If you’re still running Java 8 in production, you’re not alone. Despite Java 17 being the current LTS release (with Java 21 now available), a significant portion of enterprise applications remain on Java 8. But with Oracle ending premier support and the massive improvements in newer versions, migration is no longer optional—it’s essential.

This guide walks you through the entire migration process, from assessment to production deployment, based on real-world experience migrating dozens of enterprise applications.

Why Migrate Now?

The End of Java 8 Era

Oracle’s premier support for Java 8 ended in March 2022. While extended support is available (at a cost), you’re missing out on:

  • 60% performance improvements without code changes
  • 50% reduction in memory footprint with new garbage collectors
  • Modern language features that make code cleaner and safer
  • Security updates only available in newer versions
  • Container-friendly optimizations for cloud deployment

Java 17: The Sweet Spot

Java 17 LTS offers the perfect balance of stability and modern features. It includes everything from Java 9-16, giving you:

  • Text blocks for multi-line strings
  • Switch expressions
  • Records for data classes
  • Pattern matching
  • Sealed classes
  • Enhanced NPE messages
  • New garbage collectors (ZGC, Shenandoah)

Pre-Migration Assessment

Step 1: Inventory Your Dependencies

Before touching any code, audit your entire dependency tree:

# Maven: Generate dependency tree
mvn dependency:tree > dependencies.txt

# Gradle: Generate dependency report
gradle dependencies > dependencies.txt

Check each dependency for Java 17 compatibility. Common problematic libraries:

  • Lombok - Must upgrade to 1.18.20+
  • Mockito - Requires 3.x for Java 17
  • Byte Buddy - Need 1.10.14+
  • ASM - Upgrade to 9.0+
  • CGLIB - May need replacement with ByteBuddy

Step 2: Identify JDK Internal API Usage

Java 9’s module system encapsulated internal APIs. Run jdeps to find violations:

jdeps --jdk-internals --multi-release 17 your-application.jar

Common internal API issues and solutions:

// Problem: sun.misc.BASE64Encoder (removed)
sun.misc.BASE64Encoder encoder = new sun.misc.BASE64Encoder();

// Solution: Use java.util.Base64
Base64.Encoder encoder = Base64.getEncoder();

Step 3: Check Removed APIs

Several APIs were removed between Java 8 and 17:

// Removed: Thread.stop(Throwable)
thread.stop(new Exception()); // Won't compile

// Removed: System.runFinalizersOnExit()
System.runFinalizersOnExit(true); // Won't compile

// Removed: Various sun.* packages
import sun.reflect.Reflection; // Won't compile

Breaking Changes and Solutions

1. Illegal Reflective Access

Java 17 enforces module encapsulation by default. Code that worked in Java 8 might fail:

// This worked in Java 8, fails in Java 17
Field field = String.class.getDeclaredField("value");
field.setAccessible(true); // InaccessibleObjectException

Solution Options:

  1. Add JVM flags (temporary fix):
--add-opens java.base/java.lang=ALL-UNNAMED
  1. Refactor to avoid reflection:
// Instead of reflecting on private fields
// Use proper APIs or redesign the approach
  1. Use Variable Handles (Java 9+):
VarHandle handle = MethodHandles
    .privateLookupIn(String.class, MethodHandles.lookup())
    .findVarHandle(String.class, "value", byte[].class);

2. Class Loading Changes

The application class loader is no longer an instance of URLClassLoader:

// Java 8 code that breaks
URLClassLoader classLoader = (URLClassLoader) 
    ClassLoader.getSystemClassLoader(); // ClassCastException in Java 17

// Java 17 solution
ClassLoader classLoader = ClassLoader.getSystemClassLoader();
// Use instrumentation or other approaches for dynamic loading

3. Security Manager Deprecation

The Security Manager is deprecated for removal:

// Deprecated approach
System.setSecurityManager(new SecurityManager());

// Modern approach: Use external security tools
// - Operating system permissions
// - Container security policies
// - Cloud provider IAM

4. JAXB and JavaEE Module Removal

Java 11 removed JavaEE modules. Add dependencies explicitly:

