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:
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:
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:
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:
Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:
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:
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:
Refactor Java code safely — and automatically — with OpenRewrite.
Refactoring big codebases by hand is slow, risky, and easy to put off. That’s where OpenRewrite comes in. The open-source framework for large-scale, automated code transformations helps teams modernize safely and consistently.
Each month, the creators and maintainers of OpenRewrite at Moderne run live, hands-on training sessions — one for newcomers and one for experienced users. You’ll see how recipes work, how to apply them across projects, and how to modernize code with confidence.
Join the next session, bring your questions, and learn how to automate the kind of work that usually eats your sprint time.
Distributed systems often come with complex challenges such as service-to-service communication, state management, asynchronous messaging, security, and more.
Dapr (Distributed Application Runtime) provides a set of APIs and building blocks to address these challenges, abstracting away infrastructure so we can focus on business logic.
In this tutorial, we'll focus on Dapr's pub/sub API for message brokering. Using its Spring Boot integration, we'll simplify the creation of a loosely coupled, portable, and easily testable pub/sub messaging system:
1. Introduction
In this quick tutorial, we’re going to examine the differences between the PermGen and Metaspace memory regions in the Java environment.
It’s important to keep in mind that, starting with Java 8, the Metaspace replaces the PermGen – bringing some substantial changes.
2. PermGen
PermGen (Permanent Generation) is a special heap space separated from the main memory heap.
The JVM keeps track of loaded class metadata in the PermGen. Additionally, the JVM stores all the static content in this memory section. This includes all the static methods, primitive variables, and references to the static objects.
Furthermore, it contains data about bytecode, names, and JIT information. Before Java 7, the String Pool was also part of this memory. The disadvantages of the fixed pool size are listed in our write-up.
The default maximum memory size for 32-bit JVM is 64 MB and 82 MB for the 64-bit version.
However, we can change the default size with the JVM options:
- -XX:PermSize=[size] is the initial or minimum size of the PermGen space
- -XX:MaxPermSize=[size] is the maximum size
Most importantly, Oracle completely removed this memory space in the JDK 8 release. Therefore, if we use these tuning flags in Java 8 and newer versions, we’ll get the following warnings:
>> java -XX:PermSize=100m -XX:MaxPermSize=200m -version
OpenJDK 64-Bit Server VM warning: Ignoring option PermSize; support was removed in 8.0
OpenJDK 64-Bit Server VM warning: Ignoring option MaxPermSize; support was removed in 8.0
...With its limited memory size, PermGen is involved in generating the famous OutOfMemoryError. Simply put, the class loaders weren’t garbage collected properly and, as a result, generated a memory leak.
Therefore, we receive a memory space error; this happens mostly in the development environment while creating new class loaders.
3. Metaspace
Simply put, Metaspace is a new memory space – starting from the Java 8 version; it has replaced the older PermGen memory space. The most significant difference is how it handles memory allocation.
Specifically, this native memory region grows automatically by default.
We also have new flags to tune the memory:
- MetaspaceSize and MaxMetaspaceSize – we can set the Metaspace upper bounds.
- MinMetaspaceFreeRatio – is the minimum percentage of class metadata capacity free after garbage collection
- MaxMetaspaceFreeRatio – is the maximum percentage of class metadata capacity free after a garbage collection to avoid a reduction in the amount of space
Additionally, the garbage collection process also gains some benefits from this change. The garbage collector now automatically triggers the cleaning of the dead classes once the class metadata usage reaches its maximum metaspace size.
Therefore, with this improvement, JVM reduces the chance to get the OutOfMemory error.
Despite all of these improvements, we still need to monitor and tune the metaspace to avoid memory leaks.
4. Summary
In this quick write-up, we presented a brief description of PermGen and Metaspace memory regions. Additionally, we explained the key differences between each of them.
PermGen is still around with JDK 7 and older versions, but Metaspace offers more flexible and reliable memory usage for our applications.
