java程序中出现死锁问题，如果不了解排查方法，是束手无策的，今天咱们用三种方法找到死锁问题。
运行下面代码
package com.jvm.visualvm;
/**
* <a href="http://www.itsoku.com/archives">Java干货铺子,只生产干货，公众号：javacode2018</a>
*/
public class Demo4 {
public static void main(String[] args) {
User u1 = new User("u1");
User u2 = new User("u2");
Thread thread1 = new Thread(new SynAddRunalbe(u1, u2, 1, 2, true));
thread1.setName("thread1");
thread1.start();
Thread thread2 = new Thread(new SynAddRunalbe(u1, u2, 2, 1, false));
thread2.setName("thread2");
thread2.start();
}
/**
* 线程死锁等待演示
*/
public static class SynAddRunalbe implements Runnable {
User u1, u2;
int a, b;
boolean flag;
public SynAddRunalbe(User u1, User u2, int a, int b, boolean flag) {
this.u1 = u1;
this.u2 = u2;
this.a = a;
this.b = b;
this.flag = flag;
}
@Override
public void run() {
try {
if (flag) {
synchronized (u1) {
Thread.sleep(100);
synchronized (u2) {
System.out.println(a + b);
}
}
} else {
synchronized (u2) {
Thread.sleep(100);
synchronized (u1) {
System.out.println(a + b);
}
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static class User {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public User(String name) {
this.name = name;
}
@Override
public String toString() {
return "User{" +
"name='" + name + '\'' +
'}';
}
}
}
程序中：thread1持有u1的锁，thread2持有u2的锁，thread1等待获取u2的锁，thread2等待获取u1的锁，相互需要获取的锁都被对方持有者，造成了死锁。程序运行中一直无法结束。
通过jdk工具jps、jstack排查死锁问题
jps、jstack都是jdk提供的命令工具，方便用户排查程序的一些问题。更详细的用法见文档最后。
步骤一：使用jsp查找程序进行
jps：jdk提供的一个工具，可以查看到正在运行的java进程
C:\Users\Think>jps -l
5824 com.jvm.visualvm.Demo4
步骤二：使用jstack查看线程堆栈信息
jstack：jdk提供的一个工具，可以查看java进程中线程堆栈信息。更详细的用法见文档最后。
C:\Users\Think>jstack 5824
2019-06-04 15:34:37
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.121-b13 mixed mode):
"DestroyJavaVM" #14 prio=5 os_prio=0 tid=0x0000000002cf4000 nid=0x9fd8 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"thread2" #13 prio=5 os_prio=0 tid=0x000000002884d800 nid=0x40c4 waiting for monitor entry [0x00000000292ce000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:49)
- waiting to lock <0x00000007173d4178> (a com.jvm.visualvm.Demo4$User)
- locked <0x00000007173d41b8> (a com.jvm.visualvm.Demo4$User)
at java.lang.Thread.run(Thread.java:745)
"thread1" #12 prio=5 os_prio=0 tid=0x000000002885b800 nid=0x99c0 waiting for monitor entry [0x00000000291cf000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:42)
- waiting to lock <0x00000007173d41b8> (a com.jvm.visualvm.Demo4$User)
- locked <0x00000007173d4178> (a com.jvm.visualvm.Demo4$User)
at java.lang.Thread.run(Thread.java:745)
"Service Thread" #11 daemon prio=9 os_prio=0 tid=0x0000000027dd9000 nid=0x7f80 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C1 CompilerThread3" #10 daemon prio=9 os_prio=2 tid=0x0000000027d80800 nid=0x3b94 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x0000000027d7c000 nid=0x4c7c waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x0000000027d7b000 nid=0x8f4c waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x0000000027d73000 nid=0x9094 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x0000000027d0a800 nid=0x647c runnable [0x00000000282ce000]
java.lang.Thread.State: RUNNABLE
at 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 <0x0000000717449cd8> (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 <0x0000000717449cd8> (a java.io.InputStreamReader)
at java.io.BufferedReader.readLine(BufferedReader.java:389)
at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:64)
"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000027b61000 nid=0x7cf8 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000027b5f800 nid=0x94f4 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x0000000027af1800 nid=0x9960 in Object.wait() [0x0000000027fce000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000717188ec8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
