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Introduction to ConcurrentHashMap in Java

Joydip Kanjilal
Published: July 24, 2022

Java Developer Tutorials

ConcurrentHashMap is a subclass of HashMap and is designed to be used in multi-threaded environments. It maintains a configurable concurrency level (default value of 16), which can be specified while creating the map. The higher the concurrency level, the more threads can access its buckets at once, but this can result in more contention if all getEnumerators() are not synchronized appropriately. This Java programming tutorial discusses ConcurrentHashMap in Java with code examples wherever appropriate.

To learn more about hashing, check out our tutorial: Introduction to Hashing in Java.

What Is ConcurrentHashMap in Java?

In Java, a ConcurrentHashMap is a data structure used for storing and retrieving data in a concurrent manner. This data structure is designed to be used in multi-threaded applications where multiple threads are reading and writing to the hash map concurrently.

ConcurrentHashMap is a Java class that enables multiple threads to access the map at the same point of time. It is a part of the Java Collections Framework, and it provides a way for applications to efficiently handle concurrent updates to the map.

The ConcurrentHashMap class provides several methods for doing basic operations on the map, such as putting values into the map and getting values out of the map. Additionally, the class provides several methods for doing more advanced operations, such as iterating over the key-value pairs in the map or retrieving a set of keys that are associated with a given value.

Why Should Developers Use ConcurrentHashMap?

The ConcurrentHashMap is a powerful data structure that enables Java developers to work with concurrent data in a safe and efficient manner. The ConcurrentHashMap provides a number of features that makes it well suited for working with concurrent data, including:

  • Atomic operations: All operations on the ConcurrentHashMap are atomic, which means you can safely perform concurrent data access.
  • Lock-free: The ConcurrentHashMap is designed to be lock-free, which means that there is no need to acquire a lock in order to read or write data. This makes the ConcurrentHashMap very efficient when used in a multi-threaded environment.
  • Scalable: The ConcurrentHashMap is designed to be scalable, which means it can efficiently handle large amounts of data.

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How Does ConcurrentHashMap Work in Java?

ConcurrentHashMap is a powerful data structure that enables fast, thread-safe operations on data. The map works by allowing each thread to safely access and modify the data without having to worry about other threads interfering. This is accomplished by using a special locking mechanism that only allows one thread to access the data at a time.

This locking mechanism is called a segmented lock. It works by dividing the data into small segments, and then allowing each thread to lock one segment at a time. This way, multiple threads can work on the data concurrently, but they will never be working on the same data at the same time.

The segmented lock is implemented using a special Java class called ReentrantLock. ReentrantLock is a powerful synchronization tool that allows threads to safely lock data structures. It is used by many Java classes, including ConcurrentHashMap.

The ReentrantLock class has two main methods: lock() and unlock(). When a thread calls lock(), it will attempt to acquire the lock for the data structure. If the lock is available, the thread will acquire it and can then safely access and modify the data.

In the absence of a lock, the thread will wait until another thread releases it. Once the thread has acquired the lock, it can execute the critical section code and then release it by calling the unlock() method.

The ReentrantLock class also has a tryLock() method, which allows a thread to attempt to acquire the lock without waiting. This can be useful in situations where you do not want to block other threads if the lock is not available.

How to Program ConcurrentHashMap in Java

The ConcurrentHashMap class extends from the HashMap class and it is implemented to support concurrent access. It is also thread-safe, which means that multiple threads can access it without any issues. To create a ConcurrentHashMap, you can write the following Java code:

ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap();  

The following code listing can be used to store items in a ConcurrentHashMap and then display them at the console window:

import java.util.concurrent.*;
class MyConcurrentHashMap {
 public static void main(String[] args)
        ConcurrentHashMap<String, String> concurrentHashMap
            = new ConcurrentHashMap<>();
        concurrentHashMap.put("P001", "Lenovo Legion");
        concurrentHashMap.put("P002", "DELL Inspiron");
        concurrentHashMap.put("P003", "Lenovo ThinkPad");

When to Use ConcurrentHashMap in Java?

When multiple threads access a data structure at the same point of time, it can lead to unpredictable results or even deadlocks. This means that if you are going to implement your own version, you need to make sure that it is thread-safe by using synchronization primitives such as locks or semaphores.

ConcurrentHashMap is a great choice for high-performance applications, as it offers fast lookup and insertion times. It offers several features that make it well-suited for this type of environment, including the ability to lock and unlock individual buckets, as well as to atomically add and remove elements.

