Why Must Object.wait() Always Be Called Within a Synchronized Block?

Necessary Synchronization of Wait() to Ensure Predictability

Object.wait() must reside within a synchronized block to guarantee thread safety and prevent IllegalMonitorStateException.

Reason for Synchronization Requirement

Wait releases the monitor associated with the object, allowing other threads to acquire it. If wait() could be invoked outside a synchronized block, unpredictable behavior could arise.

Potential Dangers of Wait() Without Synchronization

Consider a blocking queue implementation without synchronized wait():

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

    public void give(String data) {
        buffer.add(data);
        notify(); // Signal waiting consumer
    }

    public String take() throws InterruptedException {
        while (buffer.isEmpty()) {
            wait(); // Suspend thread without synchronization
        }
        return buffer.remove();
    }
}

In this example:

1. A consumer thread may call take(), check that the buffer is empty, and subsequently call wait() without synchronization.
2. Before the consumer thread actually sleeps, a producer thread may add an element to the buffer and notify the waiting thread.
3. However, the consumer thread may have missed the notification due to the lack of synchronization.
4. The producer thread may never subsequently notify the consumer, resulting in a deadlock.

Universal Synchronization Issue

This synchronization issue applies universally to wait/notify mechanisms, regardless of the specific implementation. Without synchronization, there is always a risk of race conditions and unpredictable thread behavior. Hence, the rule "wait inside synchronized" ensures thread safety and prevents such issues.

The above is the detailed content of Why Must Object.wait() Always Be Called Within a Synchronized Block?. For more information, please follow other related articles on the PHP Chinese website!