Controllers

By / 4 October 2026 / 5 minutes of reading

A controller is the class that receives HTTP requests from clients, decides what to do with them, and returns a response. Spring maps each URL and HTTP method (GET, POST, PUT, DELETE) to a controller method using annotations like @GetMapping and @PostMapping. It is used to keep the web layer separate: the controller only handles request and response, and passes the real work to a service. Using one controller per resource (Student, Course) keeps each URL group and its responsibility clear.

Types

  • @Controller: returns view names for HTML pages (Spring MVC with Thymeleaf).
  • @RestController: returns JSON directly (@Controller + @ResponseBody), used in this example.
  • Mapping annotations: @RequestMapping (class level), @GetMapping, @PostMapping, @PutMapping, @DeleteMapping, @PatchMapping.
  • Input annotations: @PathVariable (/students/1), @RequestParam (?name=ravi), @RequestBody (JSON body).
  • Response control: ResponseEntity for status codes such as 201, 204, and 404.

Hands-on Experiment: Creating a spring boot application along with different controllers

The following steps are involved in developing this hands-on experiment.

Step 1: Generate the Project

Step 2: Project Structure

Step 3: pom.xml

Step 4: application.properties

Step 5: Create Entity Class

Step 6: Create Repository Interface

Step 7: Create Service Class

Step 8: Create Controller Class

Step 9: Main Application Class

Step 10: Run the Main Application Class

Step 1: Configure the Project on Spring Initializr

Go to start.spring.io and set:

  • Project: Maven
  • Language: Java
  • Spring Boot version: 3.2.x
  • Group: com.example
  • Artifact: controllers
  • Name: controllers
  • Package name: com.example.controllers
  • Packaging: Jar
  • Java: 17
  • Dependencies: Spring Web, Spring Data JPA, MySQL Driver

Click Generate to download the ZIP, then extract and import it into your IDE as a Maven project.

Step 2: Generated Project Structure

Step 3: Explore pom.xml

<?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>3.3.5</version>
        <relativePath/>
    </parent>

    <groupId>com.example</groupId>
    <artifactId>student-controllers-demo</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>
    </dependencies>

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

Step 4: application.yml

spring:
  datasource:
    url: jdbc:mysql://localhost:3306/student_controllers_db?createDatabaseIfNotExist=true&useSSL=false&allowPublicKeyRetrieval=true&serverTimezone=UTC
    username: root
    password: password
  jpa:
    hibernate:
      ddl-auto: update
    show-sql: true
server:
  port: 8082Code language: JavaScript (javascript)

Step 5: Create Entity classes

package com.example.controllers.entity;

import jakarta.persistence.*;

@Entity
public class Course {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;
    private int durationInWeeks;

    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getTitle() { return title; }
    public void setTitle(String title) { this.title = title; }
    public int getDurationInWeeks() { return durationInWeeks; }
    public void setDurationInWeeks(int durationInWeeks) { this.durationInWeeks = durationInWeeks; }
}

package com.example.controllers.entity;

import jakarta.persistence.*;

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String email;

    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }
}

Step 6: Create Repository interfaces

package com.example.controllers.repository;

import com.example.controllers.entity.Course;
import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

public interface CourseRepository extends JpaRepository<Course, Long> {
    List<Course> findByTitleContainingIgnoreCase(String title);
}

package com.example.controllers.repository;

import com.example.controllers.entity.Student;
import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

public interface StudentRepository extends JpaRepository<Student, Long> {
    List<Student> findByNameContainingIgnoreCase(String name);
}

Step 7: Create Service classes

package com.example.controllers.service;

import com.example.controllers.entity.Course;
import com.example.controllers.repository.CourseRepository;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Optional;

@Service
public class CourseService {

    private final CourseRepository repository;

    public CourseService(CourseRepository repository) {
        this.repository = repository;
    }

    public Course save(Course course) {
        return repository.save(course);
    }

    public List<Course> findAll(String title) {
        return (title == null) ? repository.findAll() : repository.findByTitleContainingIgnoreCase(title);
    }

    public Optional<Course> findById(Long id) {
        return repository.findById(id);
    }

    public boolean delete(Long id) {
        if (!repository.existsById(id)) return false;
        repository.deleteById(id);
        return true;
    }
}

package com.example.controllers.service;

import com.example.controllers.entity.Student;
import com.example.controllers.repository.StudentRepository;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Optional;