- locked <0x0000000717188ec8> (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)
"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000002deb000 nid=0xb2bc in Object.wait() [0x0000000027acf000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000717186b68> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
- locked <0x0000000717186b68> (a java.lang.ref.Reference$Lock)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
"VM Thread" os_prio=2 tid=0x0000000025be9000 nid=0xaa90 runnable
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002d0a000 nid=0x3fc0 runnable
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000002d0b800 nid=0x9cf8 runnable
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x0000000002d0d000 nid=0x51c runnable
"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x0000000002d0e800 nid=0x24d0 runnable
"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x0000000002d12000 nid=0x618c runnable
"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000002d13000 nid=0x5550 runnable
"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x0000000002d16000 nid=0x8994 runnable
"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x0000000002d17800 nid=0x39cc runnable
"VM Periodic Task Thread" os_prio=2 tid=0x0000000027dd9800 nid=0x5294 waiting on condition
JNI global references: 33
Found one Java-level deadlock:
=============================
"thread2":
waiting to lock monitor 0x0000000028846468 (object 0x00000007173d4178, a com.jvm.visualvm.Demo4$User),
which is held by "thread1"
"thread1":
waiting to lock monitor 0x0000000025bf0bb8 (object 0x00000007173d41b8, a com.jvm.visualvm.Demo4$User),
which is held by "thread2"
Java stack information for the threads listed above:
===================================================
"thread2":
at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:49)
- waiting to lock <0x00000007173d4178> (a com.jvm.visualvm.Demo4$User)
- locked <0x00000007173d41b8> (a com.jvm.visualvm.Demo4$User)
at java.lang.Thread.run(Thread.java:745)
"thread1":
at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:42)
- waiting to lock <0x00000007173d41b8> (a com.jvm.visualvm.Demo4$User)
- locked <0x00000007173d4178> (a com.jvm.visualvm.Demo4$User)
at java.lang.Thread.run(Thread.java:745)
Found 1 deadlock.
从上面的堆栈信息中我们可以发现这个内容：“Found one Java-level deadlock”，表示程序中发现了一个死锁，后面包含跟多详细的信息，重点下面：
Java stack information for the threads listed above:
===================================================
"thread2":
at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:49)
- waiting to lock <0x00000007173d4178> (a com.jvm.visualvm.Demo4$User)
- locked <0x00000007173d41b8> (a com.jvm.visualvm.Demo4$User)
at java.lang.Thread.run(Thread.java:745)
"thread1":
at com.jvm.visualvm.Demo4$SynAddRunalbe.run(Demo4.java:42)
- waiting to lock <0x00000007173d41b8> (a com.jvm.visualvm.Demo4$User)
- locked <0x00000007173d4178> (a com.jvm.visualvm.Demo4$User)
at java.lang.Thread.run(Thread.java:745)
可以死锁的代码是在Demo4.java的49行和42行，此时我们就可以去优化代码，解决死锁问题。
通过jdk提供的工具jconsole排查死锁问题
jconsole：jdk提供的一个可视化的工具，方便排查程序的一些问题，如：程序内存溢出、死锁问题等等。更详细的用法见文档最后。jconsole位于jdk的bin目录中
步骤一：打开jconsole.exe
可以看到我们的程序，点击连接。
步骤二：在jconsole窗口中查看线程堆栈信息
步骤三：堆栈中查看到死锁信息
点击“检测死锁”，可以看到程序死锁信息。
上图中可以看到详细的死锁信息，和jstack中信息类似。
通过jdk提供的工具VisualVM排查死锁问题
VisualVM：jdk提供的一个非常强大的排查java程序问题的一个工具，可以监控程序的性能、查看jvm配置信息、堆快照、线程堆栈信息。算是程序优化的必备工具。工具位于jdk的bin目录中。
步骤一：打开VisualVM.exe
步骤二：在visualvm中查看我们的目标程序
目标程序在visualvm左侧窗口中，双击即可打开。
步骤三：切换到“线程”窗口
步骤四：查看堆栈信息
在线程窗口中点击“线程Dump”按钮
可以看到产生了一个线程堆栈快照
线程堆栈快照的信息和jstack查看到的信息一样，即可发现死锁代码。
总结
程序中介绍了3中排查死锁的方法，使用到的都是jdk提供给我们的工具，这些工具对于我们优化程序来说，是非常好的，我们一定要掌握。
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