Java Migration Skill

Step-by-step guide for upgrading Java projects between major versions.

When to Use

User says "upgrade to Java 25" / "migrate from Java 8" / "update Java version"
Modernizing legacy projects
Spring Boot 2.x → 3.x → 4.x migration
Preparing for LTS version adoption

Migration Paths

Java 8 (LTS) → Java 11 (LTS) → Java 17 (LTS) → Java 21 (LTS) → Java 25 (LTS)
     │              │               │              │               │
     └──────────────┴───────────────┴──────────────┴───────────────┘
                         Always migrate LTS → LTS

Quick Reference: What Breaks

From → To	Major Breaking Changes
8 → 11	Removed `javax.xml.bind`, module system, internal APIs
11 → 17	Sealed classes (preview→final), strong encapsulation
17 → 21	Pattern matching changes, `finalize()` deprecated for removal
21 → 25	Security Manager removed, Unsafe methods removed, 32-bit dropped

Migration Workflow

Step 1: Assess Current State

bash

# Check current Java version
java -version

# Check compiler target in Maven
grep -r "maven.compiler" pom.xml

# Find usage of removed APIs
grep -r "sun\." --include="*.java" src/
grep -r "javax\.xml\.bind" --include="*.java" src/

Step 2: Update Build Configuration

Maven:

xml

<properties>
    <java.version>21</java.version>
    <maven.compiler.source>${java.version}</maven.compiler.source>
    <maven.compiler.target>${java.version}</maven.compiler.target>
</properties>

<!-- Or with compiler plugin -->
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.12.1</version>
    <configuration>
        <release>21</release>
    </configuration>
</plugin>

Gradle:

groovy

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(21)
    }
}

Step 3: Fix Compilation Errors

Run compile and fix errors iteratively:

bash

mvn clean compile 2>&1 | head -50

Step 4: Run Tests

bash

mvn test

Step 5: Check Runtime Warnings

bash

# Run with illegal-access warnings
java --illegal-access=warn -jar app.jar

Java 8 → 11 Migration

Removed APIs

Removed	Replacement
`javax.xml.bind` (JAXB)	Add dependency: `jakarta.xml.bind-api` + `jaxb-runtime`
`javax.activation`	Add dependency: `jakarta.activation-api`
`javax.annotation`	Add dependency: `jakarta.annotation-api`
`java.corba`	No replacement (rarely used)
`java.transaction`	Add dependency: `jakarta.transaction-api`
`sun.misc.Base64*`	Use `java.util.Base64`
`sun.misc.Unsafe` (partially)	Use `VarHandle` where possible

Add Missing Dependencies (Maven)

xml

<!-- JAXB (if needed) -->
<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
    <version>4.0.1</version>
</dependency>
<dependency>
    <groupId>org.glassfish.jaxb</groupId>
    <artifactId>jaxb-runtime</artifactId>
    <version>4.0.4</version>
    <scope>runtime</scope>
</dependency>

<!-- Annotation API -->
<dependency>
    <groupId>jakarta.annotation</groupId>
    <artifactId>jakarta.annotation-api</artifactId>
    <version>2.1.1</version>
</dependency>

Module System Issues

If using reflection on JDK internals, add JVM flags:

bash

--add-opens java.base/java.lang=ALL-UNNAMED
--add-opens java.base/java.util=ALL-UNNAMED

Maven Surefire:

xml

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <argLine>
            --add-opens java.base/java.lang=ALL-UNNAMED
        </argLine>
    </configuration>
</plugin>

New Features to Adopt

java

// var (local variable type inference)
var list = new ArrayList<String>();  // instead of ArrayList<String> list = ...

// String methods
"  hello  ".isBlank();      // true for whitespace-only
"  hello  ".strip();        // better trim() (Unicode-aware)
"line1\nline2".lines();     // Stream<String>
"ha".repeat(3);             // "hahaha"

// Collection factory methods (Java 9+)
List.of("a", "b", "c");     // immutable list
Set.of(1, 2, 3);            // immutable set
Map.of("k1", "v1");         // immutable map

// Optional improvements
optional.ifPresentOrElse(
    value -> process(value),
    () -> handleEmpty()
);

// HTTP Client (replaces HttpURLConnection)
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.example.com"))
    .build();
HttpResponse<String> response = client.send(request,
    HttpResponse.BodyHandlers.ofString());

Java 11 → 17 Migration

Breaking Changes

Change	Impact
Strong encapsulation	`--illegal-access` no longer works, must use explicit `--add-opens`
Sealed classes (final)	If you used preview features
Pattern matching instanceof	Preview → final syntax change

New Features to Adopt

java

// Records (immutable data classes)
public record User(String name, String email) {}
// Auto-generates: constructor, getters, equals, hashCode, toString

