Java , Spring Migration

Migrating from Java 8 to Java 17 and Spring 2.3.2 to 3.2.2: Lessons Learned and Key Challenges:

Recently, I worked on a project involving the migration of an application from Java 8 to Java 17 and from Spring 2.3.2 to 3.2.2. This upgrade brought significant improvements in terms of performance, security, and long-term support, but it also came with its fair share of challenges due to API changes and deprecations. In this post, I'll walk through some of the specific issues I encountered and how I resolved them.

Why Migrate to Java 17 and Spring 3.2.2?

Java 17 is a long-term support (LTS) release, offering several new features like sealed classes, records, and improved garbage collection, making it an ideal choice for applications needing longevity and security. Spring 3.2.2 is also updated to support the latest Java versions, bringing enhanced support for reactive programming, security updates, and other optimizations.

However, transitioning to these versions involved adjustments, especially where libraries and frameworks had shifted or deprecated classes.

Key Migration Challenges and Solutions

1. HttpStatus to HttpStatusCode In Spring 3.x, HttpStatus has been replaced by HttpStatusCode, which meant updating numerous references in our application.

Issue:

// Old code in Spring 2.x

return new ResponseEntity<>(data, HttpStatus.OK);

Solution: With HttpStatusCode, we can still access the same constants but need to use HttpStatusCode.valueOf() for compatibility:

// Updated code for Spring 3.x

**`**return new ResponseEntity<>(data, HttpStatusCode.valueOf(200));**`**

Alternatively, when possible, I replaced instances with HttpStatusCode.OK for simplicity. This minor change was necessary for
smooth integration with Spring 3.x APIs.

2. Mockito.Matcher to Mockito.ArgumentMatchers

During testing, the migration exposed issues in mocking setups due to changes in the Mockito library. The Matcher class used for defining match conditions in tests has been moved to ArgumentMatchers.

Issue:
// Old code with Mockito.Matcher

**Mockito.when(mockObject.method(Mockito.Matcher.any())).thenReturn(value);**

Solution: The ArgumentMatchers class should now be used instead of Matcher. I updated all instances of Mockito.Matcher to Mockito.ArgumentMatchers, which resolved compatibility issues in tests.

// Updated code with Mockito.ArgumentMatchers

**Mockito.when(mockObject.method(Mockito.ArgumentMatchers.any())).thenReturn(value);**

3. Transition from javax to jakarta

One of the major changes in Spring 3.x was the migration from javax packages to jakarta. This shift affected several dependencies, especially those related to Java EE, such as javax.servlet and javax.persistence.

Issue:

// Old code with javax

**import javax.servlet.ServletException;
import javax.persistence.Entity;**

Solution: The Spring 3.x ecosystem now relies on jakarta packages. This required a straightforward but extensive refactoring throughout the codebase, replacing javax imports with their jakarta counterparts.

// Updated code with jakarta

**import jakarta.servlet.ServletException;
import jakarta.persistence.Entity;**

Updating imports was time-consuming but necessary for the migration. I ensured compatibility with all jakarta imports before proceeding with further tests, as this affected several layers of the application.

Key Takeaways
This migration was challenging, but the benefits of Java 17 and Spring 3.x made it worthwhile. While dealing with issues like HttpStatusCode, ArgumentMatchers, and javax to jakarta transitions required careful planning and code adjustments, the result is a more modern, secure, and maintainable application.

If you’re planning a similar migration, I recommend thoroughly reviewing the release notes of both Java and Spring to anticipate changes. With careful refactoring and extensive testing, you can fully leverage the advantages of these new versions.