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Spring Boot helps you build Java applications fast. It removes a lot of manual setup. You write less configuration and more business logic.

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What is Auto-Configuration?

Auto-configuration means:

Spring Boot automatically configures your app based on what it finds in your project.

It checks:

• Your dependencies (libraries)
• Your settings (application.properties / yaml)

Then it decides what to create and configure.

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Example

@SpringBootApplication

public class App {

    public static void main(String[] args) {

        SpringApplication.run(App.class, args);

    }

}

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What Gets Auto-Configured?

1. Web Server (Tomcat)

If you add:

spring-boot-starter-web

Spring Boot will:

• Start an embedded Tomcat server
• Set default port (8080)
• Configure basic web setup
• No need to install Tomcat manually.

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2. Dispatcher Servlet (Spring MVC)

Spring Boot auto-configures:

• DispatcherServlet
• Request mapping
• JSON handling (via Jackson)

Example controller:

@RestController

public class HelloController {

    @GetMapping(„/hello“)

    public String hello() {

        return „Hello“;

    }

}

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3. DataSource (Database)

If you add:

spring-boot-starter-data-jpa dependency in your project

And configure:

spring.datasource.url=jdbc:mysql://localhost:3306/test

spring.datasource.username=user

spring.datasource.password=pass

Spring Boot will:

• Create a DataSource
• Configure connection pool
• Setup Hibernate / JPA

     – No manual beans required.

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4. Entity Manager / JPA

Automatically configured with default values. You can also configure it in application yaml/properties:

• EntityManager
• Transaction management
• Hibernate settings

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5. Jackson (JSON)

Spring Boot auto-configures:

• JSON serialization/deserialization
• Converts objects ↔ JSON automatically

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6. Logging

Out of the box:

• Logging framework initialized
• Default log format
• No setup needed

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7. Configuration Properties

Spring Boot auto-binds properties:

server.port= 9090

– Changes server port without code.

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How Does It Decide?

Spring Boot uses:

• @Conditional rules
• Classpath detection

Example:

• If MySQL driver is present → configure DataSource
• If web dependency exists → configure web app

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Can You Override?

Yes

You can:

• Add your own configuration
• Override defaults

Example:

@Bean

public DataSource customDataSource() {

    // your config

}

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Summary

• Spring Boot reduces setup
• Auto-config creates common components
• You focus on business logic

      Key idea: “Convention over configuration”