// Sealed classes
public sealed class Shape permits Circle, Rectangle {}
public final class Circle extends Shape {}
public final class Rectangle extends Shape {}

// Pattern matching for instanceof
if (obj instanceof String s) {
    System.out.println(s.length());  // s already cast
}

// Switch expressions
String result = switch (day) {
    case MONDAY, FRIDAY -> "Work";
    case SATURDAY, SUNDAY -> "Rest";
    default -> "Midweek";
};

// Text blocks
String json = """
    {
        "name": "John",
        "age": 30
    }
    """;

// Helpful NullPointerException messages
// a.b.c.d() → tells exactly which part was null

Java 17 → 21 Migration

Breaking Changes

Change	Impact
Pattern matching switch (final)	Minor syntax differences from preview
`finalize()` deprecated for removal	Replace with `Cleaner` or try-with-resources
UTF-8 by default	May affect file reading if assumed platform encoding

New Features to Adopt

java

// Virtual Threads (Project Loom) - MAJOR
try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> handleRequest());
}
// Or simply:
Thread.startVirtualThread(() -> doWork());

// Pattern matching in switch
String formatted = switch (obj) {
    case Integer i -> "int: " + i;
    case String s -> "string: " + s;
    case null -> "null value";
    default -> "unknown";
};

// Record patterns
record Point(int x, int y) {}
if (obj instanceof Point(int x, int y)) {
    System.out.println(x + ", " + y);
}

// Sequenced Collections
List<String> list = new ArrayList<>();
list.addFirst("first");    // new method
list.addLast("last");      // new method
list.reversed();           // reversed view

// String templates (preview in 21)
// May need --enable-preview

// Scoped Values (preview) - replace ThreadLocal
ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();
ScopedValue.where(CURRENT_USER, user).run(() -> {
    // CURRENT_USER.get() available here
});

Java 21 → 25 Migration

Breaking Changes

Change	Impact
Security Manager removed	Applications relying on it need alternative security approaches
`sun.misc.Unsafe` methods removed	Use `VarHandle` or FFM API instead
32-bit platforms dropped	No more x86-32 support
Record pattern variables final	Cannot reassign pattern variables in switch
`ScopedValue.orElse(null)` disallowed	Must provide non-null default
Dynamic agents restricted	Requires `-XX:+EnableDynamicAgentLoading` flag

Check for Unsafe Usage

bash

# Find sun.misc.Unsafe usage
grep -rn "sun\.misc\.Unsafe" --include="*.java" src/

# Find Security Manager usage
grep -rn "SecurityManager\|System\.getSecurityManager" --include="*.java" src/

New Features to Adopt

java

// Scoped Values (FINAL in Java 25) - replaces ThreadLocal
private static final ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();

public void handleRequest(User user) {
    ScopedValue.where(CURRENT_USER, user).run(() -> {
        processRequest();  // CURRENT_USER.get() available here and in child threads
    });
}

// Structured Concurrency (Preview, redesigned API in 25)
try (StructuredTaskScope.ShutdownOnFailure scope = StructuredTaskScope.open()) {
    Subtask<User> userTask = scope.fork(() -> fetchUser(id));
    Subtask<Orders> ordersTask = scope.fork(() -> fetchOrders(id));

    scope.join();
    scope.throwIfFailed();

    return new Profile(userTask.get(), ordersTask.get());
}

// Stable Values (Preview) - lazy initialization made easy
private static final StableValue<ExpensiveService> SERVICE =
    StableValue.of(() -> new ExpensiveService());

public void useService() {
    SERVICE.get().doWork();  // Initialized on first access, cached thereafter
}

// Compact Object Headers - automatic, no code changes
// Objects now use 64-bit headers instead of 128-bit (less memory)

// Primitive Patterns in instanceof (Preview)
if (obj instanceof int i) {
    System.out.println("int value: " + i);
}

// Module Import Declarations (Preview)
import module java.sql;  // Import all public types from module

Performance Improvements (Automatic)

Java 25 includes several automatic performance improvements:

Compact Object Headers: 8 bytes instead of 16 bytes per object
String.hashCode() constant folding: Faster Map lookups with String keys
AOT class loading: Faster startup with ahead-of-time cache
Generational Shenandoah GC: Better throughput, lower pauses

Migration with OpenRewrite

bash

# Automated Java 25 migration
mvn -U org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-migrate-java:LATEST \
  -Drewrite.activeRecipes=org.openrewrite.java.migrate.UpgradeToJava25

Spring Boot Migration

Spring Boot 2.x → 3.x

Requirements:

Java 17+ (mandatory)
Jakarta EE 9+ (javax.* → jakarta.*)

Package Renames:

java

// Before (Spring Boot 2.x)
import javax.persistence.*;
import javax.validation.*;
import javax.servlet.*;

