java等待并通知

Question

I'm taking one Integer variable and sharing with two threads. One thread should print even numbers and one thread should print odd number sequentially. But notify() throwing IllegalMonitorStateException.

package mywaitnotifytest;
public class App {

    public static void main(String[] args) {
        Integer i=0;
        Even even = new Even(i);
        even.setName("EvenThread");
        Odd odd = new Odd(i);
        odd.setName("OddThread");
        even.start();
        odd.start();
    }
}

class Even extends Thread{

    Integer var;

    Even(Integer var){
        this.var=var;
    }

    @Override
    public void run() {
        while(true){
            synchronized (var) {
                if(var%2==0){
                    try {
                        var.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                var++;
                System.out.println(Thread.currentThread().getName()+"  "+var);
                var.notify();
            }
        }

    }
}

class Odd extends Thread{

    Integer var;

    Odd(Integer var){
        this.var=var;
    }

    @Override
    public void run() {
        while(true){
            synchronized (var) {
                if(var%2!=0){
                    try {
                        var.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                var++;
                System.out.println(Thread.currentThread().getName()+"  "+var);
                var.notify();
            }
        }
    }
}

And the output is :

OddThread 1

Exception in thread "OddThread" java.lang.IllegalMonitorStateException

at java.lang.Object.notify(Native Method)

at mywaitnotifytest.Odd.run(App.java:67)

Answer 1

I think this is sufficiently different to the usual answer to give another one.

In this case you are using synchronized . When you apply a lock it is on a object not a reference.

---

synchronized (var) {

This locks the object var references, not on var as a field.

---

var++;

This replaces the object var points to. It is the same as

var = Integer.valueOf(var.intValue() + 1);

Note: Integer and indeed all the primitive wrappers are Immutable. When you perform any operation on them you are actually unboxing, calculating using the primitive value and re-boxing the object. It is possible to get the same object back if it is pooled. eg

Integer i = 10;
i += 0; // gives back the same object.

However, if the object is not pooled

Double d = 10;
d += 0; // creates a new object.

---

var.notify();

Attempts the call notify on the new object, not the one which was locked.

---

You shouldn't attempt to lock a field which you mutate. It won't do what it appears to do. You also shouldn't lock on a pooled object. In this case you could have another thread using the same Integer for an unrelated purpose and notify() will wake up an unrelated thread.

To use wait/notify correctly, you should

• notify() or notifyAll() after a state change in another shared field.
• you should use a while loop for wait() to check the state change.

If you don't do this

• notify can be lost if another thread is not waiting.
• wait can wake spuriously, even when no notify was called.

For the above requirement what is the edit suggested in the code? How do i share the same object for multiple threads?

public class PingPong implements Runnable {    
    static class Shared { int num; }

    private final Shared var;
    private final int bit;

    public static void main(String[] args) {
        Shared var = new Shared();
        new Thread(new PingPong(var, 0), "EvenThread").start();
        new Thread(new PingPong(var, 1), "OddThread").start();
    }

    PingPong(Shared var, int bit) {
        this.var = var;
        this.bit = bit;
    }

    @Override
    public void run() {
        try {
            String name = Thread.currentThread().getName();
            while (true) {
                synchronized (var) {
                    while (var.num % 2 == bit)
                        var.wait();

                    var.num++;
                    System.out.println(name + "  " + var.num);
                    var.notify();
                }
            }
        } catch (InterruptedException e) {
            System.out.println("Interrupted");
        }
    }
}

Answer 2

Instead of using Integer wrapper class,I created my own class and now It works fine.

 package mywaitnotifytest; public class App { public static void main(String[] args) { MyInt i = new MyInt(0); Even even = new Even(i); even.setName("EvenThread"); Odd odd = new Odd(i); odd.setName("OddThread"); even.start(); odd.start(); } } class Even extends Thread { MyInt var; Even(MyInt var) { this.var = var; } @Override public void run() { while (true) { try { Thread.sleep(200); } catch (InterruptedException e1) { // TODO Auto-generated catch block e1.printStackTrace(); } synchronized (var) { if (var.i % 2 == 0) { try { var.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } var.i++; System.out.println(Thread.currentThread().getName() + " " + var.i); var.notify(); } } } } class Odd extends Thread { MyInt var; Odd(MyInt var) { this.var = var; } @Override public void run() { while (true) { try { Thread.sleep(2000); } catch (InterruptedException e1) { // TODO Auto-generated catch block e1.printStackTrace(); } synchronized (var) { if (var.i % 2 != 0) { try { var.wait(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } var.i++; System.out.println(Thread.currentThread().getName() + " " + var.i); var.notify(); } } } } class MyInt { int i = 0; public MyInt(int i) { super(); this.i = i; } @Override public String toString() { // TODO Auto-generated method stub return "" + i; } }