Скачать или смотреть Understanding Why Only One of Your Five Threads Pops Elements from the Stack in Java Thread-Safety

Explore common multithreading pitfalls in Java and discover why only one thread can pop from your stack in this in-depth guide to synchronization and thread-safety.
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Understanding Why Only One of Your Five Threads Pops Elements from the Stack in Java Thread-Safety

Multithreading in Java can be a complex subject, especially when it comes to thread-safety and synchronization. If you have ever wondered why only one of your five threads is popping elements from a stack, you are not alone. In this post, we’ll break down the issue and provide a clear solution, making your multithreading experience smoother.

The Problem

When you create multiple threads in Java to handle a task concurrently, it is expected that each thread can perform its own operation independently. However, in the provided code snippet, when you start five threads, only the first one successfully pops all elements from the stack, leaving the others with nothing to process. This can confound developers trying to implement multithreading effectively.

Key Points of Confusion

Why does one thread dominate?

What role does the synchronized block play?

How can we leverage other data structures for better results?

The Cause

The root of the problem lies in the use of the synchronized block in conjunction with the volatile behavior of threads. Let's examine why this is a concern:

The Impact of synchronized

When a thread enters a synchronized block, it acquires a lock.

Other threads attempting to access that block have to wait until the lock is released.

In your case:

The first thread enters the lock, reads from the stack, and continues until it is empty.

The subsequent threads are stuck waiting for the first one to release the lock, which only happens after the stack is empty.

This is why only one thread is able to pop elements from the stack—the lock prevents any other thread from accessing the code meant to manipulate the stack.

The Solution

To address the issue, you need to reconsider the use of synchronization and the data structure employed for handling the stack. Instead of a standard stack, we can use Java's LinkedBlockingDeque, which is thread-safe and allows multiple threads to work with it concurrently.

Implementation with LinkedBlockingDeque

Here’s how you can modify your code. This implementation lets each thread independently access the LinkedBlockingDeque, allowing for expected concurrent behavior:

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

Explanation of the Code

LinkedBlockingDeque: This data structure is designed to handle multiple concurrent threads. It allows each thread to safely pop elements without waiting for a lock.

Poller Class: Each instance of this runnable will independently poll from the deque without interference from other threads.

Main Method: Initializes the stack with integers and starts multiple threads using the Poller.

Benefits of this Approach

Increased Concurrency: Multiple threads can access the shared stack without being blocked.

Thread-Safety: Using LinkedBlockingDeque, you can benefit from built-in thread-safety mechanisms without manually implementing your lock-handling logic.

Conclusion

Understanding how synchronized blocks work and how they impact threading behavior is crucial in Java. By transitioning to a thread-safe data structure like LinkedBlockingDeque, you enable multiple threads to collaborate effectively, resolving the problem of exclusive access by a single thread. Hopefully, this breakdown has demystified your original issue and has equipped you with the knowledge to manage threads more effectively in your applications.

Happy coding, and may your threads run smoothly!