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Tooling Intermediate

Multi-file source launcher

Launch multi-file programs without an explicit compile step.

Code Comparison
โœ• Java 8 
$ javac *.java
$ java Main
// Must compile all files first
// Need a build tool for dependencies
โœ“ 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
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Zero setup

No build tool needed for small multi-file programs.

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Auto-resolve

Referenced classes are found and compiled automatically.

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Script-like

Run multi-file programs like scripts.

Old Approach
Compile All First
Modern Approach
Source Launcher
Since JDK
22
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.

Related Documentation
Launch Multi-File Source-Code Programs (JEP 458) โ†—
Related patterns
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
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JDK 25+ โ†’
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+ โ†’
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+ โ†’
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