Migrating from Java 8/11 to Java 25: The Refactoring Checklist (2026 Edition)

Let’s be honest: simpler is better. I recently consulted for a fintech firm still running a monolithic payment gateway on Java 8. Their AWS bill was ballooning, and their “simple” bug fixes were taking weeks because of spaghetti code that modern Java features resolved years ago.

We migrated them to Java 25. The result? 40% reduction in memory usage (thanks to ZGC) and a codebase that juniors actually enjoyed working on.

This isn’t just a “news” article. This is my personal checklist from the trenches. If you are sitting on Java 8 or 11, here is exactly how you get to the promised land of 2026.

Phase 1: The “Big Bang” Preparation

Before you touch a single line of code, you need to prepare your environment. The most common mistake I see is developers trying to compile a Java 8 app with JDK 25 and trying to fix thousands of errors one by one. Don’t do that.

1. Upgrade the Toolchain First

Java 25 requires updated build tools. Your 2018 Maven/Gradle version won’t cut it.

• Maven: Upgrade to 3.9.x or 4.x.
• Gradle: You need at least Gradle 9.0 for full Java 25 support.

Phase 2: The “Jakarta” Namespace Nightmare

If you are coming from Java 8 or 11, this is the Boss Fight. In 2019, Java EE became Jakarta EE, and due to legal reasons, the package javax.* was renamed to jakarta.*.

This breaks everything: Hibernate, Spring, Tomcat, Jackson.

The Wrong Way: Using Ctrl + Shift + R to replace “javax” with “jakarta”. You will break javax.sql (which didn’t change) or javax.crypto.

The Senior Architect Way: Use OpenRewrite.

mvn org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-spring:5.x \
  -Drewrite.activeRecipes=org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_4

This script doesn’t just rename packages; it upgrades your Spring Boot version, fixes your JUnit 4 assertions to JUnit 5, and handles the Jakarta migration intelligently.

Phase 3: Refactoring for “Modern Java” (The ROI)

Once your app compiles, you shouldn’t stop there. Running Java 8 style code on Java 25 is like driving a Ferrari in a school zone. Let’s refactor.

1. Kill the Boilerplate with Records

I utilized Lombok’s @Data for a decade. I loved it. But in 2026, Java Records are superior for DTOs. They are native, immutable, and faster during serialization.

Legacy Java 8:

public class UserDTO {
    private final String name;
    private final int age;
    // ... insert 50 lines of getters, hashCode, equals, toString
}

Java 25 Refactor:

public record UserDTO(String name, int age) {}

2. Pattern Matching for Switch

We all have that one method with 15 if (obj instanceof Type) checks. Java 25 wipes this clean.

return switch (response) {
    case Success s -> "Great! " + s.message();
    case Failure f when f.isCritical() -> "Critical Error: " + f.code();
    case Failure f -> "Warning: " + f.code();
    case null -> "No response";
    default -> "Unknown state";
};

Phase 4: The 2026 Game Changers

Virtual Threads (The Performance King)

If you have any code using CompletableFuture or Reactive Streams just to handle high concurrency (like waiting for database calls), you can delete it.

With Java 25, you simply switch your thread executor. Virtual threads are so lightweight you can create millions of them.

// Old Way (Expensive Platform Threads)
ExecutorService e = Executors.newFixedThreadPool(100);
// New Way (Virtual Threads - Millions supported)
ExecutorService e = Executors.newVirtualThreadPerTaskExecutor();

Phase 5: Architect’s Watchlist (Hidden Gotchas)

• ❌ Security Manager is GONE: If your legacy app relied on `System.setSecurityManager`, it will crash. This has been removed in modern versions.
• ❌ Finalizers are TERMINATED: If you used `finalize()` to clean up resources, it’s time to learn `try-with-resources` or the `Cleaner` API.
• ✅ Docker Awareness: Java 25 is incredibly smart about container limits (`cgroup v2`). You usually don’t need to manually set heap sizes (`-Xmx`) in Kubernetes anymore; the JVM calculates it automatically based on the pod limit.

Frequently Asked Questions

Conclusion

Migrating to Java 25 is not just about chasing version numbers. It’s about reducing technical debt. The “record” classes alone reduced our DTO package size by 60%. Virtual threads removed the need for complex reactive code debugging.

Start with the toolchain, automate the boring stuff with OpenRewrite, and then enjoy the features. Good luck with the refactor!