死锁
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互相等待的现象，若无外力作用，它们都将无法推进下去。
死锁发生的原因
死锁的发生是由于资源竞争导致的，导致死锁的原因如下：
- 系统资源不足，如果系统资源充足，死锁出现的可能性就很低。
- 进程（线程）运行推进的顺序不合适。
- 资源分配不当等。
死锁发生的条件
死锁的发生的四个必要条件：
- 互斥条件：一个资源每次只能被一个进程使用。
- 占有且等待：一个进程因请求资源而阻塞时，对已获得的资源保持不放。
- 不可强行占有：进程（线程）已获得的资源，在未使用完之前，不能强行剥夺。
- 循环等待条件：若干进程（线程）之间形成一种头尾相接的循环等待资源关系。
这四个条件是死锁的必要条件，只要系统发生死锁，这些条件必然成立，而只要上述条件之一不满足，就不会发生死锁。
1：通过jstack定位死锁信息
1.1：编写死锁代码
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Lock lock1 = new ReentrantLock(); Lock lock2 = new ReentrantLock(); ExecutorService exectuorService = Executors.newFixedThreadPool(2); exectuorService.submit(() -> { lock1.lock(); try{ Thread.sleep(1000); }catch(Exception e){} try{} finally{ lock1.unlock(); lock2.unlock(); } }); exectuorService.submit(() -> { lock2.lock(); try{ Thread.sleep(1000); }catch(Exception e){} try{} finally{ lock1.unlock(); lock2.unlock(); } }); |
1.2：查看死锁线程的pid
- jps查看死锁的线程pid
- 使用 jstack -l pid 查看死锁信息
- 通过打印信息我们可以找到发生死锁的代码是在哪个位置
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"DestroyJavaVM" #13 prio=5 os_prio=31 tid=0x00007f9a1d8fe800 nid=0xd03 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None"pool-1-thread-2" #12 prio=5 os_prio=31 tid=0x00007f9a1d8fe000 nid=0xa703 waiting on condition [0x000070000ff8e000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000795768cd8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836) at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870) at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199) at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209) at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285) at com.coco.util.SlideTimeUnit.lambda$main$1(SlideTimeUnit.java:63) at com.coco.util.SlideTimeUnit$$Lambda$2/565760380.run(Unknown Source) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - <0x0000000795768d08> (a java.util.concurrent.locks.ReentrantLock$NonfairSync) - <0x0000000795a9e4e0> (a java.util.concurrent.ThreadPoolExecutor$Worker)"pool-1-thread-1" #11 prio=5 os_prio=31 tid=0x00007f9a2082c800 nid=0xa803 waiting on condition [0x000070000fe8b000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000795768d08> (a java.util.concurrent.locks.ReentrantLock$NonfairSync) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836) at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870) at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199) at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209) at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285) at com.coco.util.SlideTimeUnit.lambda$main$0(SlideTimeUnit.java:49) at com.coco.util.SlideTimeUnit$$Lambda$1/596512129.run(Unknown Source) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - <0x0000000795768cd8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync) - <0x0000000795a9ba28> (a java.util.concurrent.ThreadPoolExecutor$Worker)"Service Thread" #10 daemon prio=9 os_prio=31 tid=0x00007f9a2082c000 nid=0x4103 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None"C1 CompilerThread3" #9 daemon prio=9 os_prio=31 tid=0x00007f9a1e021800 nid=0x3f03 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE |
2：通过Arthas工具定位死锁
2.1: 下载好Arthas的jar,然后运行
有一个 thread -b 就可以查看到死锁信息
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[arthas@4182]$ thread -b"pool-1-thread-2" Id=12 WAITING on java.util.concurrent.locks.ReentrantLock$NonfairSync@2cb8a9a3 owned by "pool-1-thread-1" Id=11 at sun.misc.Unsafe.park(Native Method) - waiting on java.util.concurrent.locks.ReentrantLock$NonfairSync@2cb8a9a3 at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175) at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836) at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870) at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199) at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209) at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285) at com.coco.util.SlideTimeUnit.lambda$main$1(SlideTimeUnit.java:63) at com.coco.util.SlideTimeUnit$$Lambda$2/565760380.run(Unknown Source) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Number of locked synchronizers = 2 - java.util.concurrent.ThreadPoolExecutor$Worker@6433a2 - java.util.concurrent.locks.ReentrantLock$NonfairSync@3a855d13 <---- but blocks 1 other threads! |
3. 通过 Jvisualvm 定位死锁
Jvisualvm 是一种自带的可视化工具，往往在在本地执行。
通过 Jvisualvm 命令打开软件，选中进程，进入线程视图，会给出死锁提示:
死锁的预防
- 尽量避免使用多个锁，并且只有需要时才持有锁。
- 如果使用多个锁，一定要设计好锁的获取顺序。
- 使用带有超时的方法，为程序带来更多的可控性，比如指定获取锁的时间最多为5秒，超时就放弃。
- 通过一些代码静态检查工具发现可能存在的死锁问题，比如FindBugs。
总结
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原文链接：https://juejin.cn/post/7003546761461497893
