`wait`/`notify` called without synchronization on an objectJAVA-E0288

This method calls one of Object.wait, notify or notifyAll on a value that does not appear to be synchronized on in the current thread/method. Calling these methods on an object without a lock held on it will result in an IllegalMonitorStateException (or IMSE for short) being thrown.

This issue will be raised if a method contains a call to Object.wait(), Object.notify() or Object.notifyAll() without a corresponding synchronized block surrounding it. While it may be that the synchronized block is present somewhere higher in the call stack, the method implementation should never rely on the implied presence of a synchronized block in calling code, whether it is in private or (especially) in public API code.

Such design patterns can easily result in elusive bugs that occur in only very specific situations.

Bad Practice

Here, there is no synchronization implemented on this method at all, and if it is called outside a synchronized block specifically synchronizing on waitObj, it will throw an IMSE.

public void method(Object waitObj) {
    waitObj.wait(); // This will throw an IllegalMonitorStateException unless there is a synchronized block in calling code.
}

In this case, we are attempting to synchronize on this through a synchronized method. Within the method, we call wait on waitObj, which may point to this (that would make the call safe), but is not guaranteed to do so at all.

public synchronized void method(Object waitObj) {
    waitObj.wait(); // This will throw an IMSE because we are synchronizing on this instead of on waitObj.
}

Recommended

public void method(Object waitObj) {
    synchronized(waitObj) {
        waitObj.wait(); // Since we have ensured that we have exclusive access to waitObj, an IMSE will not be thrown.
    }
}

References

• CWE-832 - Unlock of a Resource that is not Locked
• Oracle Java Documentation - Object.wait() and Object.notify()
• SpotBugs - MWN_MISMATCHED_WAIT