6d8> (a java.lang.Object) at com.javaconcurrency.synchronized4.DeadLock.m1(DeadLock.java:14) - locked <0x0000000795780bb0> (a java.lang.Object) at com.javaconcurrency.synchronized4.DeadLockTest$1.run(DeadLockTest.java:13) Found 1 deadlock. xiongweideMacBook-Pro:wechat_jump_game xiongwei$
其中可以在这个命令的最后语句中看到如下提示：
那具体原因是怎么造成的呢？从命令行中的提示就可以分析得出：
所以如果实际项目中就可以通过这个命令去查询死锁的问题。