Ensuring Strict Thread Execution Order in Java Using Semaphores

Описание: Learn how to enforce strict ordering of thread execution in Java with semaphores, avoiding the use of `thread.join()`.
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Ensuring Strict Thread Execution Order in Java Using Semaphores

Java's multithreading capabilities can be incredibly powerful, allowing multiple tasks to run concurrently. However, when it comes to controlling thread execution order, things can get a bit tricky. In this guide, we will address the problem of enforcing strict execution order among multiple threads using semaphores, without resorting to the thread.join() method.

The Problem

Imagine you have 10 threads that need to execute strictly in order: thread-0 first, then thread-1, followed by thread-2, and so on. The goal is to ensure that each thread waits its turn no matter the order in which they arrive at the execution point. If you try to implement this with a simple semaphore, you might find that the threads are being executed out of order.

Here's a quick look at the initial implementation that creates the problem:

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As you can see in the output, the execution order can vary:

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The challenge now is to ensure that these threads execute in an orderly manner.

The Solution

To solve this problem, we can create a custom synchronization object that maintains control over the execution order. This is similar to a "take a number" system found at a deli counter, where each customer waits for their turn based on the order they arrived.

Step 1: Create a Synchronization Object

Define a class that will handle the turn management:

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Step 2: Instantiate the Synchronization Object

In your main class, you will need to declare an instance of this synchronization object:

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Step 3: Modify the Thread Execution Logic

When running each thread, you'll modify the runnable code to use the SyncThing methods for waiting and notifying:

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Complete Implementation Example

Combine these snippets to create a fully functional example that enforces the thread order:

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Conclusion

By utilizing a custom synchronization object, you can effectively manage the execution order of threads without using thread.join(). This approach leverages simple waiting and notification mechanisms to ensure each thread waits for its turn. Now, your threads can execute in the order you designed, making your Java applications more predictable and easier to manage under concurrent scenarios.

With this insight, you're equipped to tackle similar multithreading challenges that come your way!