概述

我们看到了应用卡住了。。。。怀疑是不是死锁呢？（其实没有）

jstack或者可视化工具检测是否死锁(没有)

C:\Users\Mr.Yang>E:

E:\>cd E:\Program Files\Java\jdk1.8.0_161\bin

E:\Program Files\Java\jdk1.8.0_161\bin>jps
1504 MultiProduceConsumerDemo

E:\Program Files\Java\jdk1.8.0_161\bin>jstack 1504
2019-10-01 00:44:23
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.161-b12 mixed mode):

"DestroyJavaVM" #15 prio=5 os_prio=0 tid=0x0000000002983800 nid=0x3348 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Thread-3" #14 prio=5 os_prio=0 tid=0x0000000019fa5000 nid=0x3af0 in Object.wait() [0x000000001abff000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000078b682768> (a java.lang.Object)
        at java.lang.Object.wait(Object.java:502)
        at com.artisan.test.MultiProduceConsumerDemo.consume(MultiProduceConsumerDemo.java:42)
        - locked <0x000000078b682768> (a java.lang.Object)
        at com.artisan.test.MultiProduceConsumerDemo$2.run(MultiProduceConsumerDemo.java:63)

"Thread-2" #13 prio=5 os_prio=0 tid=0x0000000019fa4000 nid=0x9e4 in Object.wait() [0x000000001aaff000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000078b682768> (a java.lang.Object)
        at java.lang.Object.wait(Object.java:502)
        at com.artisan.test.MultiProduceConsumerDemo.consume(MultiProduceConsumerDemo.java:42)
        - locked <0x000000078b682768> (a java.lang.Object)
        at com.artisan.test.MultiProduceConsumerDemo$2.run(MultiProduceConsumerDemo.java:63)

"Thread-1" #12 prio=5 os_prio=0 tid=0x0000000019fa0000 nid=0x37cc in Object.wait() [0x000000001a9ff000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000078b682768> (a java.lang.Object)
        at java.lang.Object.wait(Object.java:502)
        at com.artisan.test.MultiProduceConsumerDemo.produce(MultiProduceConsumerDemo.java:20)
        - locked <0x000000078b682768> (a java.lang.Object)
        at com.artisan.test.MultiProduceConsumerDemo$1.run(MultiProduceConsumerDemo.java:55)

"Thread-0" #11 prio=5 os_prio=0 tid=0x0000000019f9f800 nid=0x1bd4 in Object.wait() [0x000000001a8fe000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000078b682768> (a java.lang.Object)
        at java.lang.Object.wait(Object.java:502)
        at com.artisan.test.MultiProduceConsumerDemo.produce(MultiProduceConsumerDemo.java:20)
        - locked <0x000000078b682768> (a java.lang.Object)
        at com.artisan.test.MultiProduceConsumerDemo$1.run(MultiProduceConsumerDemo.java:55)

"Service Thread" #10 daemon prio=9 os_prio=0 tid=0x0000000019d2f800 nid=0x3ba4 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x0000000019ca3800 nid=0x37e4 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x0000000019c54800 nid=0x2db0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x0000000019c50800 nid=0x37bc waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x0000000019c2e000 nid=0x2ed4 runnable [0x000000001a2fe000]
   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 <0x000000078b5dad50> (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 <0x000000078b5dad50> (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=0x0000000018860000 nid=0x34f8 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000019c08800 nid=0x1514 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001883a000 nid=0x22c in Object.wait() [0x0000000019b9e000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000078b408ec0> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
        - locked <0x000000078b408ec0> (a java.lang.ref.ReferenceQueue$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=0x0000000002a73800 nid=0x29bc in Object.wait() [0x0000000019a9f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x000000078b406b68> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x000000078b406b68> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=2 tid=0x0000000018818000 nid=0x381c runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000002998000 nid=0x3024 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x000000000299a000 nid=0x229c runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x000000000299b800 nid=0x2bbc runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x000000000299d000 nid=0x255c runnable

"VM Periodic Task Thread" os_prio=2 tid=0x0000000019d67800 nid=0x1c5c waiting on condition

JNI global references: 334


E:\Program Files\Java\jdk1.8.0_161\bin>

可以看到并没有死锁的发生

或者使用 jvisualvm 、 jmc 工具来看下都行

（jmc截图）

并且可以看到4个线程均是 WAITING 状态

（jmc截图）

原因分析

为了方便观察，我们改造下，给线程起个名

package com.artisan.test;

import java.util.stream.Stream;

public class MultiProduceConsumerDemo {

    // 对象监视器-锁
    private final Object LOCK = new Object();

    // 是否生产出数据的标识
    private boolean isProduced = false;

    // volatile 确保可见性， 假设 i 就是生产者生产的数据
    private volatile int i = 0 ;

    public  void produce(){
        synchronized (LOCK){
            System.out.println(Thread.currentThread() + " GOT LOCK  " + isProduced);
            String msg = isProduced ? "已生产货物" : "没有货物可搬运";

            if (isProduced){
                try {
                    System.out.println(Thread.currentThread() + " wait becauseof  " + msg );
                    LOCK.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }else{
                i++;
                System.out.println(Thread.currentThread() + " Produce:" + i);
                LOCK.notify();
                isProduced = true;
            }
        }
    }

    public void consume(){
        // 加锁
        synchronized (LOCK){
            System.out.println(Thread.currentThread() + " GOT LOCK " + isProduced);
            String msg = isProduced ? "已生产货物" : "没有货物可搬运";

            if (isProduced){
                System.out.println(Thread.currentThread() + " Consume:" + i);
                LOCK.notify();
                isProduced = false;
            }else{
                try {
                    System.out.println(Thread.currentThread() + " wait becauseof  " + msg);
                    LOCK.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        MultiProduceConsumerDemo produceConsumerDemo = new MultiProduceConsumerDemo();
        Stream.of("P1","P2").forEach(n-> new Thread(n){
            @Override
            public void run() {
                while(true) produceConsumerDemo.produce();
            }
        }.start());


        Stream.of("C1","C2").forEach(n->new Thread(n){
            @Override
            public void run() {
                while(true) produceConsumerDemo.consume();
            }
        }.start());

        
    }
}

运行下，可以看到程序并没有持续运行，而是假死了…

（每次运行的结果都有可能不一样，这里我们以这次的运行结果来分析下）

逐步分析下：

生产者的代码

消费者代码

线程P1锁，没有货物生产，isProduce=false
线程P1,生产货物，紧接着 LOCK.notify(); isProduced = true; ，其实第一步的LOCK.notify() 是没有什么作用的，因为没有任何线程wait. 执行完以后释放锁。对应日志
紧接着，P1又抢到了锁，但是生产后没有被消费，所以直接进入LOCK.wati. 执行完以后释放锁。P1-----WAITING . 对应日志
P2同上一步P1的操作，P2-----WAITING 对应日志
C1 抢到资源，isProduce已经生产，所以C1线程直接消费，消费完成以后要通知生产者继续生产，即唤醒消费者（LOCK.notify();），将isProduce标志位置为false . 执行完以后，释放锁。C1-----WAITING日志如下
紧接着C1又抢到了锁，因没有生产，所以进入LOCK.wait（） ，释放锁。对应日志
依次类推… 直到最后C2 唤醒了C1 ,此时C1看到isProduce=false, 则C1进入了wait ,这个时候4个线程都是watiing的状态了，就出现了4个线程均是wait状态，都不执行了，出现了假死（因为notify方法，唤醒一个线程，具体是哪个线程是不确定的。）