Java Notes

Spring Boot Introduction

Complete introduction to Spring Boot - auto-configuration, starter dependencies, embedded servers, creating your first Spring Boot application, and project structure.

What is Spring Boot?

Spring Boot is a framework built on top of Spring Framework that eliminates boilerplate configuration and lets you build production-ready applications quickly. It follows the principle of "Convention over Configuration".

Creating a Spring Boot Project

Method 1: Spring Initializr (https://start.spring.io)

Select:

Project: Maven
Language: Java
Spring Boot: 3.2.x
Dependencies: Spring Web, Spring Data JPA, MySQL Driver, Lombok

Method 2: Maven pom.xml

xml example

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
         https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.4</version>
    </parent>

    <groupId>com.example</groupId>
    <artifactId>demo-app</artifactId>
    <version>1.0.0</version>

    <properties>
        <java.version>17</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

Project Structure

src/ ├── main/ │ ├── java/ │ │ └── com/example/demo/ │ │ ├── DemoApplication.java ← Main class │ │ ├── controller/ │ │ │ └── UserController.java │ │ ├── service/ │ │ │ └── UserService.java │ │ ├── repository/ │ │ │ └── UserRepository.java │ │ ├── model/ │ │ │ └── User.java │ │ ├── dto/ │ │ │ ├── UserRequest.java │ │ │ └── UserResponse.java │ │ ├── exception/ │ │ │ └── GlobalExceptionHandler.java │ │ └── config/ │ │ └── AppConfig.java │ └── resources/ │ ├── application.properties ← Configuration │ ├── application-dev.properties │ ├── application-prod.properties │ ├── static/ ← Static files │ └── templates/ ← Thymeleaf templates └── test/ └── java/ └── com/example/demo/ └── DemoApplicationTests.java

Main Application Class

java example

package com.example.demo;

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

@SpringBootApplication  // = @Configuration + @EnableAutoConfiguration + @ComponentScan
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

What @SpringBootApplication Does

Component Annotation	Purpose
`@Configuration`	Marks class as bean definition source
`@EnableAutoConfiguration`	Enables auto-config based on classpath
`@ComponentScan`	Scans current package and sub-packages

application.properties

properties example

# Server Configuration
server.port=8080
server.servlet.context-path=/api

# Database Configuration
spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=password
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

# JPA Configuration
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.format_sql=true
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect

# Logging
logging.level.root=INFO
logging.level.com.example=DEBUG
logging.file.name=app.log

# Custom Properties
app.name=Demo Application
app.version=1.0.0

Building a Complete REST API

Model

java example

@Entity
@Table(name = "users")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(nullable = false)
    private String name;
    
    @Column(unique = true, nullable = false)
    private String email;
    
    private int age;
    
    @Column(name = "created_at")
    private LocalDateTime createdAt = LocalDateTime.now();
}

Repository

java example

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    List<User> findByAgeGreaterThan(int age);
    boolean existsByEmail(String email);
}

Service

java example

@Service
@RequiredArgsConstructor
public class UserService {

    private final UserRepository userRepository;

    public List<User> getAllUsers() {
        return userRepository.findAll();
    }

    public User getUserById(Long id) {
        return userRepository.findById(id)
            .orElseThrow(() -> new RuntimeException("User not found with id: " + id));
    }

    public User createUser(User user) {
        if (userRepository.existsByEmail(user.getEmail())) {
            throw new RuntimeException("Email already exists");
        }
        return userRepository.save(user);
    }

    public User updateUser(Long id, User userDetails) {
        User user = getUserById(id);
        user.setName(userDetails.getName());
        user.setEmail(userDetails.getEmail());
        user.setAge(userDetails.getAge());
        return userRepository.save(user);
    }

    public void deleteUser(Long id) {
        User user = getUserById(id);
        userRepository.delete(user);
    }
}

Controller

java example

@RestController
@RequestMapping("/api/users")
@RequiredArgsConstructor
public class UserController {

    private final UserService userService;

    @GetMapping
    public ResponseEntity<List<User>> getAllUsers() {
        return ResponseEntity.ok(userService.getAllUsers());
    }

    @GetMapping("/{id}")
    public ResponseEntity<User> getUserById(@PathVariable Long id) {
        return ResponseEntity.ok(userService.getUserById(id));
    }

