ConcurrentHashMap vs SynchronizedMap

This post explains the features of ConcurrentHashMap and SynchronizedMap

5/6/20237 min read

Multi-threaded programming has become increasingly prevalent in software development, and managing shared data structures is a crucial aspect of writing concurrent applications. In this article, we will explore two important data structures designed to address the challenges of concurrent programming: SynchronizedHashMap and ConcurrentHashMap.

Understanding the Basics

Before we dive into a detailed comparison, let's first understand the basic principles and characteristics of SynchronizedHashMap and ConcurrentHashMap.


Synchronization: Synchronization is the process of controlling access to shared resources in a multi-threaded environment to prevent data corruption and ensure data consistency. SynchronizedHashMap is a thread-safe data structure that wraps a regular HashMap with synchronization mechanisms.

HashMap: A HashMap is a data structure that stores key-value pairs and provides fast access to values based on their associated keys. It is known for its efficient retrieval and insertion of data. SynchronizedHashMap uses a regular HashMap as its underlying data structure.

Thread Safety: SynchronizedHashMap ensures that all operations on the underlying HashMap are synchronized, making it safe for concurrent access. It achieves this through locking mechanisms.

Data Consistency: SynchronizedHashMap guarantees data consistency and prevents data corruption by enforcing synchronization for all operations on the underlying HashMap.


Concurrent Data Structure: ConcurrentHashMap is a specialized concurrent data structure designed to allow multiple threads to read and write to it concurrently without the need for extensive locking. It is optimized for high concurrency.

Segmentation: ConcurrentHashMap divides its data into segments or buckets, and each segment is independently locked. This means that different threads can access different segments concurrently, reducing lock contention.

Efficiency: ConcurrentHashMap is known for its efficiency in multi-threaded scenarios. It provides good performance and scalability for both read and write operations.

Thread Safety: ConcurrentHashMap is inherently thread-safe, and it does not require full synchronization for every operation. Instead, it uses fine-grained locks at the segment level, which enables concurrent access without bottlenecks.

Now that we have a fundamental understanding of both data structures, let's delve deeper into their characteristics, use cases, and implementation details.


Features of SynchronizedHashMap

1. Thread Safety: SynchronizedHashMap provides full thread safety, ensuring that all operations on the underlying HashMap are synchronized. This makes it safe for concurrent access.

2. Simplicity: SynchronizedHashMap offers a familiar interface to developers since it uses a regular HashMap as its underlying data structure. This means that developers can work with key-value pairs in a way they are already accustomed to.

3. Data Consistency: One of the primary features of SynchronizedHashMap is its ability to maintain data consistency across multiple threads. Any operations that involve reading or modifying the data are protected by locks, preventing race conditions.

Use Cases for SynchronizedHashMap

SynchronizedHashMap is suitable for several use cases, particularly when thread safety is crucial. Here are some scenarios where it can be applied effectively:

1. Caching: In web applications, caching frequently accessed data can significantly improve performance. SynchronizedHashMap can be used to implement a thread-safe cache.

2. Shared Resource Management: When multiple threads need to access and modify shared resources, a SynchronizedHashMap can help maintain data integrity. For instance, in a chat application, a SynchronizedHashMap can be used to manage user messages.

3. Configuration Management: Storing and accessing configuration parameters in a thread-safe manner is essential in many applications, such as server software.

4. In-Memory Databases: SynchronizedHashMaps can serve as the basis for in-memory databases that need to support concurrent read and write operations.

Implementation of SynchronizedHashMap

Let's take a look at a simplified implementation of a SynchronizedHashMap in Java:

import java.util.HashMap;

import java.util.Map;

public class SynchronizedHashMap<K, V> {

private final Map<K, V> map = new HashMap<>();

public synchronized V get(K key) {

return map.get(key);


public synchronized void put(K key, V value) {

map.put(key, value);


public synchronized V remove(K key) {

return map.remove(key);


// Other methods like size, isEmpty, containsKey, etc.



In this Java implementation, all critical operations (`get`, `put`, and `remove`) are synchronized to ensure thread safety. While this provides a basic understanding of the concept, production-ready SynchronizedHashMap implementations are more complex and optimized for high concurrency.

