LaunchMultiFileDemo.md

Java 22 Launch Multi-File Source-Code Programs (JEP 458)

Overview

Java 22 allows you to launch multi-file Java programs directly without prior compilation. This simplifies development and testing by eliminating the need to compile multiple files separately.

Basic Usage

Example: Two-File Program

Main.java:

public class Main {
    public static void main(String[] args) {
        System.out.println("Main class");
        Helper.printMessage();
        System.out.println(Helper.getMessage());
    }
}

Helper.java:

public class Helper {
    public static void printMessage() {
        System.out.println("Hello from Helper!");
    }
    
    public static String getMessage() {
        return "Helper message";
    }
}

Running the Program

# Run directly without compilation
java Main.java Helper.java

# Or specify all files
java *.java

Key Features

• No compilation step: Java compiler runs automatically
• Multiple source files: Can specify multiple .java files
• Automatic dependency resolution: Compiler resolves dependencies
• Simplified development: Faster iteration during development

Use Cases

• Quick prototyping
• Testing multi-file programs
• Learning Java
• Small scripts with multiple classes
• Development and debugging

Benefits

• Faster development cycle
• No need for build tools for simple programs
• Easier to run examples
• Better for learning and experimentation

Limitations

• Not suitable for large projects
• No separate compilation step
• All files must be in the same directory (or specify paths)
• Slower for repeated runs (compiles each time)

Example Directory Structure

project/
├── Main.java
├── Helper.java
└── Utils.java

Run with:

java Main.java Helper.java Utils.java

Notes

• This feature works with the java launcher
• Files are compiled in memory
• Dependencies are automatically resolved
• Works with standard Java source files