Concurrent containers in Java

With the popularity of multi-core processors, programmers have begun to pay attention to the problem of concurrent access to data. In order to solve the problem of thread safety, Java provides a variety of concurrent containers. This article will introduce several common Java concurrent containers.

1. ConcurrentHashMap

ConcurrentHashMap is a thread-safe hash table. Its implementation is basically the same as HashMap, but ConcurrentHashMap supports high-concurrency modification operations, so it is more suitable for multi-threaded scenarios than HashMap.

ConcurrentHashMap has multiple segment locks inside, each lock protects a hash bucket, so that multiple threads can modify different buckets concurrently. This design enables ConcurrentHashMap to achieve efficient read and write separation.

The steps to use ConcurrentHashMap are as follows:

1. Create a ConcurrentHashMap instance:

ConcurrentMap<Integer, String> map = new ConcurrentHashMap<>();

2. Add elements:

map.put(1, "one");

3. Get elements:

String value = map.get(1);

2. CopyOnWriteArrayList

CopyOnWriteArrayList is a thread-safe dynamic array. Its characteristic is that the write operation does not directly modify the original array, but creates a new array for modification, and then replaces the original array with the new array. Since modification operations and read operations do not conflict, CopyOnWriteArrayList supports high concurrent read operations.

The steps to use CopyOnWriteArrayList are as follows:

1. Create a CopyOnWriteArrayList instance:

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

2. Add elements:

list.add("one");

3. Get elements:

String value = list.get(0);

It should be noted that since each modification requires the creation of a new array, the modification operation of CopyOnWriteArrayList is relatively slow and is not suitable for high-frequency writing operations.

3. ConcurrentLinkedQueue

ConcurrentLinkedQueue is a thread-safe queue. Its implementation is based on linked lists and supports high-concurrency enqueue and dequeue operations.

ConcurrentLinkedQueue internally uses CAS operations to implement concurrent modifications to the linked list, thus avoiding performance problems caused by the use of locks.

The steps to use ConcurrentLinkedQueue are as follows:

1. Create a ConcurrentLinkedQueue instance:

Queue<String> queue = new ConcurrentLinkedQueue<>();

2. Enqueue operation:

queue.offer("one");

3. Dequeue operation:

String value = queue.poll();

It should be noted that ConcurrentLinkedQueue does not support random access, so it can only be traversed from the head of the queue.

4. ConcurrentSkipListMap

ConcurrentSkipListMap is a thread-safe ordered mapping table. Its implementation is based on skip tables and can quickly support insertion, deletion and search operations.

Similar to ConcurrentHashMap, ConcurrentSkipListMap is also divided into multiple levels. Each level has its own set of linked lists, which can improve the efficiency of concurrent access.

The steps to use ConcurrentSkipListMap are as follows:

1. Create a ConcurrentSkipListMap instance:

ConcurrentNavigableMap<Integer, String> map = new ConcurrentSkipListMap<>();

2. Add elements:

map.put(1, "one");

3. Get elements:

String value = map.get(1);

It should be noted that the implementation of ConcurrentSkipListMap is relatively complex, so in the case of small data volume, the performance may be worse than TreeMap.

Summary

Java provides a variety of concurrent containers, and programmers can choose the appropriate container according to their needs. It should be noted that different containers have different applicable scenarios, and improper use may cause performance problems. Therefore, it is recommended to choose the appropriate container according to the scenario.

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