Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Baeldung)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Microsoft – NPI EA (cat= Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat=Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag=Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – Guide Spring Cloud – NPI EA (cat=Spring Cloud)
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Let's get started with a Microservice Architecture with Spring Cloud:

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eBook – Mockito – NPI EA (tag = Mockito)
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Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.

Get started with mocking and improve your application tests using our Mockito guide:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

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eBook – Reactive – NPI EA (cat=Reactive)
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Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:

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eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Jackson – NPI EA (cat=Jackson)
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Do JSON right with Jackson

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eBook – HTTP Client – NPI EA (cat=Http Client-Side)
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Get the most out of the Apache HTTP Client

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eBook – Maven – NPI EA (cat = Maven)
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Get Started with Apache Maven:

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eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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eBook – RwS – NPI EA (cat=Spring MVC)
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Building a REST API with Spring?

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Course – LS – NPI EA (cat=Jackson)
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Get started with Spring and Spring Boot, through the Learn Spring course:

>> LEARN SPRING
Course – RWSB – NPI EA (cat=REST)
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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

>> The New “REST With Spring Boot”

Course – LSS – NPI EA (cat=Spring Security)
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Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.

I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.

You can explore the course here:

>> Learn Spring Security

Course – All Access – NPI EA (cat= Spring)
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All Access is finally out, with all of my Spring courses. Learn JUnit is out as well, and Learn Maven is coming fast. And, of course, quite a bit more affordable. Finally.

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Course – LSD – NPI EA (tag=Spring Data JPA)
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Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.

Get started with Spring Data JPA through the guided reference course:

>> CHECK OUT THE COURSE

Partner – LambdaTest – NPI EA (cat=Testing)
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End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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Course – Spring Sale 2025 – NPI (cat=Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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eBook – Java Concurrency – NPI (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

1. Overview

In this tutorial, we’ll discuss the differences between Collections.synchronizedMap() and ConcurrentHashMap.

Additionally, we’ll look at the performance outputs of the read and write operations for each.

2. The Differences

Collections.synchronizedMap() and ConcurrentHashMap both provide thread-safe operations on collections of data.

The Collections utility class provides polymorphic algorithms that operate on collections and return wrapped collections. Its synchronizedMap() method provides thread-safe functionality.

As the name implies, synchronizedMap() returns a synchronized Map backed by the Map that we provide in the parameter. To provide thread-safety, synchronizedMap() allows all accesses to the backing Map via the returned Map.

ConcurrentHashMap was introduced in JDK 1.5 as an enhancement of HashMap that supports high concurrency for retrievals as well as updates. HashMap isn’t thread-safe, so it might lead to incorrect results during thread contention.

The ConcurrentHashMap class is thread-safe. Therefore, multiple threads can operate on a single object with no complications.

In ConcurrentHashMap, read operations are non-blocking, whereas write operations take a lock on a particular segment or bucket. The default bucket or concurrency level is 16, which means 16 threads can write at any instant after taking a lock on a segment or bucket.

2.1. ConcurrentModificationException

For objects like HashMap, performing concurrent operations is not allowed. Therefore, if we try to update a HashMap while iterating over it, we will receive a ConcurrentModificationException. This will also occur when using synchronizedMap():

@Test(expected = ConcurrentModificationException.class)
public void whenRemoveAndAddOnHashMap_thenConcurrentModificationError() {
    Map<Integer, String> map = new HashMap<>();
    map.put(1, "baeldung");
    map.put(2, "HashMap");
    Map<Integer, String> synchronizedMap = Collections.synchronizedMap(map);
    Iterator<Entry<Integer, String>> iterator = synchronizedMap.entrySet().iterator();
    while (iterator.hasNext()) {
        synchronizedMap.put(3, "Modification");
        iterator.next();
    }
}

However, this is not the case with ConcurrentHashMap:

Map<Integer, String> map = new ConcurrentHashMap<>();
map.put(1, "baeldung");
map.put(2, "HashMap");
 
Iterator<Entry<Integer, String>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
    map.put(3, "Modification");
    iterator.next()
}
 
Assert.assertEquals(3, map.size());

2.2. null Support

Collections.synchronizedMap() and ConcurrentHashMap handle null keys and values differently.

