Causes and solutions to ConcurrentModificationException exceptions in Java

In Java, when multiple threads operate a collection object at the same time, a ConcurrentModificationException exception may occur. This exception usually occurs when modifying or deleting elements while traversing the collection, which will lead to inconsistency in the state of the collection. thus throwing an exception. This article will delve into the causes and solutions to this exception.

1. Reasons for exception

Usually, ConcurrentModificationException is caused by modifying or deleting a collection object while traversing it. The reason why this problem can occur is:

1. The collection needs to acquire a lock when traversing, and the modification and deletion operations also need to acquire the same lock. At this time, a deadlock situation occurs, resulting in an exception being thrown. out.
2. When traversing the collection, if other threads modify or delete it at the same time, the original iterator will be invalid, thus throwing a ConcurrentModificationException exception.
3. When using an iterator of a collection to operate, if the collection is modified in the iterator's own remove() method, a ConcurrentModificationException may also be thrown.

2. Solution

ConcurrentModificationException exception is caused by thread safety issues. We need to take corresponding measures to solve this problem. Here are several solutions:

1. Using synchronized blocks and synchronized methods

You can use synchronized blocks or synchronized methods to solve thread safety issues. For example, you can use the synchronized keyword to lock a collection during the iteration process, so that different threads cannot modify the collection at the same time. This method is relatively inefficient because you must wait until one thread completes its operation before proceeding to the next thread's operation.

2. Using CopyOnWrite container

Compared with synchronized blocks or synchronized methods, CopyOnWrite containers are more efficient. It uses a "copy-on-write" mechanism to create a new container to store data when modifying or deleting operations, thereby avoiding the problem of concurrent modifications. The CopyOnWrite container is suitable for scenarios where read operations are more frequent and write operations are less, such as the storage of cached data.

3. Use Iterator's remove method

When using iterators, it is best to use Iterator's remove method to delete elements instead of using the collection itself. method. Using the remove method of Iterator can ensure that ConcurrentModificationException will not occur during iteration, because it can not only delete elements, but also determine the elements to be deleted based on the current position of the iterator.

4. Using concurrent containers

Java's concurrent container provides some thread-safe container classes, such as ConcurrentHashMap, ConcurrentLinkedQueue, etc., which not only ensure thread safety, but also ensure concurrency Performance is also very efficient when reading and writing. Using a concurrent container can not only avoid ConcurrentModificationException exceptions, but also improve program performance.

Summary:

ConcurrentModificationException exception is one of the thread safety issues and can be handled by different solutions, such as using synchronized blocks, synchronized methods, CopyOnWrite containers, Iterator's remove method and Concurrent containers, etc. In practical applications, appropriate solutions must be selected based on specific business needs to ensure the safety and efficiency of the program.

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