How to Fix Exception in thread main – java.lang.IllegalMonitorStateException Error on Thread.wait()

While running Java program are you getting this error?

Exception in thread "main" java.lang.IllegalMonitorStateException
	at java.lang.Object.wait(Native Method)
	at crunchify.com.tutorial.CrunchifyJavaThreadPriority.main(CrunchifyJavaThreadPriority.java:50)

Today, while working on Java program I got above java.lang.IllegalMonitorStateException Exception at runtime. There wasn’t any error appeared during compile time. After finishing a complete Java program on threading when I ran the code, suddenly above error popped out.

Well, as exception stack trace clearly showed the line number from where exception appeared, I couldn’t found out the solution immediately. Exception was coming from below wait() statement.

try {
	crunchChildThread.wait(10000);   <==== from here
} catch (InterruptedException e) {
	e.printStackTrace();
}

It turned out to be a missing synchronized block. In order for Thread.wait() to work, code block needs to be in synchronized block. This happens when you try to gain access without holding the Thread/Object lock.

In order to fix this just use below code. Make sure you use synchronized keyword for method.

private synchronized void generateThreadWait() {
	final DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
	try {
		println("Inside synchronized block entry..." + dtf.format(LocalDateTime.now()));
		this.wait(10000);
		println("Inside synchronized block exit..." + dtf.format(LocalDateTime.now()));
	} catch (InterruptedException e) {
		e.printStackTrace();
	}

}

Now re-run your program again and you shouldn’t see IllegalMonitorStateException exception any more.