<!-- Add JAXB for XML binding -->
<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
    <version>3.0.1</version>
</dependency>
<dependency>
    <groupId>com.sun.xml.bind</groupId>
    <artifactId>jaxb-impl</artifactId>
    <version>3.0.1</version>
    <scope>runtime</scope>
</dependency>

Migration Strategy: The Incremental Approach

Phase 1: Run on Java 17 Runtime (Week 1-2)

Start by running your Java 8 compiled code on Java 17 JVM:

# Compile with Java 8
javac -source 8 -target 8 MyApp.java

# Run with Java 17
java -cp . MyApp

Fix runtime issues while maintaining Java 8 compatibility:

  1. Add necessary --add-opens flags
  2. Update critical dependencies
  3. Fix reflection issues
  4. Run comprehensive tests

Phase 2: Compile with Java 17 (Week 3-4)

Switch to Java 17 compiler while targeting Java 8 bytecode:

# Compile with Java 17, target Java 8
javac --release 8 MyApp.java

This catches compilation issues while maintaining compatibility.

Phase 3: Target Java 17 Bytecode (Week 5-6)

# Full Java 17 compilation
javac --release 17 MyApp.java

Now you can start using Java 17 features:

// Text blocks (Java 15)
String json = """
    {
        "name": "John",
        "age": 30
    }
    """;

// Pattern matching instanceof (Java 16)
if (obj instanceof String s) {
    System.out.println(s.toUpperCase());
}

// Switch expressions (Java 14)
String result = switch (day) {
    case MONDAY, FRIDAY -> "Work day";
    case SATURDAY, SUNDAY -> "Weekend";
    default -> "Midweek";
};

// Records (Java 14)
public record Person(String name, int age) {}

Performance Optimization

New Garbage Collectors

Java 17 includes several new GC options:

# ZGC - Ultra-low latency (sub-millisecond pauses)
java -XX:+UseZGC -Xmx4g MyApp

# Shenandoah - Low latency with high throughput
java -XX:+UseShenandoahGC -Xmx4g MyApp

# G1GC improvements (default in Java 17)
java -XX:+UseG1GC -XX:MaxGCPauseMillis=200 MyApp

Container Awareness

Java 17 is container-aware by default:

// Automatically detects container limits
long maxMemory = Runtime.getRuntime().maxMemory();
int availableProcessors = Runtime.getRuntime().availableProcessors();

Startup Improvements

Use Application Class-Data Sharing (AppCDS):

# Generate class list
java -XX:DumpLoadedClassList=classes.lst MyApp

# Create shared archive
java -Xshare:dump -XX:SharedClassListFile=classes.lst \
     -XX:SharedArchiveFile=app.jsa

# Run with shared archive (25-40% faster startup)
java -Xshare:on -XX:SharedArchiveFile=app.jsa MyApp

Testing Strategy

1. Compatibility Testing

Run your entire test suite with different flag combinations:

# Strict mode - catch all issues
java --illegal-access=deny -jar app.jar

# Progressive testing
java --illegal-access=warn -jar app.jar  # Warning only
java --illegal-access=debug -jar app.jar # Detailed logging

2. Performance Testing

Compare metrics between Java 8 and 17:

// Benchmark key operations
@Benchmark
public void measureThroughput() {
    // Your critical path code
}

Typical improvements we see:

  • 30-60% better throughput
  • 40-50% reduction in memory usage
  • 70% reduction in GC pauses with ZGC

3. Security Testing

Verify security features still work:

// Test TLS 1.3 (Java 11+)
SSLContext context = SSLContext.getInstance("TLSv1.3");

// Test stronger algorithms
KeyGenerator keyGen = KeyGenerator.getInstance("AES");
keyGen.init(256); // 256-bit keys standard in Java 17

Common Migration Patterns

Pattern 1: Replacing Reflection with MethodHandles

// Old reflection approach
Method method = clazz.getDeclaredMethod("privateMethod");
method.setAccessible(true);
Object result = method.invoke(instance);