Performance Considerations for SynchronizedHashMap

While SynchronizedHashMap offers thread safety, it comes with certain performance considerations:

1. Lock Contention: Synchronization can lead to lock contention, where multiple threads are waiting for access to the same resource. This contention can impact the scalability and performance of the application.

2. Reduced Parallelism: Synchronization can limit the parallelism of the program since only one thread can access the synchronized block at a time.

3. Blocking: In cases where one thread is performing a long operation within the synchronized block, other threads may be blocked from accessing the data structure.

4. Performance Overhead: The overhead of acquiring and releasing locks for synchronization can impact the overall performance of the data structure.


Features of ConcurrentHashMap

1. High Concurrency: ConcurrentHashMap is designed to support high levels of concurrency. It divides its data into segments, and different threads can access different segments concurrently, reducing contention.

2. Fine-Grained Locking: Instead of using a single lock for the entire data structure, ConcurrentHashMap uses fine-grained locking at the segment level. This means that multiple threads can perform operations on different segments at the same time.

3. Efficiency: ConcurrentHashMap provides good performance for both read and write operations, even in scenarios with a high number of threads. It is optimized for use cases where concurrency is a priority.

4. Scalability: Due to its segmentation and fine-grained locking, ConcurrentHashMap scales well as the number of threads and data size increase.

Use Cases for ConcurrentHashMap

ConcurrentHashMap is well-suited for scenarios that require high levels of concurrency and performance. Some common use cases include:

1. Thread-Safe Caches: When building high-performance caches that can be accessed and modified by multiple threads concurrently, ConcurrentHashMap is an excellent choice.

2. Parallel Processing: In applications that involve parallel processing of data, such as data pipelines or data analytics, ConcurrentHashMap can efficiently handle concurrent access to data structures.

3. Task Scheduling: Task schedulers often need to manage a queue of tasks that multiple threads can access. ConcurrentHashMap is an ideal choice for such task management.

4. Session Management: In web applications, managing user sessions concurrently requires a thread-safe data structure like ConcurrentHashMap to ensure data integrity.

Implementation of ConcurrentHashMap

ConcurrentHashMap is usually implemented as a combination of hash tables and linked lists or other data structures within each segment. It relies on low-level concurrency primitives provided by the language or platform to manage concurrent access.

Here is a simplified example of how ConcurrentHashMap might be implemented in Java:

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapExample {

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

public void put(String key, int value) {

map.put(key, value);



Integer get(String key) {

return map.get(key);


// Other methods for size, isEmpty, etc.


The actual implementation of ConcurrentHashMap is more complex, with mechanisms to handle resizing, concurrency control, and efficient management of segments.

Performance Considerations for ConcurrentHashMap

ConcurrentHashMap is optimized for high concurrency, but it is not without its performance considerations:

1. Memory Overhead: ConcurrentHashMap may consume more memory than a SynchronizedHashMap due to its segmentation and additional data structures.

2. Complexity: The internal implementation of ConcurrentHashMap is more complex than a simple SynchronizedHashMap, which can make it harder to understand and maintain.

3. Initial Setup Cost: Creating and initializing a ConcurrentHashMap may have a slightly higher setup cost compared to a regular HashMap.

4. Increased Complexity in Code: Working with ConcurrentHashMap might require more careful code design and error handling due to its concurrent nature.

Comparative Analysis

Now that we have a clear understanding of the features, use cases, implementations, and performance characteristics of SynchronizedHashMap and ConcurrentHashMap, let's perform a comparative analysis to help you decide which one is best suited for your specific needs.

Thread Safety

- SynchronizedHashMap: It provides full thread safety through locking mechanisms. All operations are synchronized, which ensures data consistency but may lead to lock contention.

- ConcurrentHashMap: It is inherently thread-safe and uses fine-grained locks at the segment level, allowing for high concurrency without as much lock contention.

Verdict: ConcurrentHashMap offers better thread safety with higher concurrency and less lock contention.


