目錄

• 一、產(chǎn)生死鎖的原因
• 二、如何避免死鎖

一、產(chǎn)生死鎖的原因

發(fā)生死鎖的情況： 多個線程需要同時占用多個共享資源而發(fā)生需要互相死循環(huán)等待的情況，就是，兩個線程互相等待著對象釋放鎖，一直這樣僵持下去，所以導致了死鎖的產(chǎn)生。

產(chǎn)生死鎖代碼：

package com.javaagent.thread;public class DieThread implements Runnable {public static Object obj1=new Object();public static Object obj2=new Object();private boolean flag;public DieThread(boolean bl){flag = bl;}@Overridepublic void run() {if(flag) {while(true) {synchronized(obj1) {try {Thread.sleep(1000);}catch (InterruptedException e){e.printStackTrace();}System.out.println("線程" + Thread.currentThread().getName() + "獲取obj1鎖對象，等待獲取obj2鎖對象...");synchronized(obj2) {System.out.println(Thread.currentThread().getName() + " ---- obj2.");}}}}else {while(true){synchronized(obj2) {System.out.println("線程" + Thread.currentThread().getName() + "獲取obj2鎖對象，等待獲取obj1鎖對象...");synchronized(obj1) {System.out.println(Thread.currentThread().getName() + " ---- obj1.");}}}}} } public class Main {public static void main(String[] args){DieThread d1=new DieThread(true);DieThread d2=new DieThread(false);final Thread t1 = new Thread(d1);final Thread t2 = new Thread(d2);t1.setName("DieThread_1");t2.setName("DieThread_2");t1.start();t2.start();} }

輸出結(jié)果：

線程DieThread_2獲取obj2鎖對象，等待獲取obj1鎖對象... 線程DieThread_1獲取obj1鎖對象，等待獲取obj2鎖對象...

