How to Make a Java Thread Wait for Output from Another Thread?

Waiting for Thread Output in Java: A Synchronization Dilemma

In a multithreaded Java application, coordinating the execution of multiple threads is crucial to ensure data integrity and application responsiveness. When one thread relies on the output of another, synchronization mechanisms come into play. One such scenario is described in the following question:

Question:

How can a Java thread (hereinafter referred to as the "app thread") be made to wait until another thread (the "db thread") has completed an operation and is ready to provide output? The db thread must remain active throughout the application's lifetime, and both threads must execute concurrently. Blocking the app thread until the db thread is ready is an acceptable solution.

Answer:

The key to solving this synchronization problem lies in using a CountDownLatch. CountDownLatch is a synchronization construct that allows multiple threads to wait until a specific count has been reached before proceeding. In this case:

// Create a CountDownLatch with a counter of 1
CountDownLatch latch = new CountDownLatch(1);

In the app thread where waiting is required:

// The app thread waits for the db thread to complete
latch.await();

In the db thread, once the operation is complete and the output is ready:

// The db thread signals the app thread to proceed
latch.countDown();

This mechanism ensures that the app thread will not proceed until the db thread has finished its operation. The app thread will wait until the count in the CountDownLatch reaches zero, allowing both threads to execute concurrently once the db thread is ready. This approach provides a clean and efficient solution to thread synchronization, preserving both thread execution and data integrity.

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