How do I tell Spring Boot which main class to use for the executable jar?

Question

Execution default of goal 
org.springframework.boot:spring-boot-maven-plugin:1.0.1.RELEASE:repackage 
failed: 
Unable to find a single main class from the following candidates

My project has more than one class with a main method. How do I tell the Spring Boot Maven plugin which of the classes it should use as the main class?

Solution 1

Add your start class in your pom:

<properties>
    <!-- The main class to start by executing java -jar -->
    <start-class>com.mycorp.starter.HelloWorldApplication</start-class>
</properties>

or

<build>
<plugins>
    <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>             
        <configuration>    
            <mainClass>com.mycorp.starter.HelloWorldApplication</mainClass>
        </configuration>
    </plugin>
</plugins>
</build>

Solution 2

For those using Gradle (instead of Maven) :

springBoot {
    mainClass = "com.example.Main"
}

Solution 3

If you do NOT use the spring-boot-starter-parent pom, then from the Spring documentation:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <version>1.1.3.RELEASE</version>
    <configuration>
        <mainClass>my.package.MyStartClass</mainClass>
        <layout>ZIP</layout>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Solution 4

For those using Gradle (instead of Maven), referencing here:

The main class can also be configured explicitly using the task’s mainClassName property:

bootJar {
    mainClass = 'com.example.ExampleApplication'
}

Alternatively, the main class name can be configured project-wide using the mainClassName property of the Spring Boot DSL:

springBoot {
    mainClass = 'com.example.ExampleApplication'
}

Solution 5

If you're using spring-boot-starter-parent in your pom, you simply add the following to your pom:

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

Then do your mvn package.

See this Spring docs page.

A very important aspect here is to mention that the directory structure has to be src/main/java/nameofyourpackage

View more solutions

Comments

[Evgeni Dimitrov]

java -cp myjar.jar MyClass

[hansvb]

@Evgeni: That's a runtime flag. It doesn't get that far. It fails to build.

[Dave Syer]

No need to disable anything. Or am I missing something?

[Dave Syer]

Note that this answer is correct if you use the spring-boot-starter-parent pom. In that case the "start-class" property is applied to the "mainClass" configuration parameter of the spring-boot-maven-plugin (which you can do directly if you aren't using the starter).

[Admin]

I found this solution to work without modifying the pom.xml once I replicated the package requirements for the .java classes.

[zhuguowei]

Thanks @ludo_rj, and I found this also works: mvn clean package -Dstart-class=com.foo.Application, if want to dynamically specify using which main class

[Cheloute]

I tried to use the spring-boot-maven-plugin configuration you mentionned in my multimodules maven project, composed by several Spring boot projects and including Spring Boot as BOM dependency, and it worked like a charm. Concerning the maven-compiler-plugin, I didn't specify anything as I don't want my POM platform dependent. Maven automatically forked, so I think you can just ignore this configuration.

[Thunderforge]

Spring Boot 2.x gives an error Could not set unknown property 'mainClass' for object of type org.springframework.boot.gradle.dsl.SpringBootExtension.

[Vitalik]

The answer is down this page: stackoverflow.com/a/49716696/75672

[Gerardo Roza]

One more thing to add, the parameter mentioned by @zhuguowei is also valid for the Spring Boot Maven Plugin: mvn spring-boot:run -Dstart-class=com.foo.Application. This is valid only if you haven't specified the mainClass in the pom's plugin

[zhaozhi]

main class config can be omitted

[Renaud]

@zhaozhi could you explain why / how?

[Angel O'Sphere]

Both did not work for me. I also thought it was an "AND" not an or? I see the Start-Class: correctly in the MANIFEST.MF, but spring starts a different annotated @SpringBootApplication main class. I actually need that class for bootstrapping some things so I do not really like to change the annotation. Simply removing it did not work anyway. Spring seems to start the first main() it finds. I'm using spring-boot-starter-parent 2.2.0.M3.