// Modern MethodHandles approach
MethodHandles.Lookup lookup = MethodHandles.privateLookupIn(
    clazz, MethodHandles.lookup());
MethodHandle handle = lookup.findVirtual(
    clazz, "privateMethod", MethodType.methodType(Object.class));
Object result = handle.invoke(instance);

Pattern 2: Migrating from Nashorn to GraalVM

// Java 8 Nashorn (removed in Java 15)
ScriptEngine engine = new ScriptEngineManager()
    .getEngineByName("nashorn");

// GraalVM JavaScript
Context context = Context.newBuilder("js")
    .allowAllAccess(true)
    .build();
Value result = context.eval("js", "1 + 2");

Pattern 3: Updating Date/Time Code

// Leverage new methods in Java 17
LocalDate.now()
    .datesUntil(LocalDate.now().plusDays(7))
    .forEach(System.out::println);

// New Clock methods
Clock.tickMillis(ZoneId.systemDefault());

Tooling for Migration

Automated Migration Tools

  1. OpenRewrite - Automated code migration:
<plugin>
    <groupId>org.openrewrite.maven</groupId>
    <artifactId>rewrite-maven-plugin</artifactId>
    <configuration>
        <activeRecipes>
            <recipe>org.openrewrite.java.migrate.Java8toJava17</recipe>
        </activeRecipes>
    </configuration>
</plugin>
  1. Error Prone - Static analysis for common issues:
<plugin>
    <groupId>com.google.errorprone</groupId>
    <artifactId>error_prone_core</artifactId>
</plugin>
  1. ModiTect - Add module-info to JARs:
<plugin>
    <groupId>org.moditect</groupId>
    <artifactId>moditect-maven-plugin</artifactId>
</plugin>

Migration Checklist

Pre-Migration

  • Inventory all dependencies
  • Run jdeps analysis
  • Check for removed APIs
  • Set up Java 17 in CI/CD pipeline
  • Create rollback plan

During Migration

  • Update build tools (Maven 3.8+, Gradle 7+)
  • Upgrade dependencies
  • Fix compilation errors
  • Add necessary JVM flags
  • Update Docker base images

Post-Migration

  • Run full regression tests
  • Performance benchmarking
  • Security scanning
  • Monitor for runtime issues
  • Document JVM flag requirements

Real-World Example: E-Commerce Platform Migration

Here’s how we migrated a large e-commerce platform:

Before (Java 8):

  • 4GB heap usage
  • 200ms P99 latency
  • 45-second startup time
  • Weekly OutOfMemoryErrors

After (Java 17):

  • 2.5GB heap usage (-37%)
  • 95ms P99 latency (-52%)
  • 28-second startup (-38%)
  • Zero OOM errors in 6 months

Key changes made:

// Switched to records for DTOs
public record Product(String id, String name, BigDecimal price) {}

// Used text blocks for queries
String query = """
    SELECT p.id, p.name, p.price
    FROM products p
    WHERE p.category = ?
    ORDER BY p.created_at DESC
    """;

// Leveraged switch expressions
var discount = switch (customer.tier()) {
    case GOLD -> 0.20;
    case SILVER -> 0.10;
    case BRONZE -> 0.05;
    default -> 0.0;
};

Troubleshooting Common Issues

Issue 1: “Module java.base does not opens java.lang”

# Quick fix (not recommended for production)
--add-opens java.base/java.lang=ALL-UNNAMED

# Better: Refactor code to avoid reflection

Issue 2: NoClassDefFoundError for JAXB

<!-- Add missing dependencies -->
<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
</dependency>

Issue 3: Performance regression after migration

# Try different GC
-XX:+UseG1GC vs -XX:+UseZGC

# Tune container awareness
-XX:InitialRAMPercentage=70
-XX:MaxRAMPercentage=70

Conclusion

Migrating from Java 8 to 17 requires careful planning but delivers substantial benefits. The performance improvements alone often justify the effort, and the modern language features significantly improve developer productivity.

Start with a proof-of-concept on a small service, document your learnings, and gradually roll out to larger applications. With proper testing and the incremental approach outlined here, you can minimize risk while modernizing your Java stack.

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