java通知所有: illegalMonitorStateException

Question

我想模拟一个妈妈叫三个 child 吃饭，这是我的代码:

class child extends Thread{
    String name;
    mother mom;
    public child(mother mon, String name){
        this.name = name;
        this.mom = mom;
    }

    public void run(){
        System.out.println(name+" is hungry.");
        while (!mom.finished){
            try {
                wait();
            }catch(InterruptedException e){}
        }
        washhands();
    }

    public synchronized void washhands(){
        System.out.println(name+" washed hands.");
        notifyAll();
    }
}

class mother{
    boolean finished;

    public mother(){
        finished = false;
    }
    public void cook(){
        System.out.println("Mom started cooking.");
        try{
            Thread.sleep(200);
        }catch(InterruptedException e){};
        finished = true;
        System.out.println("Mom finished cooking.");
        notifyAll();
    }
}

public class work{
    public static void main(String[] args){
        mother mom = new mother();
        child Alex = new child(mom, "Alex");
        child James = new child(mom, "James");
        child Tod = new child(mom, "Tod");

        Alex.start();
        James.start();
        Tod.start();
        mom.cook();
    }
}

所以我在类 mother 的 notifyAll() 行处得到了“线程“main”java.lang.IllegalMonitorStateException 中的异常”。我不知道会发生什么，有人可以帮助我吗？

Answer 1

考虑以下几点。

1. 您必须使用一个通用对象来保护所有需要互斥的代码块。
2. 只有当对象的监视器被当前线程锁定时，才能对对象调用 wait/notify。
3. 要获取监视器，线程必须进入由拥有监视器的对象保护的 block 。监视器由 JVM 内部使用。

除此之外，使用 Java naming conventions. 命名您的类和变量

看下面的代码

synchronized void method1 () {
}

synchronized void method2() {
}

这里method1和method2将受到调用它们的对象的监视器的保护。因此，如果我们想在 protected block 内使用相同的监视器，将按如下方式完成。

public void method3() {
    synchronized(this) {
    }
    ...
}

现在 method1()、method2() 和 method3() 中的 protected block 将受到同一对象监视器的保护。

如果我们需要跨类的保护 block ，那么我们需要在它们之间共享一个公共(public)对象。我在下面的代码中进行了相同的修改。

package filevisitor;

import java.util.Random;
import java.util.concurrent.atomic.AtomicLong;

class child extends Thread{
    String name;
    volatile mother mom;
    public child(mother mom, String name){
        this.name = name;
        this.mom = mom;
    }

    public void run(){
        System.out.println(name+" is hungry.");
        while (!mom.finished){
            try {
                synchronized(work.myLockObject) {
                    work.myLockObject.wait();
                }
            }catch(InterruptedException e){}
        }
        washhands();
    }

    public void washhands(){
        System.out.println(name+" washed hands.");
        synchronized(work.myLockObject) {
            work.myLockObject.notifyAll();
        }
    }
}

class mother{
    boolean finished;

    public mother(){
        finished = false;
    }
    public void cook(){
        System.out.println("Mom started cooking.");
        try{
            Thread.sleep(200);
        }catch(InterruptedException e){};
        finished = true;
        System.out.println("Mom finished cooking.");
        synchronized(work.myLockObject) {
            work.myLockObject.notifyAll();
        }
    }
}

public class work{
    public static final Object myLockObject = new Object();
    public static void main(String[] args){
        mother mom = new mother();
        child Alex = new child(mom, "Alex");
        child James = new child(mom, "James");
        child Tod = new child(mom, "Tod");

        Alex.start();
        James.start();
        Tod.start();
        mom.cook();
    }
}

子类中的 this.mom = mom; 还存在一个小错误。参数名称为 mon，因此此分配没有执行任何操作，导致 NullPointerException。

输出

Mom started cooking.
Alex is hungry.
Tod is hungry.
James is hungry.
Mom finished cooking.
Tod washed hands.
James washed hands.
Alex washed hands.