Spring Boot 3 and Java 21 – What’s New, What Changed, and Enterprise Use Cases

What’s New in Java 21?

Virtual Threads – A Revolution in Thread Management

As part of Project Loom, Java 21 introduces virtual threads. Traditional threads are resource-heavy, with their creation burdening the operating system, which forced developers to rely on thread pools. Virtual threads solve this problem—they are lightweight, managed by the JVM, and can run in the thousands without noticeable overhead.

What’s crucial is that developers don’t need to change their coding style—Virtual Threads look and behave like regular threads but are far more efficient. This means applications can handle significantly more concurrent requests without requiring complex concurrency mechanisms.

Pattern Matching and Record Patterns – Cleaner, More Readable Code

Pattern Matching

Pattern Matching enables both type checking and direct use of objects. Java 21 introduces this functionality to the switch block. Previously, developers had to write verbose if blocks, use instanceof, and manually cast objects. Now this can be done in a single, concise expression.

In large applications that often operate on generic types, Pattern Matching reduces errors and improves readability. The compiler also verifies whether all cases are covered, which enhances safety. Developers no longer need to worry about manual casting or potential ClassCastException.

Record Patterns

Records have already become a common way of defining simple data carrier classes in Java. Record Patterns take this further by enabling direct extraction of record fields within conditional statements. Instead of calling accessor methods manually, developers can bind values directly to variables while performing type checks.

This results in shorter, safer, and more expressive code. The feature is especially useful in large systems with numerous DTOs in circulation. Record Patterns integrate seamlessly with Pattern Matching, creating a consistent object-oriented programming model that enhances readability and simplifies maintenance.

Sealed Classes i Scoped Values –  Improved Application Safety

Sealed Classes

Sealed Classes give developers finer control over inheritance hierarchies. A class or interface marked with the sealedkeyword can only be extended by explicitly defined types. This ensures no unexpected implementations appear in the system that could disrupt application logic.

This is particularly valuable in large enterprise systems with complex class and interface structures. Sealed Classes work very well with Record Patterns and Pattern Matching, offering developers a powerful tool for safe and maintainable domain modeling.

Scoped Values

Scoped Values provide an alternative to ThreadLocal. They allow values to be shared within the scope of a thread in a controlled and predictable way. Unlike ThreadLocal, Scoped Values have a clearly defined lifecycle—they only exist within the scope of the invocation where they were defined.

This minimizes the risk of memory leaks and hard-to-detect bugs. They are especially useful with Virtual Threads, enabling the transfer of contextual data without boilerplate code. This is critical for server-side applications handling multiple users simultaneously while keeping data properly isolated. Scoped Values are simple to use and enhance both application security and predictability.

String Templates – Convenient String Interpolation

String Templates, currently available in preview, are among the most practical additions in Java 21. Until now, Java lacked a clean way to perform string interpolation. Developers had to rely on String.format or concatenation, often leading to unreadable code.

String Templates allow variables to be embedded directly in text in a clean, concise way. This improves both readability and security. In the future, the feature is expected to support automatic data sanitization for SQL queries, further enhancing application safety.