Java 25 Compact Source Files: Simplifying Micronaut Application Startup

Compact source file

In Java, a compact source file is a feature finalized in Java 25 (via JEP 512) that allows you to write a launchable Java program without explicitly declaring a class or a public static void main method. It is designed to minimize boilerplate code.

How this is useful with Micronaut application. When you create a project using micronaut application Micronaut

The project structure is always same for all the framework, either spring, micronuat, quarkus, helidon or something else

The traditional approach

package com.example;

import io.micronaut.runtime.Micronaut;

public class Application {

    public static void main(String[] args) {
        Micronaut.run(Application.class, args);
    }
}

The corresponding Gradle configuration commonly specifies the application's main class:build.gradle.kt

application {
    mainClass = "com.example.Application"
}

Using a Compact Source File
With Java 25, the same entry point can be expressed using a compact source file:

Application.java

import io.micronaut.runtime.Micronaut;

void main() {
    Micronaut.run();
}

There is no explicit:

• package declaration
• class declaration
• public static void main(...) method declaration

Configuring Micronaut

Because the compact source file does not declare a package, the fully qualified class name used in the Gradle configuration is no longer required.

application {
    mainClass = "Application"
}

The implicit class takes its name from the file name, so the file name and mainClass have to match exactly. Also, put the file in a source directory that isn't nested under a packages/ folder, or move it to the root of src/main/java. javac doesn't strictly enforce directory structure, but IDEs and incremental compilation behave better when the layout matches.

src/ 
└── main/ 
      └── java/ 
            └── Application.java