Common pitfalls in Java multithreading

Common pitfalls in Java multithreading include a variety of bugs and performance problems that arise due to concurrent execution of code. These pitfalls can lead to inconsistent results, deadlocks, or application crashes if not addressed properly. Here are some of the most important ones:

1. Race Conditions

This occurs when multiple threads access and modify shared data at the same time, leading to unpredictable results. For instance, if two threads simultaneously increment a shared counter without synchronization, the final value may be incorrect. Use synchronization mechanisms or thread-safe classes to avoid this issue.

2. Deadlocks

Deadlocks happen when two or more threads are waiting for each other to release resources, so none of them can proceed. This usually results from locking order problems. To prevent deadlocks, always acquire locks in a consistent order and consider using lock timeouts or try-lock mechanisms.

3. Use of Non-Thread-Safe Objects

Sharing non-thread-safe objects (like ArrayList) across threads without proper synchronization leads to data corruption. Always choose thread-safe alternatives such as CopyOnWriteArrayList, ConcurrentHashMap, or use synchronization wrappers.

4. Memory Consistency Errors

These errors occur when changes made by one thread are not visible to others, because of caching or reordering. Declaring variables as volatile or accessing them inside synchronized blocks ensures proper visibility across threads.

5. Improper Synchronization

• Over-synchronization: Placing unnecessary locks can degrade performance and limit parallelism.
• Under-synchronization: Not synchronizing when needed can produce race conditions and inconsistent results.
• Incorrect use of wait/notify: Calling wait() or notify() without holding the object's lock throws an exception.

6. Thread Lifecycle and Resource Leaks

Creating threads without proper management or not shutting down thread pools (ExecutorService) can lead to resource leaks and prevent the program from terminating.

7. Blocking Operations and Contention

Threads may spend too much time waiting for locks, leading to poor performance. Keep synchronized sections short and avoid nested locks whenever possible.

8. Hard-to-Reproduce Bugs

Multithreaded bugs may not appear consistently, making them difficult to reproduce and debug. Always thoroughly test concurrent code under load.

9. Using Shared Mutable State

Avoid sharing mutable variables between threads whenever possible. Favor immutable objects or thread-local variables to improve safety and simplicity.

10. Ignoring Interrupts and Exceptions

Multithreaded code should handle interrupts and exceptions correctly. Ignoring these signals can cause threads to hang or behave in unexpected ways.

Final Thoughts

Java multithreading can greatly improve performance, but it requires careful design and testing to avoid these common pitfalls.

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