Why Must `wait()` Calls Always Be Inside a Synchronized Block in Java?

Why is wait() Call Always Contained in Synchronized Block?

Object.wait() invocation requires placement within a synchronized block to prevent an IllegalMonitorStateException. This is due to the interplay between monitor acquisition and the semantics of wait():

Monitor Acquisition and Release:

In Java, each object has an associated monitor. When a thread enters a synchronized block, it acquires the monitor for that object. This prevents other threads from accessing the object simultaneously.

Wait() Semantics:

Wait() releases the monitor associated with the object it is invoked on. When a thread calls wait(), it releases the monitor and waits until another thread invokes notify() or notifyAll() on the same object, at which point the waiting thread re-acquires the monitor.

Potential Damage Without Synchronization:

If it were possible to invoke wait() outside a synchronized block, it would allow multiple threads to access the object concurrently, potentially causing inconsistencies or deadlocks.

Consider the following scenario:

• Thread A checks if a condition is met and calls wait() outside of a synchronized block.
• Before Thread A releases the monitor, Thread B modifies the object's state, satisfying the condition.
• Thread B then invokes notify() on the object.
• Thread A will not be notified of the change and will remain blocked indefinitely, causing a deadlock.

Synchronization's Role:

Enclosing wait() calls within synchronized blocks guarantees that the monitor is acquired before the condition is checked and wait() is invoked. This ensures that no other thread can modify the object's state or invoke notify() until the waiting thread has released the monitor. This synchronization ensures thread safety and prevents potential inconsistencies or deadlocks.

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