Скачать или смотреть Understanding ConcurrentModificationException in Java with CompletableFutures

Discover the nuances of `ConcurrentModificationException` in Java, especially in the context of using `CompletableFutures`. Learn strategies to avoid this exception when handling lists concurrently.
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Understanding ConcurrentModificationException in Java with CompletableFutures

Java's concurrency model is powerful, but it comes with its own set of challenges. One common problem developers encounter is the ConcurrentModificationException. In this guide, we will explore two scenarios involving this exception, particularly when working with lists and CompletableFutures. By the end, you'll have a clear understanding of how to avoid pitfalls related to concurrent modifications in your Java applications.

The Problem

Scenario 1: Parallel Calls without Modification

Imagine you have a list of params which is passed to two methods. These methods perform web service calls using data filtered from the params list in parallel using CompletableFutures. You might wonder: Can this setup throw a ConcurrentModificationException?

Scenario 2: List Modification During Parallel Execution

In a more complex scenario, one of the methods modifies the params list by adding objects while the second method processes it. In this case, it's clear that you will encounter a ConcurrentModificationException. The real question is: How can you safely manage this situation?

The Solution

Scenario 1: The Likely Outcome

In this scenario, you are less likely to encounter a ConcurrentModificationException. This is because if the list of params remains unchanged while it is being accessed by parallel threads, the operations on that list should be safe. However, if the list gets modified while it's being filtered, you can expect to see exceptions.

Scenario 2: Modifications Lead to Exceptions

When you modify a list concurrently, you are almost guaranteed to face a ConcurrentModificationException. This happens because the list is being changed while another thread is accessing it. Here's how you can handle it:

Options for Managing List Modifications

Use CopyOnWriteArrayList:

This concurrent collection allows safe iteration even while modifying the list. However, be aware that it may not be suitable for high-performance requirements due to the overhead of copying the array on modifications.

Example:

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Create a New List with a Deep Copy:

By creating a new instance of the list and populating it with the contents of the original list, you can ensure that the original list remains unchanged during processing. This can be done using constructor methods or copy methods.

Example:

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Understanding ConcurrentModificationException

The ConcurrentModificationException occurs when:

A list is changed (through operations like add, remove, etc.) while an iteration is ongoing.

The underlying structure is invalidated for the iterators that were created before the modification.

Here's a trivial example that demonstrates the exception:

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Instead, to remove items safely:

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Key Takeaways

Always Use Iterators for Safe Removal: Utilize the remove method of an iterator to avoid exceptions.

Avoid Concurrent Modifications: When using CompletableFutures, be cautious with shared lists. Prefer concurrent collections for thread-safe operations.

Understand Your Data: Knowing how your data structures behave under concurrent accesses is crucial to prevent runtime exceptions.

In summary, while ConcurrentModificationException can be a simple yet frustrating issue in Java, understanding and applying the right strategies can help you write robust concurrent applications.