meet IllegalMonitorStateException when doing multi-thread programming in Java

Question

I wrote a simple program to learn about synchronized block. The program is as follow:

public class SychronizedBlock {

    static int balance = 0;
    static Integer lock = 0;


    public static void deposit(int amt) {
        Thread t1 = new Thread(new Runnable() {
            public void run() {
                acquire_lock();
                int holdings = balance;
                balance = holdings + amt;
                System.out.println("deposit " + amt + ", balance: " + balance);
                release_lock();
            }
        });
        t1.start();

    }

    public static void acquire_lock() {
        synchronized(lock) {
            while (lock == 1) {
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            lock = 1;
        }
    }

    public static void release_lock() {
        synchronized(lock) {
            lock = 0;
            lock.notifyAll();
        }
    }

    public static void test1() {
        balance = 0;
        deposit(500);
        deposit(500);
    }

    public static void main(String[] args) {
        test1();
    }
}

However, when running the program, I met with an IllegalMonitorStateException. I think I have place the wait() and notifyAll() function in the synchronized block, and I have set the lock as parameter of synchronized. Why do I still have Exception?

Answer 1

Problem is with your release_lock method. You are reassigning lock to 0 before calling lock.notifyAll(). Which means the notifyAll will be called on a new Integer object which is not locked. Change the code to following to fix the issue.

public static void release_lock() {
    synchronized(lock) {            
        lock.notifyAll();
        lock = 0;
    }
}