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

Why wait() Must Reside Within a Synchronized Block

The enforcement of invoking Object.wait() within a synchronized block serves a crucial purpose in Java's concurrency model. Failing to adhere to this restriction leads to an IllegalMonitorStateException, but understanding the rationale behind this constraint is essential.

Reason for Synchronized Wait()

wait() releases the monitor associated with the synchronized object. Explicitly acquiring the monitor before calling wait() ensures that the calling thread exclusively holds the monitor when the wait operation occurs. This ensures that no other thread can modify the shared state while the calling thread is waiting.

Consequences of Wait() Outside a Synchronized Block

If wait() could be invoked outside a synchronized block, it could lead to various issues. Consider the following scenario:

Example: Blocking Queue

Imagine a blocking queue that allows a consumer thread to take elements from a queue, and a producer thread to add elements to the queue. Without synchronization:

class BlockingQueue {
    Queue<String> buffer = new LinkedList<>();

    void take() throws InterruptedException {
        while (buffer.isEmpty()) {
            // Suspend thread without acquiring the monitor
            wait();
        }
    }
}

Potential problems:

• Race Condition: A producer thread could add an element to the buffer, but the consumer thread might miss the notify() call because it had already entered wait().
• Deadlock: The producer thread may be blocked waiting for an empty buffer, while the consumer thread remains suspended due to an unnoticed notify().

Universal Synchronization Requirement

This synchronization issue is not limited to the blocking queue example. Any scenario involving thread communication using wait() and notify() requires synchronization to prevent race conditions and potential deadlocks.

Agreement Between Waiter and Notifier

Synchronized wait() ensures that the waiter thread (consuming) and the notifier thread (producing) agree on the state of the shared resource (predicate). This agreement guarantees that the waiter correctly checks the predicate before waiting and does not miss any notifications during that vulnerable period.

In the example above, the predicate is buffer.isEmpty(). Synchronization ensures that the consumer thread only suspends itself when the buffer is truly empty.

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