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Tooling Advanced

Class-file parsing with the standard API

Use the standard Class-File API for class-file parsing and transformation.

Tooling

Code Comparison

✕ Third-party bytecode parser

ClassReader reader = new ClassReader(
        Files.readAllBytes(classFile));
reader.accept(visitor, 0);

✓ Java 24+

ClassModel model = ClassFile.of().parse(classFile);
model.methods().forEach(method ->
        System.out.println(
                method.methodName().stringValue()));

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Why the modern way wins

🛡

Supported API

Ships and evolves with the JDK class-file format.

📦

Fewer dependencies

Handles common bytecode tasks without an external library.

🧩

Composable models

Parsing, building, and transformation share one API.

Old Approach

ASM ClassReader

Modern Approach

Class-File API

Since JDK

Difficulty

Advanced

JDK Support

Class-file parsing with the standard API

Available

Finalized in JDK 24 (JEP 484, March 2025).

How it works

The Class-File API, finalized in JDK 24, provides supported models and transformations for JVM class files. It tracks the class-file format with the JDK and removes the need for third-party libraries in many inspection and generation tasks.

AOT class preloading

Java 8

// Every startup:
// - Load 10,000+ classes
// - Verify bytecode
// - JIT compile hot paths
// Startup: 2-5 seconds

Java 25

// Training run:
$ java -XX:AOTCacheOutput=app.aot \
    -cp app.jar com.App
// Production:
$ java -XX:AOTCache=app.aot \
    -cp app.jar com.App

Hover to see modern ➜

JDK 25+ →

Tooling Advanced

Compact object headers

Java 8

// Default: 128-bit object header
// = 16 bytes overhead per object
// A boolean field object = 32 bytes!
// Mark word (64) + Klass pointer (64)

Java 25

// -XX:+UseCompactObjectHeaders
// 64-bit object header
// = 8 bytes overhead per object
// 50% less header memory
// More objects fit in cache

Hover to see modern ➜

JDK 25+ →

Tooling Intermediate

Multi-file source launcher

Java 8

$ javac *.java
$ java Main
// Must compile all files first
// Need a build tool for dependencies

Java 22+

$ java Main.java
// Automatically finds and compiles
// other source files referenced
// by Main.java

Hover to see modern ➜

JDK 22+ →