Setting up a new Spring boot project is made trivial by the Spring Initializer. IDEs like IntelliJ has integrated support for this as well. The application generated by this initializer contains three files under src.
1) The application file (e.g. DemoAppliction.java) is Spring Boot main class that bootstraps the project.
2) A properties file (application.properties) is the application configuration file.
3) A test-case file (DemoApplictionTests.java).
The main application file looks like:
package com.example.demo;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class DemoApplication {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
}
The SpringApplication.run(DemoApplication.class, args) performs bootstrapping, creates application context, and runs the application.
The configuration file looks like:
spring.application.name=demo
The test-case looks like:
package com.example.demo;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;
@SpringBootTest
class DemoApplicationTests {
@Test
void contextLoads() {
}
}
This is in fact a simple yet very useful test class and I would keep it around. The is an integration test-case that ensures that the application context loads successfully. If there are any issues this test fails. So, this can be treated like a integration smoke-test for your application. I would rename that test method name to smokeTest_contextLoads().
Spring Code TIP - Code coverage
However, once you progress on with the application and have code coverage check applied using build system like maven and its plugins Surefire, Failsafe and JaCoCo plugins, the main method is left uncovered by this test-case. Little strange!
Having a main method in the application supported by integrated test-case, one expects that the main method is invoked by the test-case. The annotation @SpringBootTest by default has this turned off. Hence @SpringBootTest calls SpringApplication.run(DemoApplication.class, args) and not the main method. So, in order to get the main method called instead when the test-case is run, we need to explicitly set a property. Once this is set, the test-case invokes the main method and the main method gets the code coverage.
The property is shown below to get the test coverage for the main method.
@SpringBootTest(useMainMethod = SpringBootTest.UseMainMethod.ALWAYS)
Not just for the code coverage, you might also want to use the useMainMethod property in scenarios where your application's main method performs additional setup or configuration that is necessary for your tests as well.
Summary
The useMainMethod property of the @SpringBootTest annotation allows you to control whether the main method of your application is invoked to start the Spring Boot application context during testing. By default, useMainMethod is set to UseMainMethod.NEVER, but you can set it to UseMainMethod.ALWAYS or UseMainMethod.WHEN_AVAILABLE to ensure that the main method is called during testing.
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