Java 17 Features

Java 17 is a Long-Term Support (LTS) release, making it a popular choice for production systems. It brings important language improvements, security enhancements, and performance/runtime upgrades.

1) Sealed Classes (Final) — Control Which Classes Can Extend You

Sealed classes let you restrict inheritance to a fixed set of subclasses. This makes your domain model safer and easier to reason about.

✅ Example: Sealed hierarchy

public sealed interface Shape permits Circle, Rectangle { }

public final class Circle implements Shape {
    double radius;
    public Circle(double radius) { this.radius = radius; }
}

public final class Rectangle implements Shape {
    double w, h;
    public Rectangle(double w, double h) { this.w = w; this.h = h; }
}

Why it matters

  • Prevents “random” classes from extending your base type
  • Great for domain modeling (Payments, States, Events, etc.)
  • Works beautifully with pattern matching (next section)

2) Pattern Matching for switch (Preview) — Cleaner Type Checks in switch

Java 17 improves switch so you can switch on types (preview feature in Java 17).

To run preview features:
--enable-preview --release 17

✅ Example: Type patterns in switch

static String describe(Object obj) {
    return switch (obj) {
        case Integer i -> "int: " + i;
        case String s -> "string: " + s;
        case null -> "null value";
        default -> "something else";
    };
}

Why it matters

  • Removes repetitive instanceof + casting
  • Makes code more readable and less error-prone

3) Strong Encapsulation of JDK Internals (Important Change)

Java 17 strongly encapsulates internal JDK APIs (like sun.misc.Unsafe). Many reflective hacks that worked before may break unless you open modules explicitly.

What you may see

  • InaccessibleObjectException
  • Reflection access warnings/errors

Fix (when you absolutely must)

Use JVM flags (as a temporary workaround):

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

Best practice

  • Prefer supported APIs and library upgrades instead of JVM opens.
  • Update frameworks/libraries to Java 17 compatible versions.

4) New Random Number Generators (Standard) — Better RNG Options

Java 17 adds new interfaces and implementations under java.util.random, offering better algorithms and flexibility.

✅ Example: RandomGenerator

import java.util.random.RandomGenerator;

public class RngDemo {
    public static void main(String[] args) {
        RandomGenerator rng = RandomGenerator.getDefault();
        System.out.println(rng.nextInt(1, 101)); // 1..100
    }
}

Why it matters

  • Better quality randomness and more choices than java.util.Random
  • Useful in simulations, testing, ML, games

5) Foreign Function & Memory API (Incubator) — Safer Native Interop (Early)

This incubator API is a modern approach to calling native libraries (C/C++) and managing off-heap memory.

This is incubator in Java 17, so it’s not final yet and requires special flags.

Why it matters

  • Long-term replacement direction for JNI
  • Safer and more performant native integration (eventually)

(Since it’s incubator and verbose, keep examples minimal in a blog unless you’re writing a deep dive.)

6) Context-Specific Deserialization Filters — Improve Security

Java has a history of deserialization risks. Java 17 makes it easier to apply deserialization filters at a more granular level.

Why it matters

  • Helps defend against deserialization gadgets
  • Useful for enterprise apps using serialization (still common in legacy systems)

7) Remove RMI Activation (Cleanup)

RMI Activation has been removed. If you use old RMI activation features, you’ll need an alternative approach.

Why it matters

  • Less legacy baggage
  • Reduced attack surface and maintenance overhead

8) macOS Rendering Pipeline Change (Metal)

Java 17 uses the Metal API for macOS graphics rendering (instead of OpenGL). Mostly relevant if you build desktop UIs.

Why it matters

  • Better performance and future compatibility on macOS

9) Performance: Improved Garbage Collectors (G1/ZGC Improvements)

Java 17 continues improvements to:

  • G1 GC (default)
  • ZGC (low latency, improved)
  • general JVM performance tuning

Practical note

If you run high-throughput services (Spring Boot microservices, APIs), Java 17 typically gives better performance than older LTS versions with minimal code changes—after library upgrades.

Java 17 Upgrade Checklist (Real-World)

✅ Step 1: Use the right build configuration

Maven

<properties>
  <maven.compiler.release>17</maven.compiler.release>
</properties>

Gradle

java {
  toolchain {
    languageVersion = JavaLanguageVersion.of(17)
  }
}

✅ Step 2: Update libraries/frameworks

  • Spring Boot / Hibernate / Jackson / Mockito / Lombok etc.
  • Watch for reflection-based libs needing updates

✅ Step 3: Run tests with illegal-access issues in mind

If you see module access errors, update libs first before adding --add-opens.

Java 17 Feature Summary (Quick Table)

  • Sealed classes ✅ Final
  • Pattern matching for switch 🧪 Preview
  • New RandomGenerator APIs ✅ Final
  • JDK internal encapsulation ✅ Major behavior change
  • Foreign Function & Memory API 🧪 Incubator
  • Security deserialization filters ✅ Improved
  • RMI Activation removed ✅ Cleanup
  • macOS Metal pipeline ✅ Runtime improvement

Conclusion

Java 17 is a strong production-friendly LTS release with safer modeling (sealed classes), cleaner code patterns (switch type matching), stronger security defaults, and runtime improvements.

If you’re coming from Java 8/11, plan for dependency upgrades and possible module access issues, and you’ll get a solid long-term platform.