Agent Skills: Spring Boot 2 to 3 Migration Skill

Spring Boot 2 to 3 Migration Skill

Overview

This skill provides guidance for migrating Spring Boot applications from version 2.x to 3.x, which is one of the most significant upgrades in Spring Boot history due to the Java EE to Jakarta EE transition.

Key Migration Steps

1. Update Spring Boot Version

Change the parent POM version:

<!-- Before: Spring Boot 2.7.x -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.7.18</version>
</parent>

<!-- After: Spring Boot 3.2.x -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.2.0</version>
</parent>

2. Update Java Version

Spring Boot 3 requires Java 17 or later:

<properties>
    <!-- Before -->
    <java.version>1.8</java.version>

    <!-- After -->
    <java.version>21</java.version>
</properties>

3. Remove Deprecated Dependencies (CRITICAL: JAXB API)

You MUST remove the old javax.xml.bind:jaxb-api dependency. This is a Java EE dependency that was commonly added for Java 9+ compatibility in Spring Boot 2.x projects, but it conflicts with Jakarta EE in Spring Boot 3.

Dependencies to REMOVE from pom.xml

<!-- REMOVE ALL OF THESE - they are incompatible with Spring Boot 3 -->

<!-- Old JAXB API - MUST BE REMOVED -->
<dependency>
    <groupId>javax.xml.bind</groupId>
    <artifactId>jaxb-api</artifactId>
</dependency>

<!-- Old JAXB Implementation - MUST BE REMOVED -->
<dependency>
    <groupId>com.sun.xml.bind</groupId>
    <artifactId>jaxb-impl</artifactId>
</dependency>

<dependency>
    <groupId>com.sun.xml.bind</groupId>
    <artifactId>jaxb-core</artifactId>
</dependency>

<!-- Old Java Activation - MUST BE REMOVED -->
<dependency>
    <groupId>javax.activation</groupId>
    <artifactId>activation</artifactId>
</dependency>

<dependency>
    <groupId>javax.activation</groupId>
    <artifactId>javax.activation-api</artifactId>
</dependency>

Why Remove These?

1. Namespace Conflict: javax.xml.bind uses the old Java EE namespace, which conflicts with Jakarta EE's jakarta.xml.bind
2. Spring Boot 3 Includes Jakarta XML Bind: If you need XML binding, Spring Boot 3 transitively includes the Jakarta versions
3. Build Failures: Having both javax.xml.bind and jakarta.xml.bind on the classpath causes ClassNotFoundException and other runtime errors

If You Need XML Binding in Spring Boot 3

Use the Jakarta versions instead:

<!-- Only add these if you actually need XML binding -->
<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
</dependency>
<dependency>
    <groupId>org.glassfish.jaxb</groupId>
    <artifactId>jaxb-runtime</artifactId>
</dependency>

Quick Check for Old JAXB Dependencies

# Check if old JAXB is in your pom.xml
grep -E "javax\.xml\.bind|jaxb-api" pom.xml

# If this returns any results, you need to remove those dependencies

Verify Removal

After removing, confirm no old JAXB references remain:

# This should return NO results
grep -E "<artifactId>jaxb-api</artifactId>" pom.xml
grep -E "javax\.xml\.bind" pom.xml

4. Update JWT Library

The old jjwt library needs to be replaced with the newer modular version:

<!-- Before -->
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt</artifactId>
    <version>0.9.1</version>
</dependency>

<!-- After -->
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-api</artifactId>
    <version>0.12.3</version>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-impl</artifactId>
    <version>0.12.3</version>
    <scope>runtime</scope>
</dependency>
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-jackson</artifactId>
    <version>0.12.3</version>
    <scope>runtime</scope>
</dependency>

Common Issues

Issue 1: Compilation Errors After Upgrade

After changing the Spring Boot version, you'll see many compilation errors related to javax.* imports. These need to be changed to jakarta.* (see Jakarta Namespace skill).

