IllegalMonitorStateException

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Explanation

The IllegalMonitorStateException is a common programming error that can show up in concurrent programs. Depending on the structure of the program, the exception may occur consistently or only occasionally. The IllegalMonitorStateException exception class extends the RuntimeException class and according to the official documentation is thrown to indicate that a thread has attempted to wait on an object’s monitor or to notify other threads waiting on an object’s monitor without owning the specified monitor. In other words if you invoke wait() or notify()/notifyAll() without synchronizing on the object i.e. outside of a synchronized method or block (with the object as the synchronization target) then IllegalMonitorStateException will be thrown. Similarly, the exception is thrown when you invoke these methods on an instance of Condition class without acquiring the associated lock with the condition. The class is part of the java.lang package and not java.util.concurrent.*.

Repro using Lock

The program below demonstrates generating IllegalMonitorStateException using a Condition object that was instantiated from a Lock object.