@Service
public class StudentService {

    private final StudentRepository repository;

    public StudentService(StudentRepository repository) {
        this.repository = repository;
    }

    public Student save(Student student) {
        return repository.save(student);
    }

    public List<Student> findAll(String name) {
        return (name == null) ? repository.findAll() : repository.findByNameContainingIgnoreCase(name);
    }

    public Optional<Student> findById(Long id) {
        return repository.findById(id);
    }

    public Optional<Student> update(Long id, Student updated) {
        return repository.findById(id).map(existing -> {
            existing.setName(updated.getName());
            existing.setEmail(updated.getEmail());
            return repository.save(existing);
        });
    }

    public boolean delete(Long id) {
        if (!repository.existsById(id)) return false;
        repository.deleteById(id);
        return true;
    }
}

Step 8: Create Controller classes

package com.example.controllers.controller;

import com.example.controllers.entity.Course;
import com.example.controllers.service.CourseService;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.List;

// Controller 2: everything about COURSES lives under /api/courses
// Same annotations, different responsibility -> one controller per resource
@RestController
@RequestMapping("/api/courses")
public class CourseController {

    private final CourseService service;

    public CourseController(CourseService service) {
        this.service = service;
    }

    @PostMapping
    public ResponseEntity<Course> add(@RequestBody Course course) {
        return ResponseEntity.status(HttpStatus.CREATED).body(service.save(course));
    }

    // GET /api/courses  or  GET /api/courses?title=java
    @GetMapping
    public List<Course> list(@RequestParam(required = false) String title) {
        return service.findAll(title);
    }

    @GetMapping("/{id}")
    public ResponseEntity<Course> get(@PathVariable Long id) {
        return service.findById(id)
                .map(ResponseEntity::ok)
                .orElse(ResponseEntity.notFound().build());
    }

    @DeleteMapping("/{id}")
    public ResponseEntity<Void> delete(@PathVariable Long id) {
        return service.delete(id) ? ResponseEntity.noContent().build()
                                  : ResponseEntity.notFound().build();
    }
}

package com.example.controllers.controller;

import com.example.controllers.entity.Student;
import com.example.controllers.service.StudentService;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.List;

// Controller 1: everything about STUDENTS lives under /api/students
@RestController
@RequestMapping("/api/students")
public class StudentController {

    private final StudentService service;

    public StudentController(StudentService service) {
        this.service = service;
    }

    // POST /api/students  -> body (JSON) becomes a Student; reply 201 Created
    @PostMapping
    public ResponseEntity<Student> register(@RequestBody Student student) {
        return ResponseEntity.status(HttpStatus.CREATED).body(service.save(student));
    }

    // GET /api/students  or  GET /api/students?name=ravi   (@RequestParam is optional)
    @GetMapping
    public List<Student> list(@RequestParam(required = false) String name) {
        return service.findAll(name);
    }

    // GET /api/students/1  -> 200 with student, or 404
    @GetMapping("/{id}")
    public ResponseEntity<Student> get(@PathVariable Long id) {
        return service.findById(id)
                .map(ResponseEntity::ok)
                .orElse(ResponseEntity.notFound().build());
    }

    // PUT /api/students/1
    @PutMapping("/{id}")
    public ResponseEntity<Student> update(@PathVariable Long id, @RequestBody Student student) {
        return service.update(id, student)
                .map(ResponseEntity::ok)
                .orElse(ResponseEntity.notFound().build());
    }

    // DELETE /api/students/1  -> 204 No Content, or 404
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> delete(@PathVariable Long id) {
        return service.delete(id) ? ResponseEntity.noContent().build()
                                  : ResponseEntity.notFound().build();
    }
}

Step 9: Main Application

package com.example.controllers;

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

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

Step 10: Run ControllerApplication

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