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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。
1.1:编写死锁代码
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();
}
});
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
"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$Worker)
"Service Thread" #10 daemon prio=9 os_prio=31 tid=0x00007f9a2082c000 nid=0x4103 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"C1 CompilerThread3" #9 daemon prio=9 os_prio=31 tid=0x00007f9a1e021800 nid=0x3f03 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
2.1: 下载好Arthas的jar,然后运行
有一个 thread -b 就可以查看到死锁信息
[arthas@4182]$ 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!
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:
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。
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