Issue 2: H2 Database Dialect

The H2 dialect class name changed:

# Before
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect

# After
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
# Note: In Hibernate 6, this is often auto-detected and may not need explicit configuration

Issue 3: Actuator Endpoints

Actuator endpoint paths have changed. Review your security configuration if you're exposing actuator endpoints.

Migration Commands

Update pom.xml with sed

# Update Spring Boot parent version
sed -i 's/<version>2\.7\.[0-9]*<\/version>/<version>3.2.0<\/version>/g' pom.xml

# Update Java version
sed -i 's/<java.version>1\.8<\/java.version>/<java.version>21<\/java.version>/g' pom.xml
sed -i 's/<java.version>8<\/java.version>/<java.version>21<\/java.version>/g' pom.xml
sed -i 's/<java.version>11<\/java.version>/<java.version>21<\/java.version>/g' pom.xml

# Remove old JAXB dependency (multi-line removal is complex - manual removal recommended)

Remove Deprecated Dependencies

# Check for old JAXB dependencies
grep -n "jaxb-api\|javax\.xml\.bind" pom.xml

# These must be manually removed from pom.xml:
# - javax.xml.bind:jaxb-api
# - com.sun.xml.bind:jaxb-impl
# - com.sun.xml.bind:jaxb-core

Using OpenRewrite for Automated Migration

OpenRewrite is the recommended tool for large-scale migrations:

Add OpenRewrite Plugin to pom.xml

<plugin>
    <groupId>org.openrewrite.maven</groupId>
    <artifactId>rewrite-maven-plugin</artifactId>
    <version>5.42.0</version>
    <configuration>
        <activeRecipes>
            <recipe>org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_2</recipe>
        </activeRecipes>
    </configuration>
    <dependencies>
        <dependency>
            <groupId>org.openrewrite.recipe</groupId>
            <artifactId>rewrite-spring</artifactId>
            <version>5.21.0</version>
        </dependency>
    </dependencies>
</plugin>

Run OpenRewrite Migration

mvn rewrite:run

This will automatically:

• Update Spring Boot version
• Migrate javax.* to jakarta.* imports
• Update deprecated Spring Security patterns
• Fix property name changes

Verification Steps

Check Spring Boot Version

# Should show 3.x version
grep -A2 "spring-boot-starter-parent" pom.xml | grep version

Check Java Version

# Should show 17 or 21
grep "java.version" pom.xml

Check for Old Dependencies

# All should return NO results
grep "jaxb-api" pom.xml
grep "javax\.xml\.bind" pom.xml
grep "<version>0\.9\.1</version>" pom.xml  # Old jjwt version

Compile and Test

# Compile the project
mvn clean compile

# Run tests
mvn test

Migration Checklist

• [ ] Update spring-boot-starter-parent version to 3.2.x
• [ ] Update java.version to 17 or 21
• [ ] Remove javax.xml.bind:jaxb-api and related JAXB dependencies
• [ ] Remove old io.jsonwebtoken:jjwt if present, replace with modular jjwt
• [ ] Run mvn clean compile to identify remaining issues
• [ ] Fix all javax.* to jakarta.* imports
• [ ] Update Spring Security configuration (see Spring Security 6 skill)
• [ ] Replace RestTemplate with RestClient (see RestClient Migration skill)
• [ ] Run tests to verify functionality

Recommended Migration Order

1. First: Upgrade to Spring Boot 2.7.x (latest 2.x) if not already
2. Second: Update Java version to 17 or 21
3. Third: Remove incompatible dependencies (JAXB, old JWT)
4. Fourth: Update Spring Boot parent to 3.2.x
5. Fifth: Fix namespace imports (javax → jakarta)
6. Sixth: Update Spring Security configuration
7. Seventh: Update HTTP clients (RestTemplate → RestClient)
8. Finally: Run full test suite

Sources

• Spring Boot 3.0 Migration Guide
• OpenRewrite Spring Boot 3 Migration
• Baeldung - Migrate to Spring Boot 3