采用 visualVM工具 來進行診斷，結(jié)果如下圖所示：

從該圖中，我們可以看出，Main的 進程ID為84114 ，紅框1中提示 檢測到死鎖 ，然后我們點擊紅框3來查看 線程Dump 數(shù)據(jù)，如下圖：

詳細信息：

2020-03-04 15:02:44 Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.121-b13 mixed mode):"RMI TCP Connection(idle)" #19 daemon prio=9 os_prio=31 tid=0x00007ff1db108800 nid=0x5f03 waiting on condition [0x0000700001961000]java.lang.Thread.State: TIMED_WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for <0x0000000796082e28> (a java.util.concurrent.SynchronousQueue$TransferStack)at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)at java.util.concurrent.SynchronousQueue$TransferStack.awaitFulfill(SynchronousQueue.java:460)at java.util.concurrent.SynchronousQueue$TransferStack.transfer(SynchronousQueue.java:362)at java.util.concurrent.SynchronousQueue.poll(SynchronousQueue.java:941)at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1066)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)Locked ownable synchronizers:- None"JMX server connection timeout 18" #18 daemon prio=9 os_prio=31 tid=0x00007ff1da87c800 nid=0x5d03 in Object.wait() [0x000070000185e000]java.lang.Thread.State: TIMED_WAITING (on object monitor)at java.lang.Object.wait(Native Method)at com.sun.jmx.remote.internal.ServerCommunicatorAdmin$Timeout.run(ServerCommunicatorAdmin.java:168)- locked <0x00000007964a9dc8> (a [I)at java.lang.Thread.run(Thread.java:745)Locked ownable synchronizers:- None"RMI Scheduler(0)" #17 daemon prio=9 os_prio=31 tid=0x00007ff1da895800 nid=0x5b03 waiting on condition [0x000070000175b000]java.lang.Thread.State: TIMED_WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for <0x00000007960ab5d8> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$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)Locked ownable synchronizers:- None"RMI TCP Connection(1)-10.0.5.39" #16 daemon prio=9 os_prio=31 tid=0x00007ff1d99ed800 nid=0x5903 runnable [0x0000700001657000]java.lang.Thread.State: RUNNABLEat java.net.SocketInputStream.socketRead0(Native Method)at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)at java.net.SocketInputStream.read(SocketInputStream.java:171)at java.net.SocketInputStream.read(SocketInputStream.java:141)at java.io.BufferedInputStream.fill(BufferedInputStream.java:246)at java.io.BufferedInputStream.read(BufferedInputStream.java:265)- locked <0x00000007963eced8> (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$0(TCPTransport.java:683)at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$3/657644190.run(Unknown Source)at java.security.AccessController.doPrivileged(Native Method)at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)Locked ownable synchronizers:- <0x0000000796299c10> (a java.util.concurrent.ThreadPoolExecutor$Worker)"RMI TCP Accept-0" #15 daemon prio=9 os_prio=31 tid=0x00007ff1d9941800 nid=0x570f runnable [0x0000700001555000]java.lang.Thread.State: RUNNABLEat java.net.PlainSocketImpl.socketAccept(Native Method)at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)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)Locked ownable synchronizers:- None"Attach Listener" #13 daemon prio=9 os_prio=31 tid=0x00007ff1db028800 nid=0x5303 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"DestroyJavaVM" #12 prio=5 os_prio=31 tid=0x00007ff1da1ac000 nid=0x1403 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"DieThread_2" #11 prio=5 os_prio=31 tid=0x00007ff1d9839800 nid=0x5103 waiting for monitor entry [0x000070000134f000]java.lang.Thread.State: BLOCKED (on object monitor)at com.javaagent.thread.DieThread.run(DieThread.java:37)- waiting to lock <0x000000079585bee0> (a java.lang.Object)- locked <0x000000079585bef0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)Locked ownable synchronizers:- None"DieThread_1" #10 prio=5 os_prio=31 tid=0x00007ff1d9835800 nid=0x4f03 waiting for monitor entry [0x000070000124c000]java.lang.Thread.State: BLOCKED (on object monitor)at com.javaagent.thread.DieThread.run(DieThread.java:26)- waiting to lock <0x000000079585bef0> (a java.lang.Object)- locked <0x000000079585bee0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)Locked ownable synchronizers:- None"Service Thread" #9 daemon prio=9 os_prio=31 tid=0x00007ff1d9835000 nid=0x4b03 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"C1 CompilerThread2" #8 daemon prio=9 os_prio=31 tid=0x00007ff1da87f800 nid=0x4903 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"C2 CompilerThread1" #7 daemon prio=9 os_prio=31 tid=0x00007ff1d98c5800 nid=0x4703 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"C2 CompilerThread0" #6 daemon prio=9 os_prio=31 tid=0x00007ff1da886000 nid=0x4503 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"Monitor Ctrl-Break" #5 daemon prio=5 os_prio=31 tid=0x00007ff1da884800 nid=0x4303 runnable [0x0000700000c3a000]java.lang.Thread.State: RUNNABLEat java.net.SocketInputStream.socketRead0(Native Method)at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)at java.net.SocketInputStream.read(SocketInputStream.java:171)at java.net.SocketInputStream.read(SocketInputStream.java:141)at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)- locked <0x00000007958fc840> (a java.io.InputStreamReader)at java.io.InputStreamReader.read(InputStreamReader.java:184)at java.io.BufferedReader.fill(BufferedReader.java:161)at java.io.BufferedReader.readLine(BufferedReader.java:324)- locked <0x00000007958fc840> (a java.io.InputStreamReader)at java.io.BufferedReader.readLine(BufferedReader.java:389)at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64)Locked ownable synchronizers:- None"Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007ff1db029800 nid=0x360f runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None"Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007ff1d9820800 nid=0x3003 in Object.wait() [0x000070000092e000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x0000000795588ec8> (a java.lang.ref.ReferenceQueue$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)Locked ownable synchronizers:- None"Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007ff1d9810800 nid=0x2e03 in Object.wait() [0x000070000082b000]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)Locked ownable synchronizers:- None"VM Thread" os_prio=31 tid=0x00007ff1da026800 nid=0x2c03 runnable "GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007ff1d9810000 nid=0x2403 runnable "GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007ff1da00d000 nid=0x2603 runnable "GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007ff1da00d800 nid=0x2803 runnable "GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007ff1da00e000 nid=0x2a03 runnable "VM Periodic Task Thread" os_prio=31 tid=0x00007ff1da80d000 nid=0x4d03 waiting on condition JNI global references: 354Found one Java-level deadlock: ============================= "DieThread_2":waiting to lock monitor 0x00007ff1da80ad58 (object 0x000000079585bee0, a java.lang.Object),which is held by "DieThread_1" "DieThread_1":waiting to lock monitor 0x00007ff1da808418 (object 0x000000079585bef0, a java.lang.Object),which is held by "DieThread_2"Java stack information for the threads listed above: =================================================== "DieThread_2":at com.javaagent.thread.DieThread.run(DieThread.java:37)- waiting to lock <0x000000079585bee0> (a java.lang.Object)- locked <0x000000079585bef0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745) "DieThread_1":at com.javaagent.thread.DieThread.run(DieThread.java:26)- waiting to lock <0x000000079585bef0> (a java.lang.Object)- locked <0x000000079585bee0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)Found 1 deadlock.

通過上面的線程Dump文件信息，我們可以看出來發(fā)生了死鎖，下面也列出了java級別的鎖問題和棧信息：

Found one Java-level deadlock: ============================= "DieThread_2":waiting to lock monitor 0x00007ff1da80ad58 (object 0x000000079585bee0, a java.lang.Object),which is held by "DieThread_1" "DieThread_1":waiting to lock monitor 0x00007ff1da808418 (object 0x000000079585bef0, a java.lang.Object),which is held by "DieThread_2"Java stack information for the threads listed above: =================================================== "DieThread_2":at com.javaagent.thread.DieThread.run(DieThread.java:37)- waiting to lock <0x000000079585bee0> (a java.lang.Object)- locked <0x000000079585bef0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745) "DieThread_1":at com.javaagent.thread.DieThread.run(DieThread.java:26)- waiting to lock <0x000000079585bef0> (a java.lang.Object)- locked <0x000000079585bee0> (a java.lang.Object)at java.lang.Thread.run(Thread.java:745)Found 1 deadlock.

并且棧信息中，可以定位到死鎖發(fā)生的地方。

如果解決死鎖，那是因為代碼的邏輯存在問題，可以調(diào)整代碼的邏輯或者業(yè)務(wù)，來避免死鎖。

二、如何避免死鎖

• 避免一個線程同時獲取多個鎖

• 避免一個線程在鎖內(nèi)同時占用多個資源，盡量保證每個鎖只占用一個資源

• 嘗試使用定時鎖，使用 lock.tryLock(timeout) 來替代使用內(nèi)部鎖機制。

• 對于數(shù)據(jù)庫鎖，加鎖和解鎖必須在一個數(shù)據(jù)庫連接里，否則會出現(xiàn)解鎖失敗的情況。

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