原因
死锁是指两个或两个以上的进程在执行过程中，因争夺资源而造成的一种相互等待的现象，如果无外力的干涉那它们都将无法推进下去，如果系统的资源充足，进程的资源请求都能够得到满足，死锁出现的可能性就很低，否则就会因争夺有限的资源而陷入死锁
代码
public class DeadLockDemo { public static void main(String[] args) { String lockA = "lockA"; String lockB = "lockB"; DeadLockDemo deadLockDemo = new DeadLockDemo(); Executor executor = Executors.newFixedThreadPool(2); executor.execute(() -> deadLockDemo.method(lockA, lockB)); executor.execute(() -> deadLockDemo.method(lockB, lockA)); } public void method(String lock1, String lock2) { synchronized (lock1) { System.out.println(Thread.currentThread().getName() + "--获取到：" + lock1 + "; 尝试获取：" + lock2); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } synchronized (lock2) { System.out.println("获取到两把锁!"); } } } }
定位分析
- jps -l 命令查定位进程号
HuandeMacBook-Air:study huanliu$ jps -l 56048 com.liuhuan.study.DeadLockDemo 484 16308 org.jetbrains.jps.cmdline.Launcher 56089 sun.tools.jps.Jps 36329 56047 org.jetbrains.jps.cmdline.Launcher
- jstack 进程号 找到死锁查看
HuandeMacBook-Air:study huanliu$ jstack 56048 2020-06-19 15:25:46 Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.151-b12 mixed mode): "Attach Listener" #15 daemon prio=9 os_prio=31 tid=0x00007fa2a9eb6800 nid=0x4c0b waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "DestroyJavaVM" #14 prio=5 os_prio=31 tid=0x00007fa2a8bed800 nid=0xe03 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "pool-1-thread-2" #13 prio=5 os_prio=31 tid=0x00007fa2aa4d4800 nid=0x3a03 waiting for monitor entry [0x000070000e086000] java.lang.Thread.State: BLOCKED (on object monitor) at com.liuhuan.study.DeadLockDemo.method(DeadLockDemo.java:33) - waiting to lock <0x00000007960de9e0> (a java.lang.String) - locked <0x00000007960dea18> (a java.lang.String) at com.liuhuan.study.DeadLockDemo.lambda$main$1(DeadLockDemo.java:20) at com.liuhuan.study.DeadLockDemo$$Lambda$2/1176735295.run(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-1" #12 prio=5 os_prio=31 tid=0x00007fa2a8beb800 nid=0x3d03 waiting for monitor entry [0x000070000df83000] java.lang.Thread.State: BLOCKED (on object monitor) at com.liuhuan.study.DeadLockDemo.method(DeadLockDemo.java:33) - waiting to lock <0x00000007960dea18> (a java.lang.String) - locked <0x00000007960de9e0> (a java.lang.String) at com.liuhuan.study.DeadLockDemo.lambda$main$0(DeadLockDemo.java:19) at com.liuhuan.study.DeadLockDemo$$Lambda$1/660879561.run(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "Service Thread" #11 daemon prio=9 os_prio=31 tid=0x00007fa2a8bc5000 nid=0x3803 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C1 CompilerThread2" #10 daemon prio=9 os_prio=31 tid=0x00007fa2a8b8c000 nid=0x3703 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread1" #9 daemon prio=9 os_prio=31 tid=0x00007fa2a8b8b800 nid=0x4103 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "C2 CompilerThread0" #8 daemon prio=9 os_prio=31 tid=0x00007fa2a8b8a800 nid=0x4303 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "JDWP Command Reader" #7 daemon prio=10 os_prio=31 tid=0x00007fa2a8819800 nid=0x3503 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "JDWP Event Helper Thread" #6 daemon prio=10 os_prio=31 tid=0x00007fa2a8819000 nid=0x4603 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "JDWP Transport Listener: dt_socket" #5 daemon prio=10 os_prio=31 tid=0x00007fa2a8818000 nid=0x3307 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007fa2a8816800 nid=0x3203 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007fa2a880c800 nid=0x5103 in Object.wait() [0x000070000d45f000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000795588ec8> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143) - locked <0x0000000795588ec8> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209) "Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007fa2aa01d000 nid=0x5303 in Object.wait() [0x000070000d35c000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000795586b68> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:502) at java.lang.ref.Reference.tryHandlePending(Reference.java:191) - locked <0x0000000795586b68> (a java.lang.ref.Reference$Lock) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153) "VM Thread" os_prio=31 tid=0x00007fa2a901a000 nid=0x2b03 runnable "GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007fa2aa002000 nid=0x1f07 runnable "GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007fa2aa005000 nid=0x2103 runnable "GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007fa2aa006000 nid=0x2303 runnable "GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007fa2aa006800 nid=0x2a03 runnable "VM Periodic Task Thread" os_prio=31 tid=0x00007fa2aa46a000 nid=0x3903 waiting on condition JNI global references: 2499 Found one Java-level deadlock: ============================= "pool-1-thread-2": waiting to lock monitor 0x00007fa2a9021ca8 (object 0x00000007960de9e0, a java.lang.String), which is held by "pool-1-thread-1" "pool-1-thread-1": waiting to lock monitor 0x00007fa2a901dc08 (object 0x00000007960dea18, a java.lang.String), which is held by "pool-1-thread-2" Java stack information for the threads listed above: =================================================== "pool-1-thread-2": at com.liuhuan.study.DeadLockDemo.method(DeadLockDemo.java:33) - waiting to lock <0x00000007960de9e0> (a java.lang.String) - locked <0x00000007960dea18> (a java.lang.String) at com.liuhuan.study.DeadLockDemo.lambda$main$1(DeadLockDemo.java:20) at com.liuhuan.study.DeadLockDemo$$Lambda$2/1176735295.run(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) "pool-1-thread-1": at com.liuhuan.study.DeadLockDemo.method(DeadLockDemo.java:33) - waiting to lock <0x00000007960dea18> (a java.lang.String) - locked <0x00000007960de9e0> (a java.lang.String) at com.liuhuan.study.DeadLockDemo.lambda$main$0(DeadLockDemo.java:19) at com.liuhuan.study.DeadLockDemo$$Lambda$1/660879561.run(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Found 1 deadlock.