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Core Concepts

Concurrent Collections

Concurrent collections are a set of classes in Java that enable safe and efficient operations on data when accessed by multiple threads. Unlike regular collections (like ArrayList, HashMap, etc.), which are not thread-safe by default, concurrent collections provide mechanisms to handle synchronization internally, so we don’t have to manage it. These collections are specifically optimized for scenarios where multiple threads might try to read from and write to a collection simultaneously. They are a part of the java.util.concurrent package, and they include:

ConcurrentList
ConcurrentMap
BlockingQueue
CopyOnWriteArrayList
CopyOnWriteArraySet
ConcurrentSkipListMap
ConcurrentSkipListSet

Pros of using Concurrent Collections

Managing thread safety manually with synchronization can become complicated and error-prone. With concurrent collections, you gain several advantages:

Thread-Safety: These collections are designed to handle multiple threads accessing and modifying the data without causing data corruption or inconsistencies.
Performance Optimizations: Some concurrent collections, like CopyOnWriteArrayList, are designed to offer better performance in certain use cases where reads are more frequent than writes.
Reduced Complexity: They manage synchronization internally, reducing the need to use explicit synchronized blocks or locks in your code.
Blocking and Non-Blocking Operations: Certain concurrent collections, such as BlockingQueue, provide thread blocking capabilities, making it easier to implement producer-consumer scenarios.

Types of Concurrent Collections

CopyOnWriteArrayList

CopyOnWriteArrayList is a thread-safe variant of the ArrayList. It works by creating a new copy of the underlying array whenever a write operation occurs (such as an add or remove). This makes it an ideal choice when you have a collection with a high read-to-write ratio.

Best used when the collection is read frequently and modified infrequently (e.g., caching, event listeners).

CopyOnWriteArraySet

CopyOnWriteArraySet is similar to CopyOnWriteArrayList, but it implements the Set interface. It is thread-safe and ensures that the underlying array is copied during write operations.

Useful when you need thread-safe sets with minimal synchronization and the set will not undergo frequent changes.

ConcurrentHashMap

ConcurrentHashMap is a thread-safe version of HashMap. Unlike a regular HashMap, it allows concurrent access and modifications by multiple threads. It uses a technique called bucketization to divide the map into segments, enabling multiple threads to update different segments simultaneously without blocking each other.

Ideal for situations where you need to store key-value pairs and handle concurrent read and write operations efficiently (e.g., caching, session storage).

ConcurrentSkipListMap

ConcurrentSkipListMap is a thread-safe version of TreeMap that supports concurrent reads and writes. It maintains the keys in sorted order using a skip list algorithm, which provides logarithmic time complexity for most operations.

Useful when you need a sorted map with fast concurrent reads and writes.

ConcurrentSkipListSet

ConcurrentSkipListSet is the thread-safe version of TreeSet. It also uses the skip list algorithm to maintain elements in sorted order.

Ideal for thread-safe, sorted sets.

BlockingQueue

BlockingQueue is a special kind of queue that supports thread-safe operations for adding and removing elements. The key feature of a BlockingQueue is that it can block a thread when trying to take an element from an empty queue or add an element to a full queue.

Often used in producer-consumer scenarios where one or more threads are producing data and other threads are consuming it. Examples include ArrayBlockingQueue, LinkedBlockingQueue, and PriorityBlockingQueue.

How to Use Concurrent Collections

Let’s take a look at a simple example using a ConcurrentHashMap and BlockingQueue.

import java.util.concurrent.*;

public class ConcurrentExample {

public static void main(String[] args) throws InterruptedException {

// Using ConcurrentHashMap

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

map.put("a", 1);

map.put("b", 2);

map.put("c", 3);

System.out.println("ConcurrentHashMap: " + map);

// Using BlockingQueue

BlockingQueue<Integer> queue = new LinkedBlockingQueue<>();

// Producer thread

Thread producer = new Thread(() -> {

try {

for (int i = 1; i <= 5; i++) {

queue.put(i);

System.out.println("Produced: " + i);

}

} catch (InterruptedException e) {

Thread.currentThread().interrupt();

}

});

// Consumer thread

Thread consumer = new Thread(() -> {

try {

for (int i = 1; i <= 5; i++) {

Integer item = queue.take();

System.out.println("Consumed: " + item);

}

} catch (InterruptedException e) {

Thread.currentThread().interrupt();

}

});

producer.start();

consumer.start();

producer.join();

consumer.join();

}

In this example:

We use a ConcurrentHashMap to store and retrieve key-value pairs safely across multiple threads.
We also use a BlockingQueue to implement a simple producer-consumer pattern, where the producer adds elements to the queue and the consumer removes them.

Best Practices for Using Concurrent Collections

Use the Right Collection for the Job: Select the most appropriate concurrent collection based on our requirements. For example, use ConcurrentHashMap for key-value storage or BlockingQueue for producer-consumer scenarios.
Avoid Synchronized Wrappers: Don’t wrap non-concurrent collections in Collections.synchronizedMap() or Collections.synchronizedList(). Instead, use the concurrent alternatives like ConcurrentHashMap or CopyOnWriteArrayList.
Minimize Contention: Even with thread-safe collections, excessive contention (when multiple threads try to access the same resource) can lead to performance degradation. Use collections that minimize locking, such as ConcurrentSkipListMap, when appropriate.