Spring boot: How to define main application class?

In this post, you are going to learn,

a.   How to define main application class

b.   Example Application

By placing the @SpringBootApplication annotation on main class, you can tell spring boot about the main application class.

The definition of @ SpringBootApplication annotation is like below.

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@SpringBootConfiguration

@EnableAutoConfiguration

@ComponentScan(excludeFilters = {

                  @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

                  @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

public @interface SpringBootApplication {

}

As you see the above definition, SpringBootApplication is annotated with @SpringBootConfiguration, @EnableAutoConfiguration and @ComponentScan. It means, if you declare any class with the annotation @SpringBootApplication, then it automatically do components scan, enable auto configuration.

Spring recommends to define main application class in a root package of all the other classes.

For example, as you see above image, Application.java is defined in root package com.sample.myApp.

Example Application

Employee.java

package com.sample.myApp.model;

import org.springframework.stereotype.Component;

@Component
public class Employee {

}

EmployeeService.java

package com.sample.myApp.service;

import org.springframework.stereotype.Component;

@Component
public class EmployeeService {

}

The Application.java file would declare the main method, along with the basic @SpringBootApplication, as follows:

Application.java

package com.sample.myApp;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.domain.EntityScan;
import org.springframework.context.ApplicationContext;

@SpringBootApplication
public class Application {

 public static void main(String args[]) {
  ApplicationContext applicationContext = SpringApplication.run(Application.class, args);

  for (String name : applicationContext.getBeanDefinitionNames()) {
   System.out.println(name);
  }
 }
}

Run the application Application.java, you can able to see the bean names, employee, employeeService in the output.

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