ConcurrentHashMap doesn’t allow null in keys or values:

@Test(expected = NullPointerException.class)
public void allowNullKey_In_ConcurrentHasMap() {
    Map<String, Integer> map = new ConcurrentHashMap<>();
    map.put(null, 1);
}

However, when using Collections.synchronizedMap(), null support depends on the input Map. We can have one null as a key and any number of null values when Collections.synchronizedMap() is backed by HashMap or LinkedHashMap, whereas if we’re using TreeMap, we can have null values but not null keys.

Let’s assert that we can use a null key for Collections.synchronizedMap() backed by a HashMap:

Map<String, Integer> map = Collections
  .synchronizedMap(new HashMap<String, Integer>());
map.put(null, 1);
Assert.assertTrue(map.get(null).equals(1));

Similarly, we can validate null support in values for both Collections.synchronizedMap() and ConcurrentHashMap.

3. Performance Comparison

Let’s compare the performances of ConcurrentHashMap versus Collections.synchronizedMap(). In this case, we’re using the open-source framework Java Microbenchmark Harness (JMH) to compare the performances of the methods in nanoseconds.

We ran the comparison for random read and write operations on these maps. Let’s take a quick look at our JMH benchmark code:

@Benchmark
public void randomReadAndWriteSynchronizedMap() {
    Map<String, Integer> map = Collections.synchronizedMap(new HashMap<String, Integer>());
    performReadAndWriteTest(map);
}

@Benchmark
public void randomReadAndWriteConcurrentHashMap() {
    Map<String, Integer> map = new ConcurrentHashMap<>();
    performReadAndWriteTest(map);
}

private void performReadAndWriteTest(final Map<String, Integer> map) {
    for (int i = 0; i < TEST_NO_ITEMS; i++) {
        Integer randNumber = (int) Math.ceil(Math.random() * TEST_NO_ITEMS);
        map.get(String.valueOf(randNumber));
        map.put(String.valueOf(randNumber), randNumber);
    }
}

We ran our performance benchmarks using 5 iterations with 10 threads for 1,000 items. Let’s see the benchmark results:

Benchmark                                                     Mode  Cnt        Score        Error  Units
MapPerformanceComparison.randomReadAndWriteConcurrentHashMap  avgt  100  3061555.822 ±  84058.268  ns/op
MapPerformanceComparison.randomReadAndWriteSynchronizedMap    avgt  100  3234465.857 ±  60884.889  ns/op
MapPerformanceComparison.randomReadConcurrentHashMap          avgt  100  2728614.243 ± 148477.676  ns/op
MapPerformanceComparison.randomReadSynchronizedMap            avgt  100  3471147.160 ± 174361.431  ns/op
MapPerformanceComparison.randomWriteConcurrentHashMap         avgt  100  3081447.009 ±  69533.465  ns/op
MapPerformanceComparison.randomWriteSynchronizedMap           avgt  100  3385768.422 ± 141412.744  ns/op

The above results show that ConcurrentHashMap performs better than Collections.synchronizedMap().

4. When to Use

We should favor Collections.synchronizedMap() when data consistency is of utmost importance, and we should choose ConcurrentHashMap for performance-critical applications where there are far more write operations than there are read operations.

This is because the Collections.synchronizedMap() requires each thread to acquire a lock on the entire object for both read/write operations. By comparison, the ConcurrentHashMap allows threads to acquire locks on separate segments of the collection, and make modifications at the same time.

5. Conclusion

In this article, we’ve demonstrated the differences between ConcurrentHashMap and Collections.synchronizedMap(). We’ve also shown the performances of both of them using a simple JMH benchmark.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.
Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

>> Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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Course – LS – NPI EA (cat=REST)

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Get started with Spring Boot and with core Spring, through the Learn Spring course:

>> CHECK OUT THE COURSE

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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Course – Spring Sale 2025 – NPI (All)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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eBook – Java Concurrency – NPI (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook Jackson – NPI EA – 3 (cat = Jackson)
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