// After (Spring Boot 3.x)
import jakarta.persistence.*;
import jakarta.validation.*;
import jakarta.servlet.*;

Find & Replace:

bash

# Find all javax imports that need migration
grep -r "import javax\." --include="*.java" src/ | grep -v "javax.crypto" | grep -v "javax.net"

Automated migration:

bash

# Use OpenRewrite
mvn -U org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-spring:LATEST \
  -Drewrite.activeRecipes=org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_0

Dependency Updates (Spring Boot 3.x)

xml

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.2.2</version>
</parent>

<!-- Hibernate 6 (auto-included) -->
<!-- Spring Security 6 (auto-included) -->

Hibernate 5 → 6 Changes

java

// ID generation strategy changed
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)  // preferred
private Long id;

// Query changes
// Before: createQuery returns raw type
// After: createQuery requires type parameter

// Before
Query query = session.createQuery("from User");

// After
TypedQuery<User> query = session.createQuery("from User", User.class);

Common Migration Issues

Issue: Reflection Access Denied

Symptom:

java.lang.reflect.InaccessibleObjectException: Unable to make field accessible

Fix:

bash

--add-opens java.base/java.lang=ALL-UNNAMED
--add-opens java.base/java.lang.reflect=ALL-UNNAMED

Issue: JAXB ClassNotFoundException

Symptom:

java.lang.ClassNotFoundException: javax.xml.bind.JAXBContext

Fix: Add JAXB dependencies (see Java 8→11 section)

Issue: Lombok Not Working

Fix: Update Lombok to latest version:

xml

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.30</version>
</dependency>

Issue: Test Failures with Mockito

Fix: Update Mockito:

xml

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>5.8.0</version>
    <scope>test</scope>
</dependency>

Migration Checklist

Pre-Migration

Document current Java version
List all dependencies and their versions
Identify usage of internal APIs (sun.*, com.sun.*)
Check framework compatibility (Spring, Hibernate, etc.)
Backup / create branch

During Migration

Update build tool configuration
Add missing Jakarta dependencies
Fix javax.* → jakarta.* imports (if Spring Boot 3)
Add --add-opens flags if needed
Update Lombok, Mockito, other tools
Fix compilation errors
Run tests

Post-Migration

Remove unnecessary --add-opens flags
Adopt new language features (records, var, etc.)
Update CI/CD pipeline
Document changes made

Quick Commands

bash

# Check Java version
java -version

# Find internal API usage
grep -rn "sun\.\|com\.sun\." --include="*.java" src/

# Find javax imports (for Jakarta migration)
grep -rn "import javax\." --include="*.java" src/

# Compile and show first errors
mvn clean compile 2>&1 | head -100

# Run with verbose module warnings
java --illegal-access=debug -jar app.jar

# OpenRewrite Spring Boot 3 migration
mvn org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-spring:LATEST \
  -Drewrite.activeRecipes=org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_0

Version Compatibility Matrix

Framework	Java 8	Java 11	Java 17	Java 21	Java 25
Spring Boot 2.7.x	✅	✅	✅	⚠️	❌
Spring Boot 3.2.x	❌	❌	✅	✅	✅
Spring Boot 3.4+	❌	❌	✅	✅	✅
Hibernate 5.6	✅	✅	✅	⚠️	❌
Hibernate 6.4+	❌	❌	✅	✅	✅
JUnit 5.10+	✅	✅	✅	✅	✅
Mockito 5+	❌	✅	✅	✅	✅
Lombok 1.18.34+	✅	✅	✅	✅	✅

LTS Support Timeline:

Java 21: Oracle free support until September 2028
Java 25: Oracle free support until September 2033

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SKILL.md

Java Migration Skill

When to Use

Migration Paths

Quick Reference: What Breaks

Migration Workflow

Step 1: Assess Current State

Step 2: Update Build Configuration

Step 3: Fix Compilation Errors

Step 4: Run Tests

Step 5: Check Runtime Warnings

Java 8 → 11 Migration

Removed APIs

Add Missing Dependencies (Maven)

Module System Issues

New Features to Adopt

Java 11 → 17 Migration

Breaking Changes

New Features to Adopt

Java 17 → 21 Migration

Breaking Changes

New Features to Adopt

Java 21 → 25 Migration

Breaking Changes

Check for Unsafe Usage

New Features to Adopt

Performance Improvements (Automatic)

Migration with OpenRewrite

Spring Boot Migration

Spring Boot 2.x → 3.x

Dependency Updates (Spring Boot 3.x)

Hibernate 5 → 6 Changes

Common Migration Issues

Issue: Reflection Access Denied

Issue: JAXB ClassNotFoundException

Issue: Lombok Not Working

Issue: Test Failures with Mockito

Migration Checklist

Pre-Migration

During Migration

Post-Migration

Quick Commands

Version Compatibility Matrix