Spring - Stereotype Annotations - GeeksforGeeks

03 Dec, 2021
Spring is one of the most popular Java EE frameworks. It is an open-source lightweight framework that allows Java EE 7 developers to build simple, reliable, and scalable enterprise applications. This framework mainly focuses on providing various ways to help you manage your business objects. Now talking about Spring Annotation, Spring Annotations are a form of metadata that provides data about a program. Annotations are used to provide supplemental information about a program. It does not have a direct effect on the operation of the code they annotate. It does not change the action of the compiled program. 
Spring Framework provides us with some special annotations. These annotations are used to create Spring beans automatically in the application context. @Component annotation is the main Stereotype Annotation. There are some Stereotype meta-annotations which is derived from @Component those are
@Service@Repository@Controller
@Service
@Repository
@Controller
1: @Service: We specify a class with @Service to indicate that they’re holding the business logic. Besides being used in the service layer, there isn’t any other special use for this annotation. The utility classes can be marked as Service classes.
2: @Repository: We specify a class with @Repository to indicate that they’re dealing with CRUD operations, usually, it’s used with DAO (Data Access Object) or Repository implementations that deal with database tables.
3: @Controller: We specify a class with @Controller to indicate that they’re front controllers and responsible to handle user requests and return the appropriate response. It is mostly used with REST Web Services.
So the stereotype annotations in spring are @Component, @Service, @Repository, and @Controller.
@Component Annotation
@Component is a class-level annotation. It is used to denote a class as a Component. We can use @Component across the application to mark the beans as Spring’s managed components. A component is responsible for some operations.
Illustration: Let’s create a very simple Spring boot application to showcase the use of Spring Component annotation and how Spring autodetects it with annotation-based configuration and classpath scanning.
Step 1: Create a Simple Spring Boot Project. Geek, you need pre-requisite of creating and setting up Spring Boot Project 
Refer to this article Create and Setup Spring Boot Project in Eclipse IDE and create a simple spring boot project. 
Step 2: Add the spring-context dependency in your pom.xml file. Go to the pom.xml file inside your project and add the following spring-context dependency.
XML
<dependency>    <groupId>org.springframework</groupId>    <artifactId>spring-context</artifactId>    <version>5.3.13</version></dependency>
Step 3: Create a simple component class
Go to the src > main > java > your package name > right-click > New > Java Class and create your component class and mark it with @Component annotation. 
Java
// Java Program to Illustrate Component classpackage com.example.demo;  import org.springframework.stereotype.Component;  // Annotation@Component  // Classpublic class ComponentDemo {      // Method    public void demoFunction()    {          // Print statement when method is called        System.out.println("Hello GeeksForGeeks");    }}
Step 4: Create an annotation-based spring context
Now go to your Application (@SpringBootApplication) file and here in this file create an annotation-based spring context and get the ComponentDemo bean from it.
Java
// Java Program to Illustrate Application class  // Importing package herepackage com.example.demo;// Importing required classesimport org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.context.annotation.AnnotationConfigApplicationContext;  // Annotation@SpringBootApplication  // Classpublic class DemoApplication {      // Main driver method    public static void main(String[] args)    {          // Annotation based spring context        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();        context.scan("com.example.demo");        context.refresh();          // Getting the Bean from the component class        ComponentDemo componentDemo = context.getBean(ComponentDemo.class);        componentDemo.demoFunction();          // Closing the context        // using close() method        context.close();    }}
Output:
So you can see the power of @Component annotation, we didn’t have to do anything to inject our component to spring context. 
In an application, the business logic resides within the service layer so we use the @Service Annotation to indicate that a class belongs to that layer. It is also a specialization of @Component Annotation like the @Repository Annotation. One most important thing about the @Service Annotation is it can be applied only to classes. It is used to mark the class as a service provider. So overall @Service annotation is used with classes that provide some business functionalities. Spring context will autodetect these classes when annotation-based configuration and classpath scanning is used.
Example
Step 1: Create a Simple Spring Boot Project
Refer to this article Create and Setup Spring Boot Project in Eclipse IDE and create a simple spring boot project. 
Step 2: Add the spring-context dependency in your pom.xml file. Go to the pom.xml file inside your project and add the following spring-context dependency.
XML
<dependency>    <groupId>org.springframework</groupId>    <artifactId>spring-context</artifactId>    <version>5.3.13</version></dependency>
Step 3: In your project create one package and name the package as “service”. In the service package create a class and name it as MyServiceClass. This is going to be our final project structure.
Below is the code for the MyServiceClass.java file.
Java
package com.example.demo.service;  import org.springframework.stereotype.Service;  @Servicepublic class MyServiceClass {      // Simple program for    // factorial of a number    public int factorial(int n)    {        if (n == 0)            return 1;          return n*factorial(n-1);    }  }
In this code notice that it’s a simple java class that provides functionalities to calculate the factorial of a number. So we can call it a service provider. We have annotated it with @Service annotation so that spring-context can autodetect it and we can get its instance from the context.
Step 4: Spring Repository Test
So now our Spring Repository is ready, let’s test it out. Go to the DemoApplication.java file and refer to the below code. 
Java
package com.example.demo;  import com.example.demo.service.MyServiceClass;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.context.annotation.AnnotationConfigApplicationContext;  @SpringBootApplicationpublic class DemoApplication {      public static void main(String[] args) {          AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();        context.scan("com.example.demo");        context.refresh();                  MyServiceClass myServiceClass = context.getBean(MyServiceClass.class);          // Testing the factorial method        int factorialOf5 = myServiceClass.factorial(5);        System.out.println("Factorial of 5 is: " + factorialOf5);          // close the spring context        context.close();    }  }
Output:
@Repository Annotation is a specialization of @Component annotation which is used to indicate that the class provides the mechanism for storage, retrieval, update, delete and search operation on objects. Though it is a specialization of @Component annotation, so Spring Repository classes are autodetected by spring framework through classpath scanning. This annotation is a general-purpose stereotype annotation which very close to the DAO pattern where DAO classes are responsible for providing CRUD operations on database tables. 