- SynchronizedHashMap: It offers a simple and familiar interface since it is based on a regular HashMap. Developers can work with it using common key-value pair operations.

- ConcurrentHashMap: It has a more complex internal implementation due to its segmentation and fine-grained locks, which may make it less straightforward for developers to work with.

Verdict: SynchronizedHashMap is simpler to work with in terms of code and understanding.


- SynchronizedHashMap: It suffers from lock contention, reduced parallelism, and potential blocking, which can affect performance in highly concurrent scenarios.

- ConcurrentHashMap: It is optimized for high concurrency and offers good performance even with a large number of threads. It provides scalability and efficiency for both read and write operations.

Verdict: ConcurrentHashMap excels in terms of performance, especially in high-concurrency scenarios.

Use Cases

- SynchronizedHashMap: It is well-suited for scenarios where thread safety is essential, but high concurrency is not a primary concern. Use cases include simple caches, shared resource management, and configuration storage.

- ConcurrentHashMap: It shines in use cases where high concurrency, scalability, and efficient multi-threaded access are required. Examples include thread-safe caches, parallel processing, task scheduling, and session management in web applications.

Verdict: The choice depends on the specific use case; ConcurrentHashMap is better for high-concurrency scenarios, while SynchronizedHashMap is suitable for basic thread safety needs.

Code Complexity

- SynchronizedHashMap: Code using SynchronizedHashMap is simpler and more straightforward since it follows the familiar HashMap interface.

- ConcurrentHashMap: Code using ConcurrentHashMap may be more complex due to the need to manage concurrency, but it also provides more advanced control over multi-threaded access.

Verdict: SynchronizedHashMap leads in terms of code simplicity, but ConcurrentHashMap provides more control over concurrency.

### Memory Overhead

- SynchronizedHashMap: It generally has lower memory overhead since it doesn't require the additional data structures for segmentation used in ConcurrentHashMap.

- ConcurrentHashMap: Due to its segmentation and fine-grained locking, ConcurrentHashMap may consume more memory.

Verdict: SynchronizedHashMap is generally more memory-efficient.

Choosing Between SynchronizedHashMap and ConcurrentHashMap

The choice between SynchronizedHashMap and ConcurrentHashMap should be based on the specific requirements of your project. Here are some guidelines to help you decide:

1. Use Case: If you need a simple and straightforward solution for basic thread safety and you are not dealing with extremely high levels of concurrency, SynchronizedHashMap is a suitable choice.

2. High Concurrency: If your application demands high concurrency, especially in scenarios with a large number of threads, then ConcurrentHashMap is the better option. It excels in performance, scalability, and efficiency.

3. Performance Critical Applications: For performance-critical applications, especially in concurrent processing and data access, ConcurrentHashMap is preferred. It minimizes lock contention and offers better parallelism.

4. Code Simplicity: If you prioritize code simplicity and maintainability over performance, SynchronizedHashMap provides a more straightforward and familiar interface.

5. Memory Efficiency: If you are concerned about memory usage and want to minimize overhead, SynchronizedHashMap is a more memory-efficient choice.

6. Complexity Tolerance: If your development team is comfortable with handling more complex code and you require advanced control over concurrency, then ConcurrentHashMap may be a better fit.

In summary, SynchronizedHashMap is well-suited for simpler use cases where basic thread safety is sufficient, while ConcurrentHashMap is ideal for applications with high concurrency requirements and performance-critical scenarios. Ultimately, your choice should align with your project's specific needs and constraints.


SynchronizedHashMap and ConcurrentHashMap are both valuable tools in the realm of concurrent programming, each with its own strengths and weaknesses. The choice between them should be based on the specific requirements of your project, considering factors like thread safety, performance, code simplicity, memory efficiency, and concurrency needs.

It's important to conduct real-world tests and benchmarking to validate the performance characteristics of these data structures in your specific use case. Ultimately, your choice should align with your application's current needs and future scalability requirements. Whether you opt for the simplicity of SynchronizedHashMap or the high concurrency of ConcurrentHashMap, both data structures play essential roles in ensuring the reliable and efficient operation of multi-threaded applications.

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