使用多线程运行奇数偶程序时获取异常

Question

I am trying to run below written program. But Here I am getting exception as

Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
at java.lang.Object.notify(Native Method)
at main.java.OddEven$even.run(OddEven.java:16)
at java.lang.Thread.run(Unknown Source)

I am unable to find reason behind exception.

Execption is occuring in notify method. We get IllegalMonitorStateException in notify method only when current thread does not own the lock object.

public class OddEven {

private Integer count = 0;
Object ob = new Object();

class even implements Runnable {

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

            }
        }

    }
}

class odd implements Runnable {
    @Override
    public void run() {
            while (count % 2 != 0) {
                synchronized (ob) {
                if (count % 2 != 0) {
                    System.out.println(count++);
                    notify();
                    try {
                        wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }

            }
        }

    }

}

public static void main(String[] args) throws CloneNotSupportedException {
    OddEven t1 = new OddEven();
    Thread e = new Thread(t1.new even());
    Thread o = new Thread(t1.new odd());
    e.start();
    o.start();
}

}

Answer 1

To call notify() on an object you need to have a lock on that object, ie be in a block synchronized on that object. You are in a synchronized block, but you are synchronizing on ob , while you are calling notify() on this .

You either have to also use ob for your notify() and wait() calls, or synchronize on this .