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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.

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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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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.

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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.

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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:

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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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eBook – Persistence – NPI EA (cat=Persistence)
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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:

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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

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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:

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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.

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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:

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Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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1. Overview

In this article, we will be looking at a WeakHashMap from the java.util package.

In order to understand the data structure, we’ll use it here to roll out a simple cache implementation. However, keep in mind that this is meant to understand how the map works, and creating your own cache implementation is almost always a bad idea.

Simply put, the WeakHashMap is a hashtable-based implementation of the Map interface, with keys that are of a WeakReference type.

An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use, meaning that there is no single Reference that point to that key. When the garbage collection (GC) process discards a key, its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations.

2. Strong, Soft, and Weak References

To understand how the WeakHashMap works, we need to look at a WeakReference class – which is the basic construct for keys in the WeakHashMap implementation. In Java, we have three main types of references, which we’ll explain in the following sections.

2.1. Strong References

The strong reference is the most common type of Reference that we use in our day to day programming:

Integer prime = 1;

The variable prime has a strong reference to an Integer object with value 1. Any object which has a strong reference pointing to it is not eligible for GC.

2.2. Soft References

Simply put, an object that has a SoftReference pointing to it won’t be garbage collected until the JVM absolutely needs memory.

Let’s see how we can create a SoftReference in Java:

Integer prime = 1;  
SoftReference<Integer> soft = new SoftReference<Integer>(prime); 
prime = null;

The prime object has a strong reference pointing to it.

Next, we are wrapping prime strong reference into a soft reference. After making that strong reference null, a prime object is eligible for GC but will be collected only when JVM absolutely needs memory.

2.3. Weak References

The objects that are referenced only by weak references are garbage collected eagerly; the GC won’t wait until it needs memory in that case.

We can create a WeakReference in Java in the following way:

Integer prime = 1;  
WeakReference<Integer> soft = new WeakReference<Integer>(prime); 
prime = null;

When we made a prime reference null, the prime object will be garbage collected in the next GC cycle, as there is no other strong reference pointing to it.

References of a WeakReference type are used as keys in WeakHashMap.

3. WeakHashMap as an Efficient Memory Cache

Let’s say that we want to build a cache that keeps big image objects as values, and image names as keys. We want to pick a proper map implementation for solving that problem.

Using a simple HashMap will not be a good choice because the value objects may occupy a lot of memory. What’s more, they’ll never be reclaimed from the cache by a GC process, even when they are not in use in our application anymore.

Ideally, we want a Map implementation that allows GC to automatically delete unused objects. When a key of a big image object is not in use in our application in any place, that entry will be deleted from memory.

Fortunately, the WeakHashMap has exactly these characteristics. Let’s test our WeakHashMap and see how it behaves:

WeakHashMap<UniqueImageName, BigImage> map = new WeakHashMap<>();
BigImage bigImage = new BigImage("image_id");
UniqueImageName imageName = new UniqueImageName("name_of_big_image");

map.put(imageName, bigImage);
assertTrue(map.containsKey(imageName));

imageName = null;
System.gc();

await().atMost(10, TimeUnit.SECONDS).until(map::isEmpty);

We’re creating a WeakHashMap instance that will store our BigImage objects. We are putting a BigImage object as a value and an imageName object reference as a key. The imageName will be stored in a map as a WeakReference type.

Next, we set the imageName reference to be null, therefore there are no more references pointing to the bigImage object. The default behavior of a WeakHashMap is to reclaim an entry that has no reference to it on next GC, so this entry will be deleted from memory by the next GC process.

We are calling a System.gc() to force the JVM to trigger a GC process. After the GC cycle, our WeakHashMap will be empty:

WeakHashMap<UniqueImageName, BigImage> map = new WeakHashMap<>();
BigImage bigImageFirst = new BigImage("foo");
UniqueImageName imageNameFirst = new UniqueImageName("name_of_big_image");

BigImage bigImageSecond = new BigImage("foo_2");
UniqueImageName imageNameSecond = new UniqueImageName("name_of_big_image_2");

map.put(imageNameFirst, bigImageFirst);
map.put(imageNameSecond, bigImageSecond);
 
assertTrue(map.containsKey(imageNameFirst));
assertTrue(map.containsKey(imageNameSecond));

imageNameFirst = null;
System.gc();

await().atMost(10, TimeUnit.SECONDS)
  .until(() -> map.size() == 1);
await().atMost(10, TimeUnit.SECONDS)
  .until(() -> map.containsKey(imageNameSecond));

Note that only the imageNameFirst reference is set to null. The imageNameSecond reference remains unchanged. After GC is triggered, the map will contain only one entry – imageNameSecond.

4. Conclusion

In this article, we looked at types of references in Java to fully understand how java.util.WeakHashMap works. We created a simple cache that leverages behavior of a WeakHashMap and test if it works as we expected.

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)
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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 – 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:

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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 – 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:

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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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Course – LS – NPI EA (cat=REST)

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

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