Top 10 Most Useful Spring Boot Annotations for Beginners in 2024

Quick Summary:

This detailed and well-researched guide will help you understand the basics of Spring Boot Annotations, featuring the top 10 most useful annotations for beginners and professional developers alike. Each annotation listed below is effective in building scalable and efficient applications by addressing key issues and controlling the configuration. With a comprehensive knowledge of using these annotations, you can ensure a smooth workflow and improved efficiency in Spring Boot development.

Introduction

Spring Boot Annotation might be a new term for beginners, but for skilled and experienced web developers, it is nothing short of a boon. Even a recent survey by Jrebel revealed that more than 80% of web developers use Spring Boot for development projects. Although there are tons of Spring Boot Annotations, knowing just a few useful ones is enough to get started. But how would you know which annotations to begin with? So, we curated a list of the top 10 most useful Spring Boot Annotations in 2024. But before that, here’s what you need to know about Spring Boot. 

Spring Boot is one of the most powerful Java frameworks. It can streamline web app development processes by making them more productive and efficient. It allows developers to concentrate exclusively on coding while seamlessly handling all the underlying processes.

Now that you might have understood its importance, let’s move on to the essential components of Spring Boot, i.e., annotations. This detailed guide will introduce you to the top 10 Spring Boot Annotations list, alongside explanations, example codes, and the best practices to make you understand more precisely. So, let’s get started, but before that…

What are Spring Boot Annotations?

Spring Boot Annotations are common components of this Java framework that help in the effective development of a software system by providing supplemental information about a program. These annotations in Spring Boot can help with the scalability, improved performance, and maintainability of web apps. 

SpringBoot Annotations come equipped with advanced features and functionalities, such as component scanning, dependency injection, defining REST Endpoints, controller supports, and more. 

In simple words, Annotations can be understood as instructions for Spring Boot to set up and run the web app. For instance, you can use Spring boot annotations to start your web app, set up databases, and handle requests. 

Top 10 Spring Boot Annotations to Know in 2024

Now that you have understood the basics of Spring Boot Annotations, it’s time to explore the most useful ones for beginner developers. So, here’s a list of the top 10 Spring Boot Annotations: 

#1. @SpringBootApplication

This annotation is crucial to starting and running a Spring Boot Application using the Java main method. It sets up the application context from the classpath and starts the application. It helps developers by automatically configuring and scanning components, making setup easier. Here are its main features:

• Component Scanning: Registers Java Beans with the Spring Context.
• Auto-Configuration: Configures data sources and external dependencies automatically.
• Bean Definition: Allows defining Java Beans within the current class.

This annotation includes three other annotations that provide these functionalities:

• @EnableAutoConfiguration: Automatically configures the Spring Boot application using defaults based on the classpath dependencies.
• @ComponentScan: Scans the package and its sub-packages to register classes with the Spring Context.
• @Configuration: Marks a class as a configuration class that contains bean definitions.

Example Code of @SpringBootApplication

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication

public class MyApplication {

    public static void main(String[] args) {

        SpringApplication.run(MyApplication.class, args);

    }

}

#2. @Configuration

The `@Configuration` annotation in Spring Core indicates that a class contains definitions for various Java Beans marked with `@Bean.` The Spring IoC Container will manage these beans. Additionally, this annotation includes the functionality of `@Component,` meaning that the class will be detected during Component Scanning by the Spring framework.

Example of Code of @Configuration

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

@Configuration

public class MyConfig {

    @Bean

    public MyBean myBean1() {

        return new MyBean();

    }

      // Other Bean Definitions

}

#3. @Bean

The `@Bean` annotation is used on the top of a method to tell the Spring Framework that this method defines a Java Bean. The process will return a Java Bean that the Spring IoC Container will manage.

Example Code of @Bean

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

@Configuration

public class MyConfig {

      // Bean - 1

    @Bean

    public User theUser() {

        return new User();

    }

      // Bean - 2 

      @Bean 

      public Admin theAdmin() {

      return new Admin();

    }

      // Other bean definitions

}

• In the example above, we created a basic `MyConfig` class. Every class that defines beans must be annotated with `@Configuration.` 
• The `@Bean` annotation is used on both methods that return `User` and `Admin` Java Beans. 
• We can define multiple beans within this class.

#4. @Autowired

This annotation's main purpose is to support the key feature of the Spring Framework, 'Dependency Injection.' It is the process of linking one object to another so they can share data and use each other's functions.

 Let's look at a simple example to understand its advantages.

Manual Dependency Injection

Suppose we have a car and an engine. We need to add the engine as it is dependent on the car to work. Assume we have two classes - 'Car' and 'Engine'. 

Here's how to do it in Java:

public class MyApp {

    public static void main(String[] args) {

          // Engine is a non-dependent object.

          Engine engine = new Engine();

          // Passing the engine object as argument to attach engine with the car.

