[FIXED] Following Spring-Boot Intro, "Unable to find a suitable main class, please add a 'mainClass' property"

Issue

Following Spring-Boot tutorial, try to run with $ mvn spring-boot:run, gives me and error: "Unable to find a suitable main class..." How do I move past this?

(Working with Ubuntu)

I have followed this tutorial to try and work with Spring-Boot:

https://docs.spring.io/spring-boot/docs/current/reference/html/getting-started-first-application.html

up until 11.4. When I enter

$ mvn spring-boot:run

I end up with this error:

[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.1.3.RELEASE:run (default-cli) on project myproject: Unable to find a suitable main class, please add a 'mainClass' property 

I thought I'd followed every direction to the T. The Example.java, which is supposed to be running at this step, is located here,

folder/src/main/java/Example.java 

The one called "folder" has the pom file for maven, and everything else. Maven doesn't have any problem reading that pom file.

From the stack trace:

Caused by: org.apache.maven.plugin.MojoExecutionException: Unable to find a suitable main class, please add a 'mainClass' property

What do I do? All the other questions seem to have to do with the directory structure, or something more complicated. Where did I go wrong here?

Solution

I dont know what you did, it appears that you dont have a main function to start the application. I just made a project folder, then I created the pom.xml file with

<?xml version="1.0" encoding="UTF-8"?>
<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>com.example</groupId>
    <artifactId>myproject</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.3.RELEASE</version>
    </parent>

    <!-- Additional lines to be added here... -->

</project>

Then I modified my pom.xml to the content below

<?xml version="1.0" encoding="UTF-8"?>
<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>com.example</groupId>
    <artifactId>myproject</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.3.RELEASE</version>
    </parent>

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

    <!-- Additional lines to be added here... -->

</project>

And created my project/src/main/java/Example.java as follows (equals to the Exmaple.java from the reference)

import org.springframework.boot.*;
import org.springframework.boot.autoconfigure.*;
import org.springframework.web.bind.annotation.*;

@RestController
@EnableAutoConfiguration
public class Example {

    @RequestMapping("/")
    String home() {
        return "Hello World!";
    }

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

}

then I went to the project root directory and typed

$mvn spring-boot:run

and it worked.

Answered By - thiagoyf