Java避免死锁的几个常见方法(有测试代码和分析过程)

目录

Java避免死锁的几个常见方法

死锁产生的条件

上死锁代码

然后 ：jstack 14320 >> jstack.text

Java避免死锁的几个常见方法

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Java避免死锁的几个常见方法

• 避免一个线程同时获取多个锁。
• 避免一个线程在锁内同时占用多个资源，尽量保证每个锁只占用一个资源。
• 尝试使用定时锁，使用lock.tryLock（timeout）来替代使用内部锁机制。
• 对于数据库锁，加锁和解锁必须在一个数据库连接里，否则会出现解锁失败的情况。

死锁产生的条件

1. 互斥条件：进程对所分配到的资源不允许其他进程进行访问，若其他进程访问该资源，只能等待，直至占有该资源的进程使用完成后释放该资源
2. 请求和保持条件：进程获得一定的资源之后，又对其他资源发出请求，但是该资源可能被其他进程占有，此事请求阻塞，但又对自己获得的资源保持不放
3. 不可剥夺条件：是指进程已获得的资源，在未完成使用之前，不可被剥夺，只能在使用完后自己释放
4. 环路等待条件：是指进程发生死锁后，必然存在一个进程–资源之间的环形链死锁问题

上死锁代码

package cn.net.cdsz.ccb.test;public class TestDeadLock {private static Object obj1 = new Object();private static Object obj2 = new Object();public static void main(String[] args) {new Thread(new Thread1()).start();new Thread(new Thread2()).start();}private static class Thread1 implements Runnable {@Overridepublic void run() {synchronized (obj1) {System.out.println("Thread1 拿到 obj1 的锁");try {//停顿2秒的意义在于，让thread2线程拿到obj2的锁Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (obj2) {System.out.println("Thread1 拿到 obj2 的锁");}}}}private static class Thread2 implements Runnable {@Overridepublic void run() {synchronized (obj2) {System.out.println("Thread2 拿到 obj2 的锁");try {//停顿2秒的意义在于，让thread1线程拿到obj1的锁Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (obj1) {System.out.println("Thread2 拿到 obj1 的锁");}}}}}

