Java's concurrency exception - what to do with java.util.ConcurrentModificationException?

As a high-level language, Java is widely used in programming languages. In the development of Java applications and frameworks, we often encounter concurrency problems. Concurrency problems mean that when multiple threads operate on the same object at the same time, some unexpected results will occur. These problems are called concurrency problems. One of the common exceptions is the java.util.ConcurrentModificationException exception. So how do we effectively solve this exception during the development process?

Java.util.ConcurrentModificationException is a runtime exception that will be thrown when traversing a collection if other threads change the structure of the collection during the traversal process. To put it simply, when one thread is using the iterator of the Collection class to traverse the collection, another thread modifies the collection, causing the iterator to work abnormally and eventually throwing an exception. Since this exception will cause the program to run abnormally or even crash, solving this exception is an urgent problem that we need to solve during development.

Then we can take the following methods to solve the java.util.ConcurrentModificationException exception.

1. Use Iterator to traverse collections

In Java, collections (List, Set, Map) all implement the Iterable interface and can be traversed using Iterator instead. This exception will occur. Because when using an Iterator to traverse a collection, there is a variable modCount in the iterator that records changes in the collection structure. Once it is found that the modCount value of the iterator is inconsistent with the modCount value of the collection, a ConcurrentModificationException exception will be thrown, which can effectively avoid Add, delete, and modify the collection during collection.

Sample code:

List<String> list = new ArrayList<>();
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    String str = iterator.next();
    //...
}

2. Use the Collections.synchronizedList() method for synchronization

The Collections.synchronizedList() method can turn the List collection into a thread-safe collection. When the List collection is modified concurrently, the List will be locked to avoid ConcurrentModificationException. However, because this method uses object-level locks, it may affect performance.

Sample code:

List<String> list = new ArrayList<>();
List<String> synchronizedList = Collections.synchronizedList(list);

3. Use CopyOnWriteArrayList

CopyOnWriteArrayList is a new thread-safe class in Java 5. Its design idea is to use CopyOnWriteArrayList when there are many read operations and write operations. When there are few operations, the copy mechanism is used. When there is a write operation, a new array is copied for modification, and the read operation is performed in the old array, which avoids the problem of data inconsistency during concurrent access by multiple threads.

Sample code:

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();

Summary:

In Java development, we need to fully understand the concept of concurrency and related knowledge to avoid concurrency problems caused by data competition. The above are several methods to solve the java.util.ConcurrentModificationException exception. Different methods have different applicable scenarios and need to be selected according to the specific situation. At the same time, during the concurrent development process, you also need to pay attention to issues such as thread safety, lock usage, and deadlocks to ensure the correctness and efficiency of the program.

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