java程序死锁，3种方式快速找到死锁代码

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。