一直等待下了。。。

 去找pid：14320

然后 ：jstack 14320 >> jstack.text

得到以下内容：

2023-04-14 14:51:33
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.31-b07 mixed mode):"JMX server connection timeout 21" #21 daemon prio=5 os_prio=0 tid=0x000000002167d800 nid=0x6108 in Object.wait() [0x00000000225ef000]java.lang.Thread.State: TIMED_WAITING (on object monitor)at java.lang.Object.wait(Native Method)at com.sun.jmx.remote.internal.ServerCommunicatorAdmin$Timeout.run(ServerCommunicatorAdmin.java:168)- locked <0x000000076e3b32b8> (a [I)at java.lang.Thread.run(Thread.java:745)"RMI Scheduler(0)" #20 daemon prio=5 os_prio=0 tid=0x0000000021674800 nid=0x3d78 waiting on condition [0x00000000224ee000]java.lang.Thread.State: TIMED_WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for  <0x000000076dc34d08> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093)at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)"RMI TCP Connection(1)-192.168.136.113" #19 daemon prio=5 os_prio=0 tid=0x0000000021a3b800 nid=0x10c0 runnable [0x00000000223ee000]java.lang.Thread.State: RUNNABLEat java.net.SocketInputStream.socketRead0(Native Method)at java.net.SocketInputStream.read(SocketInputStream.java:150)at java.net.SocketInputStream.read(SocketInputStream.java:121)at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)at java.io.BufferedInputStream.read(BufferedInputStream.java:265)- locked <0x000000076e281978> (a java.io.BufferedInputStream)at java.io.FilterInputStream.read(FilterInputStream.java:83)at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:550)at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$240(TCPTransport.java:683)at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$1/410101441.run(Unknown Source)at java.security.AccessController.doPrivileged(Native Method)at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)"RMI TCP Accept-0" #18 daemon prio=5 os_prio=0 tid=0x00000000212dc000 nid=0x8584 runnable [0x000000000153e000]java.lang.Thread.State: RUNNABLEat java.net.DualStackPlainSocketImpl.accept0(Native Method)at java.net.DualStackPlainSocketImpl.socketAccept(DualStackPlainSocketImpl.java:131)at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:404)at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:199)- locked <0x000000076dc69d98> (a java.net.SocksSocketImpl)at java.net.ServerSocket.implAccept(ServerSocket.java:545)at java.net.ServerSocket.accept(ServerSocket.java:513)at sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:52)at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:400)at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:372)at java.lang.Thread.run(Thread.java:745)"DestroyJavaVM" #16 prio=5 os_prio=0 tid=0x000000000365b000 nid=0xae0 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Thread-1" #15 prio=5 os_prio=0 tid=0x000000001ec17000 nid=0x69ac waiting for monitor entry [0x0000000020a1f000]java.lang.Thread.State: BLOCKED (on object monitor)at cn.net.cdsz.ccb.test.TestDeadLock$Thread2.run(TestDeadLock.java:46)- waiting to lock <0x000000076c88add0> (a java.lang.Object)- locked <0x000000076c88ade0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)"Thread-0" #14 prio=5 os_prio=0 tid=0x000000001ec24800 nid=0x8d08 waiting for monitor entry [0x000000002091f000]java.lang.Thread.State: BLOCKED (on object monitor)at cn.net.cdsz.ccb.test.TestDeadLock$Thread1.run(TestDeadLock.java:26)- waiting to lock <0x000000076c88ade0> (a java.lang.Object)- locked <0x000000076c88add0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)"Service Thread" #13 daemon prio=9 os_prio=0 tid=0x000000001eb16000 nid=0x8ac8 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C1 CompilerThread3" #12 daemon prio=9 os_prio=2 tid=0x000000001ea92800 nid=0x67b4 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread2" #11 daemon prio=9 os_prio=2 tid=0x000000001ea8f800 nid=0x5c68 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread1" #10 daemon prio=9 os_prio=2 tid=0x000000001ea8f000 nid=0x6b4c waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread0" #9 daemon prio=9 os_prio=2 tid=0x000000001ea8d800 nid=0x7974 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"JDWP Command Reader" #8 daemon prio=10 os_prio=0 tid=0x000000001ea53800 nid=0x3634 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"JDWP Event Helper Thread" #7 daemon prio=10 os_prio=0 tid=0x000000001ea4d800 nid=0x86e0 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"JDWP Transport Listener: dt_socket" #6 daemon prio=10 os_prio=0 tid=0x000000001ea41000 nid=0x540 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x000000001e9e6000 nid=0x83a0 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001e9e5000 nid=0x8dec runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001d2eb800 nid=0x8670 in Object.wait() [0x000000001fd1f000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x000000076c3862f8> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:142)- locked <0x000000076c3862f8> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:158)at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x000000001d2ea000 nid=0x8ba4 in Object.wait() [0x000000001fc1e000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x000000076c385d68> (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 <0x000000076c385d68> (a java.lang.ref.Reference$Lock)"VM Thread" os_prio=2 tid=0x000000001e9a1800 nid=0x6398 runnable "GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000003670000 nid=0x7e64 runnable "GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000003671800 nid=0x8218 runnable "GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000003673000 nid=0x7830 runnable "GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000003674800 nid=0x8e8c runnable "GC task thread#4 (ParallelGC)" os_prio=0 tid=0x0000000003676800 nid=0x8114 runnable "GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000003678000 nid=0x8220 runnable "GC task thread#6 (ParallelGC)" os_prio=0 tid=0x000000000367b000 nid=0x6fc0 runnable "GC task thread#7 (ParallelGC)" os_prio=0 tid=0x000000000367c000 nid=0x26d8 runnable "GC task thread#8 (ParallelGC)" os_prio=0 tid=0x000000000367d800 nid=0x8238 runnable "GC task thread#9 (ParallelGC)" os_prio=0 tid=0x000000000367e800 nid=0x82a4 runnable "VM Periodic Task Thread" os_prio=2 tid=0x000000001eb18000 nid=0x8900 waiting on condition JNI global references: 4894Found one Java-level deadlock:
=============================
"Thread-1":waiting to lock monitor 0x000000001d2f21b8 (object 0x000000076c88add0, a java.lang.Object),which is held by "Thread-0"
"Thread-0":waiting to lock monitor 0x000000001d2ef668 (object 0x000000076c88ade0, a java.lang.Object),which is held by "Thread-1"Java stack information for the threads listed above:
===================================================
"Thread-1":at cn.net.cdsz.ccb.test.TestDeadLock$Thread2.run(TestDeadLock.java:46)- waiting to lock <0x000000076c88add0> (a java.lang.Object)- locked <0x000000076c88ade0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)
"Thread-0":at cn.net.cdsz.ccb.test.TestDeadLock$Thread1.run(TestDeadLock.java:26)- waiting to lock <0x000000076c88ade0> (a java.lang.Object)- locked <0x000000076c88add0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)Found 1 deadlock.

以上jstack.text内容的最后：

可以清晰的看到：

Thread-1获取了 <0x000000076c88ade0> 的锁，等待获取 <0x000000076c88add0> 这个锁
Thread-0获取了 <0x000000076c88add0> 的锁，等待获取 <0x000000076c88ade0> 这个锁
由此可见，发生了死锁。
 

避免死锁的几个常见方法

• 避免一个线程同时获取多个锁。
• 避免一个线程在锁内同时占用多个资源，尽量保证每个锁只占用一个资源。
• 尝试使用定时锁，使用lock.tryLock（timeout）来替代使用内部锁机制。
• 对于数据库锁，加锁和解锁必须在一个数据库连接里，否则会出现解锁失败的情况。

参考：【JVM】面试题之死锁及问题是怎么定位_jvm线程锁定定位_it噩梦的博客-CSDN博客