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Tooling advanced

AOT class preloading

Cache class loading and compilation for instant startup.

Code Comparison
✕ 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
Why the modern way wins
⚡

Faster startup

Skip class loading, verification, and linking.

📦

Cached state

Training run captures the ideal class state.

🔧

No code changes

Works with existing applications — just add JVM flags.

Old Approach
Cold Start Every Time
Modern Approach
AOT Cache
Since JDK
25
Difficulty
advanced
JDK Support
AOT class preloading
Available

Available in JDK 25 LTS (JEPs 514/515, Sept 2025). Experimental.

How it works

AOT class preloading caches loaded and linked classes from a training run. On subsequent starts, classes are loaded from the cache, skipping verification and linking. Combined with AOT compilation, this dramatically reduces startup time.

Related patterns

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]
Hover to see modern ➜
JDK 9+ →
Tooling beginner

Single-file execution

Java 8
$ javac HelloWorld.java
$ java HelloWorld
// Two steps every time
Java 11+
$ java HelloWorld.java
// Compiles and runs in one step
// Also works with shebangs:
#!/usr/bin/java --source 25
Hover to see modern ➜
JDK 11+ →
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+ →
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