How to test main class of Spring-boot application

All these answers seem overkill.
You don't add tests to make a metric tool happy.
Loading a Spring context of the application takes time. Don't add it in each developer build just to win about 0.1% of coverage in your application.
Here you don't cover only 1 statement from 1 public method. It represents nothing in terms of coverage in an application where thousands of statements are generally written.

First workaround : make your Spring Boot application class with no bean declared inside. If you have them, move them in a configuration class (for make them still cover by unit test). And then ignore your Spring Boot application class in the test coverage configuration.

Second workaround : if you really need to to cover the main() invocation (for organizational reasons for example), create a test for it but an integration test (executed by an continuous integration tool and not in each developer build) and document clearly the test class purpose :

import org.junit.Test;

// Test class added ONLY to cover main() invocation not covered by application tests.
public class MyApplicationIT {
   @Test
   public void main() {
      MyApplication.main(new String[] {});
   }
}

I solved in a different way here. Since this method is there only as a bridge to Spring's run, I annotated the method with @lombok.Generated and now sonar ignores it when calculating the test coverage.

Other @Generated annotations, like javax.annotation.processing.Generated or javax.annotation.Generated might also work but I can't test now because my issue ticket was closed.

package com.stackoverflow;

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

@SpringBootApplication
public class Application {

    @Generated
    public static void main(String... args) {
        SpringApplication.run(Application.class, args);
    }

}

I had the same goal (having a test that runs the main() method) and I noticed that simply adding a test method like @fg78nc said will in fact "start" the application twice : once by spring boot test framework, once via the explicit invocation of mainApp.main(new String[] {}), which I don't find elegant.

I ended up writing two test classes : one with @SpringBootTest annotation and the empty test method applicationContextLoaded(), another one without @SpringBootTest (only RunWith(SpringRunner.class)) that calls the main method.

SpringBootApplicationTest

package example;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.boot.test.context.SpringBootTest;

@RunWith(SpringRunner.class)
@SpringBootTest
public class SpringBootApplicationTest {

  @Test
  public void contextLoads() {
  }
}

ApplicationStartTest

package example;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
public class ApplicationStartTest {
  @Test
  public void applicationStarts() {
    ExampleApplication.main(new String[] {});
  }
}

Overall, the application is still started two times, but because there is now two test classes. Of course, with only these two tests methods, it seems overkill, but usually more tests will be added to the class SpringBootApplicationTest taking advantage of @SpringBootTest setup.

You can do something like this

@Test
public void applicationContextLoaded() {
}

@Test
public void applicationContextTest() {
    mainApp.main(new String[] {});
}