概述
高并发编程-线程通信_使用wait和notify进行线程间的通信 - 遗留问题
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)
代码语言：javascript复制C:UsersMr.Yang>E:
E:>cd E:Program FilesJavajdk1.8.0_161bin
E:Program FilesJavajdk1.8.0_161bin>jps
1504 MultiProduceConsumerDemo
E:Program FilesJavajdk1.8.0_161bin>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 FilesJavajdk1.8.0_161bin>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
代码语言：javascript复制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
3. 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
4. P2同上一步P1的操作 ，P2-----WAITING 对应日志
5. 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
6. 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
7. 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。）
那如何解决呢？ 下篇博文我们一起来探讨下
