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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:
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:
Get started with Spring and Spring Boot, through the Learn Spring course:
>> LEARN SPRINGExplore 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.
In this brief article, we’re going to discuss a common Exception that we may encounter when working with the Stream class in Java 8:
IllegalStateException: stream has already been operated upon or closed.We’ll discover the scenarios when this exception occurs, and the possible ways of avoiding it, all along with practical examples.
In Java 8, each Stream class represents a single-use sequence of data and supports several I/O operations.
A Stream should be operated on (invoking an intermediate or terminal stream operation) only once. A Stream implementation may throw IllegalStateException if it detects that the Stream is being reused.
Whenever a terminal operation is called on a Stream object, the instance gets consumed and closed.
Therefore, we’re only allowed to perform a single operation that consumes a Stream, otherwise, we’ll get an exception that states that the Stream has already been operated upon or closed.
Let’s see how this can be translated to a practical example:
Stream<String> stringStream = Stream.of("A", "B", "C", "D");
Optional<String> result1 = stringStream.findAny();
System.out.println(result1.get());
Optional<String> result2 = stringStream.findFirst();As a result:
A
Exception in thread "main" java.lang.IllegalStateException:
stream has already been operated upon or closedAfter the #findAny() method is invoked, the stringStream is closed, therefore, any further operation on the Stream will throw the IllegalStateException, and that’s what happened after invoking the #findFirst() method.
Simply put, the solution consists of creating a new Stream each time we need one.
We can, of course, do that manually, but that’s where the Supplier functional interface becomes really handy:
Supplier<Stream<String>> streamSupplier
= () -> Stream.of("A", "B", "C", "D");
Optional<String> result1 = streamSupplier.get().findAny();
System.out.println(result1.get());
Optional<String> result2 = streamSupplier.get().findFirst();
System.out.println(result2.get());As a result:
A
AWe’ve defined the streamSupplier object with the type Stream<String>, which is exactly the same type which the #get() method returns. The Supplier is based on a lambda expression that takes no input and returns a new Stream.
Invoking the functional method get() on the Supplier returns a freshly created Stream object, on which we can safely perform another Stream operation.
In this quick tutorial, we’ve seen how to perform terminal operations on a Stream multiple times, while avoiding the famous IllegalStateException that is thrown when the Stream is already closed or operated upon.
Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:
Once the early-adopter seats are all used, the price will go up and stay at $33/year.
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:
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:
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:
Get started with Spring Boot and with core Spring, through the Learn Spring course:
Modern Java teams move fast — but codebases don’t always keep up. Frameworks change, dependencies drift, and tech debt builds until it starts to drag on delivery. OpenRewrite was built to fix that: an open-source refactoring engine that automates repetitive code changes while keeping developer intent intact.
The monthly training series, led by the creators and maintainers of OpenRewrite at Moderne, walks through real-world migrations and modernization patterns. Whether you’re new to recipes or ready to write your own, you’ll learn practical ways to refactor safely and at scale.
If you’ve ever wished refactoring felt as natural — and as fast — as writing code, this is a good place to start.
