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.

>> 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 – 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. Introduction

In this article, we’ll give a guide to the CountDownLatch class and demonstrate how it can be used in a few practical examples.

Essentially, by using a CountDownLatch we can cause a thread to block until other threads have completed a given task.

2. Usage in Concurrent Programming

Simply put, a CountDownLatch has a counter field, which you can decrement as we require. We can then use it to block a calling thread until it’s been counted down to zero.

If we were doing some parallel processing, we could instantiate the CountDownLatch with the same value for the counter as a number of threads we want to work across. Then, we could just call countdown() after each thread finishes, guaranteeing that a dependent thread calling await() will block until the worker threads are finished.

3. Waiting for a Pool of Threads to Complete

Let’s try out this pattern by creating a Worker and using a CountDownLatch field to signal when it has completed:

public class Worker implements Runnable {
    private List<String> outputScraper;
    private CountDownLatch countDownLatch;

    public Worker(List<String> outputScraper, CountDownLatch countDownLatch) {
        this.outputScraper = outputScraper;
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        doSomeWork();
        outputScraper.add("Counted down");
        countDownLatch.countDown();
    }
}

Then, let’s create a test in order to prove that we can get a CountDownLatch to wait for the Worker instances to complete:

@Test
public void whenParallelProcessing_thenMainThreadWillBlockUntilCompletion()
  throws InterruptedException {

    List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
    CountDownLatch countDownLatch = new CountDownLatch(5);
    List<Thread> workers = Stream
      .generate(() -> new Thread(new Worker(outputScraper, countDownLatch)))
      .limit(5)
      .collect(toList());

      workers.forEach(Thread::start);
      countDownLatch.await(); 
      outputScraper.add("Latch released");

      assertThat(outputScraper)
        .containsExactly(
          "Counted down",
          "Counted down",
          "Counted down",
          "Counted down",
          "Counted down",
          "Latch released"
        );
    }

Naturally “Latch released” will always be the last output – as it’s dependant on the CountDownLatch releasing.

Note that if we didn’t call await(), we wouldn’t be able to guarantee the ordering of the execution of the threads, so the test would randomly fail.

4. A Pool of Threads Waiting to Begin

If we took the previous example, but this time started thousands of threads instead of five, it’s likely that many of the earlier ones will have finished processing before we have even called start() on the later ones. This could make it difficult to try and reproduce a concurrency problem, as we wouldn’t be able to get all our threads to run in parallel.

To get around this, let’s get the CountdownLatch to work differently than in the previous example. Instead of blocking a parent thread until some child threads have finished, we can block each child thread until all the others have started.

Let’s modify our run() method so it blocks before processing:

public class WaitingWorker implements Runnable {

    private List<String> outputScraper;
    private CountDownLatch readyThreadCounter;
    private CountDownLatch callingThreadBlocker;
    private CountDownLatch completedThreadCounter;

    public WaitingWorker(
      List<String> outputScraper,
      CountDownLatch readyThreadCounter,
      CountDownLatch callingThreadBlocker,
      CountDownLatch completedThreadCounter) {

        this.outputScraper = outputScraper;
        this.readyThreadCounter = readyThreadCounter;
        this.callingThreadBlocker = callingThreadBlocker;
        this.completedThreadCounter = completedThreadCounter;
    }

    @Override
    public void run() {
        readyThreadCounter.countDown();
        try {
            callingThreadBlocker.await();
            doSomeWork();
            outputScraper.add("Counted down");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            completedThreadCounter.countDown();
        }
    }
}

Now, let’s modify our test so it blocks until all the Workers have started, unblocks the Workers, and then blocks until the Workers have finished:

@Test
public void whenDoingLotsOfThreadsInParallel_thenStartThemAtTheSameTime()
 throws InterruptedException {
 
    List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
    CountDownLatch readyThreadCounter = new CountDownLatch(5);
    CountDownLatch callingThreadBlocker = new CountDownLatch(1);
    CountDownLatch completedThreadCounter = new CountDownLatch(5);
    List<Thread> workers = Stream
      .generate(() -> new Thread(new WaitingWorker(
        outputScraper, readyThreadCounter, callingThreadBlocker, completedThreadCounter)))
      .limit(5)
      .collect(toList());

    workers.forEach(Thread::start);
    readyThreadCounter.await(); 
    outputScraper.add("Workers ready");
    callingThreadBlocker.countDown(); 
    completedThreadCounter.await(); 
    outputScraper.add("Workers complete");

    assertThat(outputScraper)
      .containsExactly(
        "Workers ready",
        "Counted down",
        "Counted down",
        "Counted down",
        "Counted down",
        "Counted down",
        "Workers complete"
      );
}

This pattern is really useful for trying to reproduce concurrency bugs, as can be used to force thousands of threads to try and perform some logic in parallel.

5. Terminating a CountdownLatch Early

Sometimes, we may run into a situation where the Workers terminate in error before counting down the CountDownLatch. This could result in it never reaching zero and await() never terminating:

@Override
public void run() {
    if (true) {
        throw new RuntimeException("Oh dear, I'm a BrokenWorker");
    }
    countDownLatch.countDown();
    outputScraper.add("Counted down");
}

Let’s modify our earlier test to use a BrokenWorker, in order to show how await() will block forever:

@Test
public void whenFailingToParallelProcess_thenMainThreadShouldGetNotGetStuck()
  throws InterruptedException {
 
    List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
    CountDownLatch countDownLatch = new CountDownLatch(5);
    List<Thread> workers = Stream
      .generate(() -> new Thread(new BrokenWorker(outputScraper, countDownLatch)))
      .limit(5)
      .collect(toList());

    workers.forEach(Thread::start);
    countDownLatch.await();
}

Clearly, this is not the behavior we want – it would be much better for the application to continue than infinitely block.

To get around this, let’s add a timeout argument to our call to await().

boolean completed = countDownLatch.await(3L, TimeUnit.SECONDS);
assertThat(completed).isFalse();

As we can see, the test will eventually time out and await() will return false.

6. Conclusion

In this quick guide, we’ve demonstrated how we can use a CountDownLatch in order to block a thread until other threads have finished some processing.

We’ve also shown how it can be used to help debug concurrency issues by making sure threads run in parallel.

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)
announcement - icon

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)
announcement - icon

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

eBook – Java Concurrency – NPI (cat=Java Concurrency)
announcement - icon

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)