    @PostMapping
    public ResponseEntity<User> createUser(@RequestBody User user) {
        User created = userService.createUser(user);
        return ResponseEntity.status(HttpStatus.CREATED).body(created);
    }

    @PutMapping("/{id}")
    public ResponseEntity<User> updateUser(@PathVariable Long id, @RequestBody User user) {
        return ResponseEntity.ok(userService.updateUser(id, user));
    }

    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
        userService.deleteUser(id);
        return ResponseEntity.noContent().build();
    }
}

Running the Application

bash example

# Using Maven
mvn spring-boot:run

# Using JAR
mvn clean package
java -jar target/demo-app-1.0.0.jar

# With profile
java -jar app.jar --spring.profiles.active=prod

# With custom port
java -jar app.jar --server.port=9090

Auto-Configuration Explained

When you add spring-boot-starter-web:

Tomcat is auto-configured as embedded server
DispatcherServlet is registered
Jackson is configured for JSON serialization
Error handling is set up

When you add spring-boot-starter-data-jpa:

DataSource is auto-configured from properties
EntityManagerFactory is created
Transaction management is enabled
Spring Data repositories are detected

Common Starter Dependencies

Starter	Purpose
`spring-boot-starter-web`	REST APIs and web apps
`spring-boot-starter-data-jpa`	Database access with JPA
`spring-boot-starter-security`	Authentication and authorization
`spring-boot-starter-validation`	Input validation
`spring-boot-starter-test`	Unit and integration testing
`spring-boot-starter-mail`	Email sending
`spring-boot-starter-actuator`	Production monitoring
`spring-boot-starter-cache`	Caching support

Interview Key Points

Spring Boot = Spring Framework + Auto-configuration + Embedded Server
@SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan
Auto-configuration works based on classpath dependencies
Embedded Tomcat eliminates need for external server
application.properties or application.yml for configuration
Spring Boot starters are curated dependency groups
Fat JAR includes all dependencies for standalone deployment
DevTools enables hot reload during development

Summary

In this chapter, we learned about Spring Boot Introduction in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Spring Boot Introduction.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

Spring FrameworkCRUD ApplicationBuild a complete CRUD application with Spring Boot - project setup, layered architecture, full CRUD operations, testing, and deployment.Spring FrameworkException Handling in Spring BootComplete guide to exception handling in Spring Boot - @ExceptionHandler, @ControllerAdvice, custom exceptions, error responses, and validation error handling.Spring FrameworkJWT Authentication — Java ProgrammingImplementing JWT authentication in Spring Boot - token generation, validation, refresh tokens, security filter, and complete authentication flow.Spring FrameworkREST API with Spring BootBuilding RESTful APIs with Spring Boot - REST principles, controller annotations, request/response handling, pagination, HATEOAS, and API versioning.Spring FrameworkSpring SecurityComplete guide to Spring Security - authentication, authorization, security filter chain, role-based access control, password encoding, and security configuration.Spring FrameworkSpring Data JPAComplete guide to Spring Data JPA - entity mapping, repository interfaces, query methods, JPQL, native queries, relationships, and database operations.

PreviousSpring Security NextSpring Data JPA

Java Notes

Spring Boot Introduction

Complete introduction to Spring Boot - auto-configuration, starter dependencies, embedded servers, creating your first Spring Boot application, and project structure.

What is Spring Boot?

Creating a Spring Boot Project

Method 1: Spring Initializr (https://start.spring.io)

Select:

Project: Maven
Language: Java
Spring Boot: 3.2.x
Dependencies: Spring Web, Spring Data JPA, MySQL Driver, Lombok

Method 2: Maven pom.xml

xml example

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
         https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.4</version>
    </parent>

    <groupId>com.example</groupId>
    <artifactId>demo-app</artifactId>
    <version>1.0.0</version>

    <properties>
        <java.version>17</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

Project Structure

Main Application Class

java example

package com.example.demo;

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

@SpringBootApplication  // = @Configuration + @EnableAutoConfiguration + @ComponentScan
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

What @SpringBootApplication Does

Component Annotation	Purpose
`@Configuration`	Marks class as bean definition source
`@EnableAutoConfiguration`	Enables auto-config based on classpath
`@ComponentScan`	Scans current package and sub-packages

application.properties

properties example

# Server Configuration
server.port=8080
server.servlet.context-path=/api

# Database Configuration
spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=password
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

