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Discover the inner workings of Java multithreading and what happens to the `main thread` when a `callable` is executed. Understand blocking, waiting states, and how they impact thread performance.
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Understanding Callable and Future in Java

In the world of Java multithreading, managing multiple tasks at once can significantly enhance application performance. One common method to achieve this is by using Callable and Future. But what really happens to the main thread when you execute a Callable? In this guide, we will explore this concept in detail and clarify the roles of the main thread and callable threads in a multithreaded Java application.

The Main Issue: What Happens to the Main Thread?

When a Callable is executed in a separate thread, the main thread may block while it waits for a result. This raises several questions:

Is the main thread sleeping?

What happens to its status during this time?

How does the Java thread scheduler manage these states?

Let’s delve deeper into these aspects to get a clearer picture.

Using Callable and Future

To understand the behavior of the main thread, let’s consider a simple example:

[[See Video to Reveal this Text or Code Snippet]]

Execution Flow

Creation of Executor Service: The program creates an instance of ExecutorService and submits a Callable task.

Blocking the Main Thread: When the main method calls result.get(), it blocks and waits for the callable to finish executing.

Thread State: During this time, the main thread is not active in the run queue; instead, it is moved to a wait state.

What Happens in Wait State?

Removal from Run Queue: The main thread is temporarily removed from the run queue of the scheduler.

Wait Queue: It is then placed in a wait queue, which holds it until the Callable task is completed.

Resumed Execution: Once the callable finishes its computation, the main thread is reintegrated into the run queue. Due to the completely fair scheduler, it is likely to execute soon after being reinserted.

Important Points to Note:

Blocking: The main thread does not "sleep" in the traditional sense; instead, it waits actively in the wait queue.

Performance Impact: The blocking of the main thread can impact the overall performance if not managed correctly, especially in more complex applications with multiple threads.

Observing Thread Status with Tools

Many developers wonder if they can visualize the state of threads executing their Java programs. Tools like VisualVM allow you to see the status of threads (e.g., running, sleeping, waiting) while your application is running. This can be particularly useful to monitor performance metrics and identify bottlenecks.

Summary

In summary, when using Callable and Future in Java, the main thread enters a wait state while the Callable executes, blocking any further operations until the result is available. This behavior is crucial to understand as it impacts application responsiveness and performance.

By leveraging Java’s built-in concurrency utilities, you can effectively manage multithreaded operations to optimize the responsiveness of your applications while being mindful of thread states.

Understanding these mechanisms not only aids in better application design but also prepares you to diagnose performance issues when they arise.

Feel free to share your thoughts and experiences with Java multithreading in the comments below!