What happen if you don't use @SpringBootApplication annotation.

If you don't use the @SpringBootApplication annotation in a Spring Boot project, you will lose several key features and conveniences that Spring Boot provides. Here's what happens and how it affects your application:

Error: org.springframework.context.ApplicationContextException: Unable to start web server; nested exception is org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.

What is @SpringBootApplication?

@SpringBootApplication is a combination of three annotations:

1. @SpringBootConfiguration: It is equivalent to @Configuration and allows you to define Spring beans using Java-based configuration.

2. @EnableAutoConfiguration: This enables Spring Boot's auto-configuration mechanism, which automatically configures many Spring components (e.g., web, security, data sources) based on classpath dependencies.

3. @ComponentScan: This enables component scanning, so Spring will automatically discover and register beans (like @Service, @Controller, etc.) in the package where the main class is located and its sub-packages.
   
   

What Happens Without @SpringBootApplication?

If you don't use @SpringBootApplication, here's what you will need to do manually:

1. No Auto-Configuration (@EnableAutoConfiguration)

Without @EnableAutoConfiguration, Spring Boot won't automatically configure components based on the classpath. This means:

• You'll have to manually configure beans for components like:

  • Data sources (JDBC, JPA)

  • Web server (like Tomcat or Jetty)

  • Message queues, caches, etc.

• Dependency injection and other conveniences provided by Spring Boot won't work without manual configuration.

Example: If you're using Spring Data JPA, without auto-configuration, you'll need to configure the EntityManager, transaction management, and database connection settings manually.

2. No Component Scanning (@ComponentScan)

Without @ComponentScan, Spring Boot won't automatically find and register components (e.g., @Controller, @Service, @Repository, etc.). You would have to manually specify the packages to scan for components.

Example:

import org.springframework.context.annotation.ComponentScan;

import org.springframework.context.annotation.Configuration;

@Configuration

@ComponentScan(basePackages = "com.example")

public class AppConfig {

    // Manually configure beans if needed

}

If you don't do this, Spring will not detect your classes annotated with @Controller, @Service, or @Repository, and they won't be registered as Spring beans.

3. No Configuration Class (@SpringBootConfiguration or @Configuration)

If you don't have @SpringBootConfiguration (which is itself a meta-annotation for @Configuration), Spring Boot will not know where to look for your application's configuration. You will need to add @Configuration manually to any class where you're defining beans.

Example:

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

@Configuration

public class MyConfig {

    @Bean

    public MyService myService() {

        return new MyService();

    }

}

Running the Application Without @SpringBootApplication

You can still run the application, but you'll need to manually configure everything that Spring Boot would normally handle for you.

Here’s how you would manually replace the functionality provided by @SpringBootApplication:

Add @EnableAutoConfiguration: If you still want Spring Boot's auto-configuration without using @SpringBootApplication, you can add @EnableAutoConfiguration to a configuration class manually:

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;

import org.springframework.context.annotation.ComponentScan;

import org.springframework.context.annotation.Configuration;

@Configuration

@EnableAutoConfiguration

@ComponentScan(basePackages = "com.example")  // Ensure to specify the correct package to scan

public class MySpringBootApplication {

    public static void main(String[] args) {

        SpringApplication.run(MySpringBootApplication.class, args);

    }

}

Add @ComponentScan: If you don’t use @SpringBootApplication, you must explicitly use @ComponentScan to tell Spring where to look for your components. This is critical for automatic bean discovery (like controllers and services).
java
Copy code
@ComponentScan(basePackages = "com.example")

Example Without @SpringBootApplication

Here's how your main application class might look like without @SpringBootApplication:

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;

import org.springframework.context.annotation.ComponentScan;

import org.springframework.context.annotation.Configuration;

@Configuration

@EnableAutoConfiguration  // Enables Spring Boot auto-configuration

@ComponentScan(basePackages = "com.example")  // Scans for @Controller, @Service, etc.

public class MySpringBootApplication {

    public static void main(String[] args) {

        SpringApplication.run(MySpringBootApplication.class, args);

    }

}

package com.pankaj;

import com.panku.HelloWorldController;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;

import org.springframework.context.annotation.ComponentScan;

@EnableAutoConfiguration

@ComponentScan(basePackages = {"com.panku", "com.pankaj"})

public class MySpringBootApplication {

   @Autowired

   private HelloWorldController helloWorldController;

   public static void main(String[] args) {

       SpringApplication.run(MySpringBootApplication.class, args);

   }

}

Summary

If you don’t use @SpringBootApplication, you lose the following conveniences:

1. Auto-Configuration: You'll need to manually configure things like the data source, message queues, security, web server, etc.

2. Component Scanning: You'll have to explicitly declare the package(s) to scan for Spring-managed components.

3. Configuration: You'll need to manually define your configuration classes using @Configuration.