# JPA Configuration
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.format_sql=true
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect

# Logging
logging.level.root=INFO
logging.level.com.example=DEBUG
logging.file.name=app.log

# Custom Properties
app.name=Demo Application
app.version=1.0.0

Building a Complete REST API

Model

java example

@Entity
@Table(name = "users")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(nullable = false)
    private String name;
    
    @Column(unique = true, nullable = false)
    private String email;
    
    private int age;
    
    @Column(name = "created_at")
    private LocalDateTime createdAt = LocalDateTime.now();
}

Repository

java example

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    List<User> findByAgeGreaterThan(int age);
    boolean existsByEmail(String email);
}

Service

java example

@Service
@RequiredArgsConstructor
public class UserService {

    private final UserRepository userRepository;

    public List<User> getAllUsers() {
        return userRepository.findAll();
    }

    public User getUserById(Long id) {
        return userRepository.findById(id)
            .orElseThrow(() -> new RuntimeException("User not found with id: " + id));
    }

    public User createUser(User user) {
        if (userRepository.existsByEmail(user.getEmail())) {
            throw new RuntimeException("Email already exists");
        }
        return userRepository.save(user);
    }

    public User updateUser(Long id, User userDetails) {
        User user = getUserById(id);
        user.setName(userDetails.getName());
        user.setEmail(userDetails.getEmail());
        user.setAge(userDetails.getAge());
        return userRepository.save(user);
    }

    public void deleteUser(Long id) {
        User user = getUserById(id);
        userRepository.delete(user);
    }
}

Controller

java example

@RestController
@RequestMapping("/api/users")
@RequiredArgsConstructor
public class UserController {

    private final UserService userService;

    @GetMapping
    public ResponseEntity<List<User>> getAllUsers() {
        return ResponseEntity.ok(userService.getAllUsers());
    }

    @GetMapping("/{id}")
    public ResponseEntity<User> getUserById(@PathVariable Long id) {
        return ResponseEntity.ok(userService.getUserById(id));
    }

    @PostMapping
    public ResponseEntity<User> createUser(@RequestBody User user) {
        User created = userService.createUser(user);
        return ResponseEntity.status(HttpStatus.CREATED).body(created);
    }

    @PutMapping("/{id}")
    public ResponseEntity<User> updateUser(@PathVariable Long id, @RequestBody User user) {
        return ResponseEntity.ok(userService.updateUser(id, user));
    }

    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
        userService.deleteUser(id);
        return ResponseEntity.noContent().build();
    }
}

Running the Application

bash example

# Using Maven
mvn spring-boot:run

# Using JAR
mvn clean package
java -jar target/demo-app-1.0.0.jar

# With profile
java -jar app.jar --spring.profiles.active=prod

# With custom port
java -jar app.jar --server.port=9090

Auto-Configuration Explained

When you add spring-boot-starter-web:

Tomcat is auto-configured as embedded server
DispatcherServlet is registered
Jackson is configured for JSON serialization
Error handling is set up

When you add spring-boot-starter-data-jpa:

DataSource is auto-configured from properties
EntityManagerFactory is created
Transaction management is enabled
Spring Data repositories are detected

Common Starter Dependencies

Starter	Purpose
`spring-boot-starter-web`	REST APIs and web apps
`spring-boot-starter-data-jpa`	Database access with JPA
`spring-boot-starter-security`	Authentication and authorization
`spring-boot-starter-validation`	Input validation
`spring-boot-starter-test`	Unit and integration testing
`spring-boot-starter-mail`	Email sending
`spring-boot-starter-actuator`	Production monitoring
`spring-boot-starter-cache`	Caching support

Interview Key Points

Spring Boot = Spring Framework + Auto-configuration + Embedded Server
@SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan
Auto-configuration works based on classpath dependencies
Embedded Tomcat eliminates need for external server
application.properties or application.yml for configuration
Spring Boot starters are curated dependency groups
Fat JAR includes all dependencies for standalone deployment
DevTools enables hot reload during development

Summary

In this chapter, we learned about Spring Boot Introduction in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Spring Boot Introduction.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

PreviousSpring Security NextSpring Data JPA