How to Synchronize Threads in Java: How Can an Application Logic Thread Wait for a Database Thread to Finish Initialization?

Waiting for Thread Output in Java

Developing Java applications often involves multiple threads running concurrently, necessitating coordination mechanisms to ensure proper flow and data synchronization. This article addresses a specific scenario where one thread must wait for the output of another thread.

In this case, the application employs two threads: an application logic thread and a database access thread. While both threads must persist throughout the application's lifetime and operate simultaneously, the application logic thread must initially wait until the database access thread is ready.

Potential Solutions

The question raises challenges in finding a suitable solution. Using Thread.join() is not an option as the database thread only exits during application shutdown. Additionally, creating an empty loop to poll for database readiness leads to unnecessary CPU consumption.

CountDownLatch Approach

A recommended solution leverages the CountDownLatch class, which uses a counter mechanism. The counter is initialized to 1, representing one operation required to complete.

App Thread Execution

In the application logic thread, the code includes:

CountDownLatch latch = new CountDownLatch(1);
latch.await();

The latch.await() method blocks further execution until the latch counter reaches 0, signaling completion of the database initialization.

Database Thread Execution

Once the database thread completes initialization, it executes:

latch.countDown();

Decrementing the latch counter releases the app thread from waiting.

This approach effectively synchronizes the two threads, ensuring that the application logic thread proceeds only after the database thread is ready. It offers an efficient and non-blocking mechanism for thread coordination, avoiding the drawbacks of blocking loops or interrupting the database thread.

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