        Car theCar = new Car(engine);

    }

}

Using @Autowired

Now, let's see how we can achieve the same thing in Spring Boot using the `@Autowired` annotation. There are several methods to do this, such as constructor injection, field injection, or setter injection.

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

public class Car {

      // Engine Object

      private Engine theEngine;

      @Autowired

      public Car(Engine theEngine) {

        this.engine = theEngine;

    }

      //  Car class methods

}

• Adding an annotation at the top of a constructor, field, or setter method tells the Spring Framework that this is a dependency injection point. 
• In our example above, the engine is the class being injected into the car class.

#5. @Qualifier

This annotation in Spring Boot helps programmers specify which beans they want from the Spring Container. To explain further, if we have a general class in our Spring application with three specialized types, the Spring container might get confused about which one to provide when we request the general class. It will then prompt us to be more specific.

Let's look at an example:

• We have an interface called `Teacher`.
• We have three classes implementing `Teacher`: `MathTeacher`, `PhysicsTeacher`, and `ChemistryTeacher.` Each of these classes is annotated with `@Component`.

When you tell the Spring framework to inject a dependency using `@Autowired,` it will throw a `NoUniqueBeanDefinitionException` if there are multiple beans available for auto wiring and you haven't specified which one to use with `@Qualifier.`

Example Code of @Qualifier

package com.adwitiya.springcorequalifier.REST;

import com.adwitiya.springcorequalifier.common.Teacher;

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

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

import org.springframework.web.bind.annotation.RestController;

@RestController

public class DemoController {

      // Teacher Object

    private Teacher myTeacher;

    //* Constructor injections & @Qualifier to specify that we're requesting a Math Teacher's Bean

    @Autowired

    DemoController(@Qualifier("mathTeacher") Teacher theTeacher) {

        this.myTeacher = theTeacher;

    }

    // Rest Mappings

}

• The code above is a basic Rest Controller with mappings for various operations. 
• `@Autowired` injects the `Teacher` dependency into the `Teacher` object. 
• We use constructor injection to specify which `Teacher` bean we want. In this example, it's `MathTeacher`. Note that the initial letter is in lowercase by default in Spring Boot unless the bean name is explicitly changed using the `@Bean("BeanName")` annotation.

#6. @Component

This annotation tells the Spring framework to manage this class using the Spring Container. The class will be treated as a Bean, and the Spring Container will create its instance. This instance can then be used in different parts of our application through dependency injection.

For example, if we have a Java Bean called 'MathTeacher' and we annotate it with @Component, the Spring framework will create and manage its instance for dependency injection.

Example of Code @Component

@Component

class MathTeacher implement Teacher {

      @Override

      public getSchedule(){

        return "Study maths for 2 hours";

    }

}

• We have created a MathTeacher class that implements the Teacher interface. 
• To keep things simple, we haven't added any fields. There's one method, marked with @Override, which other objects can call when MathTeacher is injected as a dependency.
• The @Component annotation is important because it registers this bean with the Spring context, allowing it to be automatically injected into other parts of our application.

#7. @Controller

This is a class-based annotation in Spring Boot that identifies the class as the 'Controller'. In the Spring Framework, a Controller is an important part that manages incoming requests and gives a specific response for each one. This annotation is used with Spring MVC along with @RequestMapping.

Example Code of @Controller

import org.springframework.stereotype.Controller;

import org.springframework.web.bind.annotation.RequestMapping;

// Mark the class as Controller

@Controller

public class TestController {

      // Request mapping defined

    @RequestMapping("/api")

    public String sayHello()

    {

        return "Hello!";

    }

}

• In the code above, we've created a class called TestController. This class is marked as a controller for managing incoming requests. 
• For simplicity, there's only one method, which can be accessed through the endpoint 'https://localhost:8080/api'.

#8. @RestController

The `@RestController` annotation is used to create RESTful web services. It's a specialized version of the `@Controller` annotation we discussed earlier. This annotation is applied at the class level and is used to handle various REST APIs like PUT, GET, POST, and DELETE. When a class is annotated with `@RestController,` it is considered to define multiple REST endpoints.

The `@RestController` annotation includes two sub-annotations: `@Controller` and `@ResponseBody.`

@ResponseBody: By default, a Spring application expects each REST endpoint method to return a view page like HTML or JSP, which a view resolver should resolve. However, the `@ResponseBody` annotation changes this default behavior, indicating that all methods in this controller will return content instead of a view page.

Example Code of @RestController

import org.springframework.web.bind.annotation.GetMapping;

import org.springframework.web.bind.annotation.RestController;

@RestController

@RequestMapping("/api")

public class TestController {

      // Simple REST Endpoint - /hello

    @GetMapping("/hello")

    public String greet() {

        return "Hello!";

    }

}

We have created a basic test REST controller with a REST endpoint at `/api/hello.` When you run this on `localhost:8080` (the default port), you will see the output: "Hello!"

Note: The method name can be anything; the important part is that the endpoint must be known to access a specific resource.