Basically, you should use it whenever you need a concurrent map that doesn’t block when used. For example, if you’re working with a lot of thread-safe data, or if you need to access the map from multiple threads, ConcurrentHashMap is a good option. It’s also a good choice if you need to store a large amount of data and want it to be accessible from multiple threads.

The ConcurrentHashMap is designed to be accessed by multiple threads for reading or writing data. You can access it from multiple threads without locking the entire object so you don’t have to wait when one thread locks another part of your program while accessing it.

ConcurrentHashMap also supports atomic putIfAbsent() and get() operations which allows you to lock only that item if it exists in order to add new data or retrieve existing data from map respectively without any deadlocks if other threads are already running these operations on some other entry within this map.

When using a Hashtable or synchronized collection, the entire collection is locked during reads and writes. With a ConcurrentHashMap, only the required element is locked for reading or writing. This allows for better performance because more than one thread can read data at once.

Do not use it if you need sequential access to elements in the map. A concurrent map does not guarantee that an item will be available immediately after another item has been modified; there could be other threads modifying different parts of it at any given time point in time this means that there is no guaranteed ordering after modifications.

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ConcurrentHashMap vs HashMap in Java

First and foremost, the ConcurrentHashMap class is designed for use in concurrent data access, whereas the Hashtable class is not. This means that the ConcurrentHashMap class can be safely used by multiple threads concurrently, without the risk of data corruption or race conditions.

Second, the ConcurrentHashMap class is much more efficient than the Hashtable class. It uses a hash-based algorithm to store and retrieve data, which makes it faster and more scalable. Finally, the ConcurrentHashMap class offers a number of powerful features that are not available in the Hashtable class. These include support for locking, evicting and iterating over entries, and adding and removing entries from the map.

There are a few different scenarios in which it makes sense to use a ConcurrentHashMap over a regular HashMap. One common scenario is when you need to support multiple readers and writers simultaneously. This could be the case in a web application, for example, where multiple users are accessing data at the same time.

Another scenario in which ConcurrentHashMap is useful is when you need to perform frequent reads and infrequent writes. In this case, it can be more efficient to use a ConcurrentHashMap because writes will not block reads.

ConcurrentHashMap is ideal for high-performance, multi-threaded applications. If you need to access and update a map from multiple threads, ConcurrentHashMap is the best option. It provides all the operations of a HashMap and additionally allows concurrent access for read, write, and update.

ConcurrentHashMap vs Hashtable in Java

ConcurrentHashMap uses multiple locks, so different parts of the map can be updated concurrently without blocking each other. Hashtable, on the other hand, uses a single lock for the entire map, which can lead to performance issues. However, unlike Hashtable, accesses to ConcurrentHashMap are generally not blocked when a lock is unavailable. This allows for much higher concurrency in retrieving data from a ConcurrentHashMap.

When updating a ConcurrentHashMap, multiple threads can lock different portions of the map for updates. The number of locks is configurable at creation time, and can be adjusted if needed.

Locking a portion of the map only blocks other threads from updating the locked portion; it does not block other threads from reading other parts of the map that are not locked. Unlike a Hashtable, null keys and values are not allowed in a ConcurrentHashMap.

When Not to use ConcurrentHashMap in Java

ConcurrentHashMap is not a good choice if you would need to maintain insertion order of the elements. This is because the internal structure of a ConcurrentHashMap is based on hash codes, which means that the order in which elements are inserted is not preserved.

Another situation in which it is not ideal to use a ConcurrentHashMap is when you need to synchronize access to the entire map. This is because each segment of the map is locked independently, so locking the entire map would require locking each segment individually, which would be inefficient.

Final Thoughts on ConcurrentHashMap in Java

ConcurrentHashMap is a Java class that allows multiple threads to access the map simultaneously. It is a part of the Java Collections Framework, and it provides a way for applications to efficiently handle concurrent updates to the map. ConcurrentHashMap is different from traditional HashMap in that it uses a lock to ensure that only one thread can modify the map at a time. This prevents data corruption and race conditions.

ConcurrentHashMap is an extension of the Map interface that can hold multiple entries at the same time. You can take advantage of ConcurrentHashMap to perform concurrent updates on its collection of keys, and values without locking out readers or writers for any significant period of time. The key features of ConcurrentHashMap are: performance, scalability and thread safety, which are critical for concurrent programming.

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