java.lang.IllegalMonitorStateException: (m=null) Failed to get monitor for

Question

Why may this happen? The thing is that monitor object is not null for sure, but still we get this exception quite often:

java.lang.IllegalMonitorStateException: (m=null) Failed to get monitor for (tIdx=60)
        at java.lang.Object.wait(Object.java:474)
        at ...

The code that provokes this is a simple pool solution:

    public Object takeObject() {
        Object obj = internalTakeObject();
        while (obj == null) {
            try {
                available.wait();
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            obj = internalTakeObject();
        }
        return obj;
    }

    private Object internalTakeObject() {
        Object obj = null;
        synchronized (available) {
            if (available.size() > 0) {
                obj = available.keySet().iterator().next();
                available.remove(obj);
                synchronized (taken) {
                    taken.put(obj, Boolean.valueOf(true));
                }
            }
        }
        return obj;
    }

    public void returnObject(Object obj) {
        synchronized (taken) {
            taken.remove(obj);
        }
        synchronized (available) {
            if (available.size() < size) {
                available.put(obj, Boolean.valueOf(true));
                available.notify();
            }
        }
    }

Am I missing something?

EDIT: The exception happens in available.wait(); line.

Answer 1

available.wait(); must be in a synchronized(available) section

Answer 2

See the javadoc for Object.wait.

in particular "The current thread must own this object's monitor." and "[throws] IllegalMonitorStateException - if the current thread is not the owner of the object's monitor." That is, you need to synchronize on the object you are going to call wait on.

so your code should be:

synchronized (available) {
    available.wait();
}