[FIXED] Specifying Main class in pom.xml for spring boot projects in Kotlin

Issue

I have created a Spring Boot project using Kotlin. I would like to create a .jar file with all the dependencies so that I can run the application from the command line. The FQCN for the main class is: com.example.Application.kt. I have following configuration in my pom.xml:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <mainClass>com.example.Application</mainClass>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>

The application fails to start complaining that the mainClass was not found. Here is the sample exception:

Exception in thread "main" java.lang.ClassNotFoundException: com.example.Application
        at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:471)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
        at org.springframework.boot.loader.LaunchedURLClassLoader.loadClass(LaunchedURLClassLoader.java:93)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
        at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:46)
        at org.springframework.boot.loader.Launcher.launch(Launcher.java:87)
        at org.springframework.boot.loader.Launcher.launch(Launcher.java:51)
        at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:52)

What am I missing?

Solution

I am just attempting to learn Kotlin myself, but I suspect you may need to append Kt to the end of your classname. For example:

<manifest>
    <mainClass>com.example.ApplicationKt</mainClass>
</manifest>

Unrelated to Spring, I encountered this issue myself when attempting to build an executable JAR. If you unzip your JAR and look at the class files, I believe you will find an ApplicationKt.class.

If so, this is because you did not define the main function within a class. In this scenario, Kotlin generates a static method belonging to a <filename>Kt.class version of your file. According to this SO answer:

Unlike Java, Kotlin allows you to define functions that are not inside a class. But if you want Java to talk to Kotlin that becomes a problem. How to solve it? Convert those functions into static methods of a class named as the file with the "Kt" suffix

You can find more info about that in the documentation

Answered By - Miles_Dowe