This Code can Throw an IllegalMonitorStateException

Question

void waitForSignal(){
    Object ob =new Object();

    synchronized (Thred.currentThread()) {
        try {
            ob.wait();
            ob.notify();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

This Methods Throws an IllegalMonitorStateException. Can some one explain why so.

Thanks in advance

Answer 1

You should only invoke wait on the object that you have acquired lock on.

In your code, you have acquried lock on Thread.currentThread() , but you are invoking it on ob , which will throw IllegalMonitorStateException .

So, you should rather change your synchronized block to: -

synchronized (ob) {

}

Answer 2

You want to synchronize on the object you are waiting:

synchronized (ob) {
        try {
            ob.wait();
            ob.notify();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

Answer 3

If even you fixed your problem with IllegalMonitorException like

void waitForSignal() {
    Object ob = new Object();
    synchronized (ob) {

your code won't work. In your code each thread creates a new lock, invisible outside the method. No chance for other threads to notify waiting threads.