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:

>> Join Pro and download the eBook

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:

>> Download the eBook

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.

>> GET THE COURSE
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:

>> EXPLORE ACCESS NOW

eBook – HTTP Client – NPI (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

1. Overview

In this tutorial, we’re going to compare two of Spring’s web client implementations — RestTemplate and new Spring 5’s reactive alternative WebClient.

2. Blocking vs Non-Blocking Client

It’s a common requirement in web applications to make HTTP calls to other services. So, we need a web client tool.

2.1. RestTemplate Blocking Client

For a long time, Spring has been offering RestTemplate as a web client abstraction. Under the hood, RestTemplate uses the Java Servlet API, which is based on the thread-per-request model.

This means that the thread will block until the web client receives the response. The problem with the blocking code is due to each thread consuming some amount of memory and CPU cycles.

Let’s consider having a lot of incoming requests, which are waiting for some slow service needed to produce the result.

Sooner or later, the requests waiting for the results will pile up. Consequently, the application will create many threads, which will exhaust the thread pool or occupy all the available memory. We can also experience performance degradation because of the frequent CPU context (thread) switching.

2.2. WebClient Non-Blocking Client

On the other side, WebClient uses an asynchronous, non-blocking solution provided by the Spring Reactive framework.

While RestTemplate uses the caller thread for each event (HTTP call), WebClient will create something like a “task” for each event. Behind the scenes, the Reactive framework will queue those “tasks” and execute them only when the appropriate response is available.

The Reactive framework uses an event-driven architecture. It provides means to compose asynchronous logic through the Reactive Streams API. As a result, the reactive approach can process more logic while using fewer threads and system resources, compared to the synchronous/blocking method.

WebClient is part of the Spring WebFlux library. So, we can also write client code using a functional, fluent API with reactive types (Mono and Flux) as a declarative composition.

3. Comparison Example

To demonstrate the differences between these two approaches, we’d need to run performance tests with many concurrent client requests.

We would see a significant performance degradation with the blocking method after a certain number of parallel client requests.

However, the reactive/non-blocking method should give constant performances, regardless of the number of requests.

For this article, we’ll implement two REST endpoints, one using RestTemplate and the other using WebClient. Their task is to call another slow REST web service, which returns a list of tweets.

To start, we’ll need the Spring Boot WebFlux starter dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

And here’s our slow service REST endpoint:

@GetMapping("/slow-service-tweets")
private List<Tweet> getAllTweets() {
    Thread.sleep(2000L); // delay
    return Arrays.asList(
      new Tweet("RestTemplate rules", "@user1"),
      new Tweet("WebClient is better", "@user2"),
      new Tweet("OK, both are useful", "@user1"));
}

3.1. Using RestTemplate to Call a Slow Service

Let’s now implement another REST endpoint that will call our slow service via the web client.

First, we’ll use RestTemplate:

@GetMapping("/tweets-blocking")
public List<Tweet> getTweetsBlocking() {
    log.info("Starting BLOCKING Controller!");
    final String uri = getSlowServiceUri();

    RestTemplate restTemplate = new RestTemplate();
    ResponseEntity<List<Tweet>> response = restTemplate.exchange(
      uri, HttpMethod.GET, null,
      new ParameterizedTypeReference<List<Tweet>>(){});

    List<Tweet> result = response.getBody();
    result.forEach(tweet -> log.info(tweet.toString()));
    log.info("Exiting BLOCKING Controller!");
    return result;
}

When we call this endpoint, due to the synchronous nature of RestTemplate, the code will block waiting for the response from our slow service. The rest of the code in this method will be run only when the response has been received.

Here’s what we’ll see in the logs:

Starting BLOCKING Controller!
Tweet(text=RestTemplate rules, username=@user1)
Tweet(text=WebClient is better, username=@user2)
Tweet(text=OK, both are useful, username=@user1)
Exiting BLOCKING Controller!

3.2. Using WebClient to Call a Slow Service

Second, let’s use WebClient to call the slow service:

@GetMapping(value = "/tweets-non-blocking", 
            produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<Tweet> getTweetsNonBlocking() {
    log.info("Starting NON-BLOCKING Controller!");
    Flux<Tweet> tweetFlux = WebClient.create()
      .get()
      .uri(getSlowServiceUri())
      .retrieve()
      .bodyToFlux(Tweet.class);

    tweetFlux.subscribe(tweet -> log.info(tweet.toString()));
    log.info("Exiting NON-BLOCKING Controller!");
    return tweetFlux;
}

In this case, WebClient returns a Flux publisher, and the method execution gets completed. Once the result is available, the publisher will start emitting tweets to its subscribers.

Note that a client (in this case, a web browser) calling this /tweets-non-blocking endpoint will also be subscribed to the returned Flux object.

Let’s observe the log this time:

Starting NON-BLOCKING Controller!
Exiting NON-BLOCKING Controller!
Tweet(text=RestTemplate rules, username=@user1)
Tweet(text=WebClient is better, username=@user2)
Tweet(text=OK, both are useful, username=@user1)

Note that this endpoint method completed before the response was received.

4. Conclusion

In this article, we explored two different ways of using web clients in Spring.

RestTemplate uses Java Servlet API and is therefore synchronous and blocking.

Conversely, WebClient is asynchronous and will not block the executing thread while waiting for the response to come back. The notification will be produced only when the response is ready.

RestTemplate will still be used. But in some cases, the non-blocking approach uses much fewer system resources compared to the blocking one. So, WebClient is a preferable choice in those cases.

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)
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.

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?

Explore the eBook

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:

>> EXPLORE ACCESS NOW

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:

>> EXPLORE ACCESS NOW

Course – LS – NPI (cat=HTTP Client-Side)
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Get started with Spring Boot and with core Spring, through the Learn Spring course:

>> CHECK OUT THE COURSE

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

>> CHECK OUT THE COURSE

Partner – Microsoft – NPI (cat=Spring)
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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.

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