Mastering ConcurrentHashMap in Java: A Comprehensive Guide

Introduction:

In the world of multithreaded programming, thread-safety is a critical concern. Java’s built-in collections, such as HashMap, are not thread-safe, which means that concurrent access from multiple threads can lead to data corruption or unexpected behavior. This is where ConcurrentHashMap comes into play, providing a thread-safe implementation of the Map interface that is designed to handle concurrent access efficiently.

What is ConcurrentHashMap?

ConcurrentHashMap is a part of the java.util.concurrent package, introduced in Java 5. It is a highly efficient, thread-safe implementation of the Map interface that allows multiple threads to perform read and write operations concurrently without the need for external synchronization mechanisms like locks.

Characteristics of Java’s ConcurrentHashMap (CHM) : Check following point how CHM allows concurrent read and write operations, its locking mechanism, concurrency level, and other characteristics that make it suitable for multi-threaded applications^[1].

1. underlying DS is Hashtable
2. CHM allows concurrent read and thread-safe operations
3. To perform read operation thread won’t require any lock. But to perform update operation thread requires lock. but it is lock of only a particular part of map (segment lock) instead of total map (Bucket level lock)
4. Concurrent update achieved by internally dividing map into smaller portions, which is defined by Concurrency level
5. The default concurrency level is 16
6. CHM allows any no. of read operations but 16 update operations at a time by default
7. null is not allowed for both keys and values
8. While one thread iterating, the other thread can perform update operation and CHM never throw ConcurrentModificationException

Internal Structure:

ConcurrentHashMap achieves thread-safety by partitioning the map into multiple segments, each with its own lock. This partitioning is controlled by the concurrencyLevel variable, which determines the number of segments (and, consequently, the number of locks) to use.

Each segment is essentially a separate HashMap instance, and threads can operate on different segments concurrently without interfering with each other. This approach reduces contention and improves performance in multithreaded environments.

The concurrencyLevel should be chosen based on the expected number of concurrent threads and the level of contention in the application. A higher concurrencyLevel value generally improves concurrency but also increases memory overhead.

https://itsromiljain.medium.com/curious-case-of-concurrenthashmap-90249632d335

https://itsromiljain.medium.com/curious-case-of-concurrenthashmap-90249632d335

Concurrent Modification Detection:

ConcurrentHashMap provides a thread-safe iterator that supports concurrent modification, allowing other threads to modify the map while iterating over it. This is achieved through the use of the baseCount variable.

The baseCount variable stores the number of mappings in the map at the time the iterator was created. During iteration, if the baseCount value changes (due to concurrent modifications), the iterator can detect this change and throw a ConcurrentModificationException.

However, if the modifications are made in a thread-safe manner (e.g., using the replace, put, or remove methods of ConcurrentHashMap), the baseCount is updated accordingly, allowing the iteration to continue without throwing an exception.

Key Features and Methods:

ConcurrentHashMap provides several useful methods for efficient concurrent access, including:

1. putIfAbsent(K key, V value): Associates the specified value with the specified key if the key is not already associated with a value.
2. remove(Object key, Object value): Removes the entry for the specified key only if it is currently mapped to the specified value.
3. replace(K key, V oldValue, V newValue): Replaces the entry for the specified key only if it is currently mapped to the specified value.
4. computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction): Computes a mapping for the specified key if the key is not already associated with a value, using the specified mapping function.
5. forEach(BiConsumer<? super K, ? super V> action): Performs the given action for each entry in the map until all entries have been processed or the action throws an exception.

Performance Considerations:

In single-threaded scenarios, HashMap generally performs better than ConcurrentHashMap because it doesn’t have the overhead of managing thread safety. However, in multithreaded environments, ConcurrentHashMap can outperform HashMap due to its ability to handle concurrent access without the need for external synchronization mechanisms.

It’s important to note that ConcurrentHashMap generally has a higher memory footprint than HashMap due to the additional overhead required for managing thread safety and concurrency.

Choosing Between HashMap and ConcurrentHashMap:

When deciding between using HashMap or ConcurrentHashMap, consider the following factors:

• If your application is single-threaded or if you can ensure that the map will only be accessed by a single thread, use HashMap for better performance and simpler code.
• If your application is multithreaded and multiple threads need to access and modify the map concurrently, use ConcurrentHashMap to ensure thread safety and avoid potential data corruption or race conditions.
• If you need to iterate over the map while allowing concurrent modifications, ConcurrentHashMap is the preferred choice.

Conclusion:

ConcurrentHashMap is a powerful and efficient data structure in Java, designed to handle concurrent access from multiple threads while ensuring thread safety. By understanding its internal structure, features, and performance characteristics, developers can make informed decisions and leverage the capabilities of ConcurrentHashMap to build robust and scalable multithreaded applications.

Ref: 1. https://www.youtube.com/watch?v=IUo1Ym-Uj_I