Specify main class for spring boot application jar

This is continuation to my previous post. In my previous post, I explained how to generate UBER jar for a spring boot application using spring boot maven plugin.

Suppose, if your application has more than one main class, how can you specify the main class to be executed in the application jar.

Using <configuration> element, you can specify the main class.

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <mainClass>com.sample.app.App1</mainClass>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>repackage</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Find the below working application.

App1.java

package com.sample.app;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class App1 {

    public static void main(String[] args) {
        System.out.println("I am in App1 class");
        SpringApplication.run(App1.class, args);
    }

}

App2.java

package com.sample.app;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class App2 {

    public static void main(String[] args) {
        System.out.println("I am in App2 class");
        SpringApplication.run(App1.class, args);
    }

}

HomeController.java

package com.sample.app.controller;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HomeController {

    @RequestMapping("/")
    public String homePage() {
        return "Welcome to Spring boot Application Development";
    }

}

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>springbootMisc</groupId>
    <artifactId>springbootMisc</artifactId>
    <version>1</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.6.RELEASE</version>
    </parent>

    <packaging>jar</packaging>

    <dependencies>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>


    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <mainClass>com.sample.app.App2</mainClass>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>repackage</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

Total project structure looks like below.

Generate UBER jar file

Right click on project.

Run As -> Run Configurations…

Mention ‘package’ in goals section and click on Run button.

It generates a jar file in project target directory.

$ls *jar

springbootMisc-1.jar

Run the jar file by executing the command ‘java -jar {jarFile}’

$java -jar springbootMisc-1.jar

You can see below messages in console.

$java -jar springbootMisc-1.jar
I am in App2 class

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/

You can download the complete working application from this link.

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