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Tooling Intermediate

Multi-file source launcher

Launch multi-file programs without an explicit compile step.

Tooling

Code Comparison

✕ 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

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Why the modern way wins

🚀

Zero setup

No build tool needed for small multi-file programs.

🔗

Auto-resolve

Referenced classes are found and compiled automatically.

📝

Script-like

Run multi-file programs like scripts.

Old Approach

Compile All First

Modern Approach

Source Launcher

Since JDK

Difficulty

Intermediate

JDK Support

Multi-file source launcher

Available

Available since JDK 22 (March 2024)

How it works

Java 22+ can automatically compile referenced source files when launching from a .java file. This makes small multi-file programs as easy to run as scripts, without needing Maven or Gradle.

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

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JDK 25+ →

Tooling Advanced

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

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JDK 25+ →

Tooling Beginner

JShell for prototyping

Java 8

// 1. Create Test.java
// 2. javac Test.java
// 3. java Test
// Just to test one expression!

Java 9+

$ jshell
jshell> "hello".chars().count()
$1 ==> 5
jshell> List.of(1,2,3).reversed()
$2 ==> [3, 2, 1]

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JDK 9+ →