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#9. @RequestMapping

This annotation can be used at both the class and method levels to define the URL path for our endpoints. It is the main annotation for specifying the path that will return a response containing the resource requested by the client. 

However, using this annotation at the class level is generally preferred because methods should perform a single task and be annotated with something more specific, like @GetMapping or @PostMapping.

For example, a controller can be annotated with @RequestMapping, while a method can be annotated with @GetMapping to indicate that the method is only returning a resource.

10. @PathVariable

This is a method-level annotation that helps us be more precise based on the context and get a specific resource. For instance, if we need the data of a student with id = 5, we can mention this in the URL while fetching the resource.

@PathVariable is used to pass and extract parameters from the endpoint to the method parameters, allowing us to perform operations on them.

Example Code of @PathVariable

import org.springframework.web.bind.annotation.GetMapping;

import org.springframework.web.bind.annotation.PathVariable;

import org.springframework.web.bind.annotation.RestController;

@RestController

public class ProductController {

      // id is the parameter passed in URL

    @GetMapping("/api/product/{id}")

    public Product getProduct(@PathVariable Long id) {

        // Retrieve product with the ID specified in the URL from the Database.

        return productService.getProductById(id);

    }

}

• Let's assume we have a `Product` class already defined as a POJO and a service layer object called `productService.` When a client accesses the endpoint `/api/product/{id}`, the `{id}` will be a positive integer representing the specific product's ID. 
• For example, `/api/product/5` means the client is requesting the product with ID 5. This number can change based on the client's request.

5 Best Practices for Effectively Using Spring Boot Annotations

As we have discussed some of the most common Spring bot Annotations, here are some tips and best practices for using them more efficiently and effectively. 

Avoid Using Too Many Annotations

While there are many annotations available, it’s important to use only the necessary Spring Boot annotations. Too many annotations can make your code difficult to read and maintain.

Use @ConditionalOnProperty for Simple Conditions

In Spring applications, the @ConditionalOnProperty annotation can enable or disable customizations based on a property's value. This makes creating applications that adapt to different conditions easier, making your work more efficient.

Testing with @ComponentScan

The @ComponentScan annotation is useful for integration testing. It helps identify any issues that might arise during testing and ensures that your tests are executed correctly according to your application's behavior.

Global Exception Handling with @ControllerAdvice

The @ControllerAdvice annotation allows you to set global exception handlers for all controllers in Spring applications. This helps provide consistent and accurate error messages across your Spring Boot applications.

Conclusion

As we have seen in this blog, Spring Boot Annotations hold potential to enhance your web development's performance, scalability, and maintainability. It simplifies development by enabling web developers to focus solely on code writing. 

This blog featured the top 10 Spring Boot Annotations that would be a great start to elevate productivity and streamline your development project. Each annotation has a different function that adds value to your project. 

If you’re still doubtful about using annotations for Spring Boot development, you can connect with our adapt web developer. At Mtoag Technologies, we use the latest Java frameworks like Spring Boot to build scalable and advanced web applications. For further details, you can book a free consultation with our expert team. 

FAQs

What is @SpringBootApplication Annotation in Spring Boot?

The `@SpringBootApplication` annotation combines `@Configuration`, `@EnableAutoConfiguration,` and `@ComponentScan.` It simplifies the configuration of a Spring Boot application by enabling auto-configuration and component scanning and allowing the definition of extra configuration in the application class.

What is the Difference Between @EnableAutoConfiguration and @SpringBootConfiguration?

@EnableAutoConfiguration automatically configures your Spring application based on the dependencies you have added. `@SpringBootConfiguration` is a specialized form of `@Configuration` used to denote that a class provides Spring Boot application configuration. Essentially, `@SpringBootConfiguration` is part of `@SpringBootApplication.`

What is @Controller Annotation?

The `@Controller` annotation defines a controller in Spring MVC. It marks a class as a web controller capable of handling HTTP requests and returning views. It is typically used in combination with `@RequestMapping` to map web requests using specific handler methods.

What is @Transactional in Spring Boot?

The `@Transactional` annotation manages transaction boundaries in Spring. It ensures that a method or class is executed within a transactional context, providing features like transaction propagation, isolation, and rollback on exceptions, which help maintain data integrity.

When to Use @Repository Annotation?

The `@Repository` annotation indicates that a class is a Data Access Object (DAO). It provides CRUD operations on the database and translates database exceptions into Spring's DataAccessException hierarchy, making it easier to handle database errors.

What is the @Service Annotation?

 The `@Service` annotation marks a class as a service provider. It is a specialization of `@Component` and defines business logic and service-layer beans. It helps create a clear separation between the service layer and other layers in the application.

What is DAO in Spring Boot?

DAO, or Data Access Object, is a design pattern used to abstract and encapsulate all access to the data source. In Spring Boot, DAOs are typically annotated with `@Repository` and provide methods for CRUD operations, ensuring a clean separation between the data access logic and business logic.