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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
|
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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
|
"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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
|
[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
到此这篇关于java定位死锁的文章就介绍到这了,更多相关java定位死锁内容请搜索服务器之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持服务器之家!
原文链接:https://juejin.cn/post/7003546761461497893