Скачать или смотреть How to Fix Synchronized ArrayList Issues in Java Multithreading

Learn how to resolve issues with empty lists returned from a synchronized ArrayList in a Java multithreading context. Explore synchronized methods for effective thread management.
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This video is based on the question https://stackoverflow.com/q/70663244/ asked by the user 'Ram' ( https://stackoverflow.com/u/14208877/ ) and on the answer https://stackoverflow.com/a/70663629/ provided by the user 'Vikas Verma' ( https://stackoverflow.com/u/6621503/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem with Synchronized ArrayLists in Java

In the world of multithreading in Java, managing shared resources can be tricky. One common issue that developers face is when a Synchronized ArrayList returns an empty list during concurrent access by multiple threads. This can lead to inconsistent and unexpected behaviors in applications. Let’s dive deeper into understanding how this happens, using an example, and explore a solution to ensure thread safety.

The Example

Consider the following Java class implementing the Singleton pattern with a Synchronized ArrayList:

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

In the App class, two threads are created to test the behavior of our Singleton:

One thread (t1) adds elements to the list.

The second thread (t2) retrieves and prints the list elements.

The issue arises when the output sometimes unexpectedly displays empty lists. The key part is:

Thread t1 clears the list and then adds elements to it.

Thread t2, in the meantime, is trying to fetch the list which could potentially still be executing the clear operation of Thread t1. Hence, it might print an empty list.

The Root Cause

The methods addElement and getStrList are not synchronized, leading to a race condition between the two threads. The sequence of operations is crucial here; if t2 attempts to read the list after t1 has cleared it but before it has added anything back, it will face empty results.

Why is This Happening?

addElement consists of two operations:

strList.clear(); (clears the current list)

strList.addAll(list); (adds new elements)

getStrList() simply returns a copy of the current list.

Without synchronization, these operations can interleave between the two threads, causing t2 to access an empty list.

The Solution

To fix this issue, we need to ensure that both methods that modify or read from the shared strList are synchronized. This will ensure that when one thread is executing addElement, the other thread cannot run getStrList, thus protecting our shared resource from concurrent modifications.

Implementing the Fix

Here’s how you can synchronize the methods:

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

Conclusion

In conclusion, managing multithreading requires careful strategies to synchronize access to shared resources. By modifying the Singleton class to synchronize the crucial methods, we can prevent the issue of empty lists being returned during concurrent operations. This not only enhances the stability of your application but also provides a solid foundation for building thread-safe components in Java.

Remember, synchronization is key when working with shared data in a multithreading environment!