Example
Step 1: Create a Simple Spring Boot Project
Refer to this article Create and Setup Spring Boot Project in Eclipse IDE and create a simple spring boot project. 
Step 2: Add the spring-context dependency in your pom.xml file. Go to the pom.xml file inside your project and add the following spring-context dependency.
XML
<dependency>    <groupId>org.springframework</groupId>    <artifactId>spring-context</artifactId>    <version>5.3.13</version></dependency>
Step 3: In your project create two packages and name the package as “entity” and “repository”. In the entity package create a class name it as Student. In the repository, package create a Generic Interface name it as DemoRepository and a class name it as StudentRepository. This is going to be our final project structure.
Step 4: Create an entity class for which we will implement a spring repository. Here our entity class is Student. Below is the code for the Student.java file. This is a simple POJO (Plain Old Java Object) class in java. 
Java
package com.example.demo.entity;  public class Student {      private Long id;    private String name;    private int age;      public Student(Long id, String name, int age) {        this.id = id;        this.name = name;        this.age = age;    }      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 int getAge() {        return age;    }      public void setAge(int age) {        this.age = age;    }      @Override    public String toString() {        return "Student{" +                "id=" + id +                ", name='" + name + '\'' +                ", age=" + age +                '}';    }}
Step 5: Before implementing the Repository class we have created a generic DemoRepository interface to provide the contract for our repository class to implement. Below is the code for the DemoRepository.java file.
Java
// Java Program to illustrate DemoRepository File  package com.example.demo.repository;  public interface DemoRepository<T> {      // Save method    public void save(T t);      // Find a student by its id    public T findStudentById(Long id);  }
Step 6: Now let’s look at our StudentRepository class implementation. 
Java
// Java Program to illustrate StudentRepository File  package com.example.demo.repository;  import com.example.demo.entity.Student;import org.springframework.stereotype.Repository;  import java.util.HashMap;import java.util.Map;  @Repositorypublic class StudentRepository implements DemoRepository<Student> {      // Using an in-memory Map    // to store the object data    private Map<Long, Student> repository;      public StudentRepository() {        this.repository = new HashMap<>();    }      // Implementation for save method    @Override    public void save(Student student) {        repository.put(student.getId(), student);    }      // Implementation for findStudentById method    @Override    public Student findStudentById(Long id) {        return repository.get(id);    }}
In this StudentRepository.java file, you can notice that we have added the @Repository annotation to indicate that the class provides the mechanism for storage, retrieval, update, delete and search operation on objects.
Note: Here we have used an in-memory Map to store the object data, you can use any other mechanisms too. In the real world, we use Databases to store object data. 
Step 7: Spring Repository Test
So now our Spring Repository is ready, let’s test it out. Go to the DemoApplication.java file and refer to the below code. 
Java
package com.example.demo;  import com.example.demo.entity.Student;import com.example.demo.repository.StudentRepository;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.context.annotation.AnnotationConfigApplicationContext;  @SpringBootApplicationpublic class DemoApplication {      public static void main(String[] args) {                  AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();        context.scan("com.example.demo");        context.refresh();          StudentRepository repository = context.getBean(StudentRepository.class);          // testing the store method        repository.save(new Student(1L, "Anshul", 25));        repository.save(new Student(2L, "Mayank", 23));          // testing the retrieve method        Student student = repository.findStudentById(1L);        System.out.println(student);          // close the spring context        context.close();    }  }
Output: Lastly, run your application and you should get the following output as shown below as follows:
Spring @Controller annotation is also a specialization of @Component annotation. The @Controller annotation indicates that a particular class serves the role of a controller. Spring Controller annotation is typically used in combination with annotated handler methods based on the @RequestMapping annotation. It can be applied to classes only. It’s used to mark a class as a web request handler. It’s mostly used with Spring MVC applications. This annotation acts as a stereotype for the annotated class, indicating its role. The dispatcher scans such annotated classes for mapped methods and detects @RequestMapping annotations. Let’s understand all of these by example. 
Example
Step 1: Create a Simple Spring Boot Project
Refer to this article Create and Setup Spring Boot Project in Eclipse IDE and create a simple spring boot project. 
Step 2: Add the spring-web dependency in your pom.xml file. Go to the pom.xml file inside your project and add the following spring-web dependency.
XML
<dependency>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-starter-web</artifactId></dependency>
Step 3: In your project create one package and name the package as “controller”. In the controller package create a class and name it as DemoController. This is going to be our final project structure.
Below is the code for the DemoController.java file.
Java
package com.example.demo.controller;  import org.springframework.stereotype.Controller;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.ResponseBody;  @Controllerpublic class DemoController {      @RequestMapping("/hello")    @ResponseBody    public String helloGFG()    {        return "Hello GeeksForGeeks";    }}
We have used the below annotations in our controller layer. Here in this example, the URI path is /hello.
@Controller: This is used to specify the controller.
@RequestMapping: This is used to map to the Spring MVC controller method.
@ResponseBody: Used to bind the HTTP response body with a domain object in the return type.
Now, our controller is ready. Let’s run our application inside the DemoApplication.java file. There is no need to change anything inside the DemoApplication.java file. 
Java
package com.example.demo;  import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;  @SpringBootApplicationpublic class DemoApplication {      public static void main(String[] args) {        SpringApplication.run(DemoApplication.class, args);    }  }
Output:
Try this Tomcat URL, which is running on http://localhost:8989/hello
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