In a multithreaded environment, using concurrent collections in the java.util.concurrent package can improve efficiency and security. 1. ConcurrentHashMap is suitable for high-concurrent read and write scenarios, and uses segmented locking or CAS mechanisms to improve performance; 2. CopyOnWriteArrayList is suitable for List operations with more read and less read, such as event listener list; 3. BlockingQueue supports blocking operations and is often used in producer-consumer models; 4. Others such as ConcurrentSkipListMap, LinkedTransferQueue, etc. are also suitable for specific concurrent scenarios. When choosing, trade-offs should be made based on factors such as read and write frequency, consistency requirements, etc.

Java's java.util.concurrent package provides many practical collection classes for concurrent programming. These classes are more efficient and safer than traditional synchronous collections in a multi-threaded environment. If you are writing a multi-threaded program and need multiple threads to operate the collection at the same time, directly using a normal HashMap or ArrayList may cause data inconsistency or performance bottlenecks. At this time, the concurrent collection provided by java.util.concurrent can come in handy.

Here are some of the most commonly used concurrent collection types and scenarios where they apply.

ConcurrentHashMap: Thread-safe and efficient Map

In a multi-threaded environment, if you need a Map that can be read and written concurrently by multiple threads, ConcurrentHashMap is the first choice. It does not lock the entire Map like Collections.synchronizedMap() , but uses segmented locks (JDK 1.7) or CAS synchronized (JDK 1.8) to improve concurrency performance.

Applicable scenarios:

• Multiple threads read frequently and occasionally write
• Cache systems, counters, etc. where shared state is needed

Common methods:

• putIfAbsent(key, value) : Insert only if the key does not exist
• computeIfPresent(key, remappingFunction) : recalculate the value if the key exists
• forEach(BiConsumer) : Concurrently and safely traverse the Map

For example, you want to count the number of visits per user:

 ConcurrentHashMap<String, Integer> counts = new ConcurrentHashMap<>();
counts.computeIfPresent("user1", (k, v) -> v 1);
counts.putIfAbsent("user1", 1);

CopyOnWriteArrayList: Applicable to List with more reads and fewer reads

CopyOnWriteArrayList is a thread-safe List implementation. Its characteristic is that a new array copy is created every time it is modified, so read operations do not require locking at all. This makes it very suitable for scenarios where more reads, less writes.

Applicable scenarios:

• List of listeners (such as event listening)
• Cache list for configuration information
• Data structures that are not frequently updated but read frequently

Note:

• Write operations are expensive, especially when the list is large
• The iterator does not throw ConcurrentModificationException
• Real-time consistency is not guaranteed (read may see old data)

For example, when registering an event listener:

 CopyOnWriteArrayList<EventListener> listeners = new CopyOnWriteArrayList<>();
listeners.add(myListener);

This way, even if multiple threads are adding listeners, it will not cause concurrent exceptions.

BlockingQueue: for producer-consumer model

BlockingQueue is a queue interface that supports blocking operations, which is often used to implement the producer-consumer model. When the queue is full, the producer thread will be blocked; when the queue is empty, the consumer thread will be blocked.

Common implementations are:

• ArrayBlockingQueue : Bounded queue, based on array
• LinkedBlockingQueue : can set capacity, default unbounded
• SynchronousQueue : No elements are stored, producers must wait for the consumer to take them away

Example of usage:

 BlockingQueue<String> queue = new ArrayBlockingQueue<>(10);

// Producer thread new Thread(() -> {
    try {
        queue.put("item");
    } catch (InterruptedException e) {}
}).start();

// Consumer thread new Thread(() -> {
    try {
        String item = queue.take();
    } catch (InterruptedException e) {}
}).start();

This mechanism is very suitable for scenarios such as task scheduling and message queueing.

Other useful concurrent collections

In addition to the several main classes mentioned above, java.util.concurrent also provides some other tool classes to assist concurrent operations:

• ConcurrentSkipListMap / ConcurrentSkipListSet : Ordered concurrent collection, supporting sorting operations under high concurrency
• LinkedTransferQueue : Powerful unbounded queue, supporting "transfer" operation (transfer)
• Concurrent implementation of Deque interface (such as LinkedBlockingDeque) : used for dual-ended queue operations

In general, the concurrent collections provided by Java can already meet the needs of most multithreaded environments. Choosing the right collection type can not only improve performance, but also reduce the occurrence of concurrent errors. When using it, remember to weigh the reading and writing frequency, whether order is required, and whether strong consistency is required according to the specific business scenario.

Basically that's it.

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