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Spring Boot Annotations Everyone Should Know [2024]

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24th Jan, 2024
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Spring Boot Annotations Everyone Should Know [2024]

Summary:

In this article, you will learn Spring Boot Annotations like

  1. @Bean
  2. @Service
  3. @Repository
  4. @Configuration
  5. @Controller
  6. @RequestMapping
  7. @Autowired
  8. @Component
  9. @SpringBootApplication
  10. @EnableAutoConfiguration

Read the full article to know more in detail.

Spring Boot Annotations are a form of metadata that provides data about a program that is not a part of the program itself. They do not have any direct effect on the operation of the code they annotate. Spring Boot Annotations do not use XML and instead use the convention over configuration principle. 

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The use of annotations extends beyond mere code organization. Spring Annotations have become integral to the development process, pivotal in simplifying and elevating the overall coding experience.

These annotations are crucial in the software development industry that runs on adaptability and scalability. They provide a mechanism for developers to express intentions and configurations concisely. 

This blog thoroughly explores Spring Boot Annotations, providing detailed insights into their uses. It also offers a helpful Spring Boot Annotations list to understand how these annotations work in practice.

Spring Boot Annotations Explained

Before delving into the different types, it is important to address the fundamental question, “What are Annotations in Spring Boot?” 

These annotations are critical in enhancing the development process by simplifying configuration tasks and fostering a more intuitive and readable codebase. Whether categorizing components, defining URL mappings, or specifying dependencies, Spring Boot Annotations in Java contribute to a modular and adaptable architecture.

Advantages of Using Annotations in Spring Boot

Spring Boot Annotations have certain advantages attached to them such as:-

  • Works well the servlet containers

In Java, Spring Boot, the servlet containers are the atmosphere where the Java web applications can survive/ live. Servlet Container or Web Container is the application server where it applies various Java versions like Java Servlet, JSP, etc. 

These containers play a crucial role in executing and managing the lifecycle of web components, handling requests and responses, and ensuring the seamless functioning of Java-based web applications. 

  • Saves memory space due to boostrapping

Bootstrapping in Spring Boot annotations is nothing but a process of initialising an application. The application can be initialised by using the Spring Initializer. Bootstrapping allows the users to utilise the space in their respective devices while they give the scope to applications to load quickly. 

  • No requirement for WAR files

WAR files in software engineering stands for Web Application Resource or Web Application Archive). Although Spring Boot can use WAR files they use JAR files for various reasons, such as the compressed file size which helps the developers to connect the applications with tools. Also, the JAR files are easier to handle, create, update, etc.

JAR or Java Archive files serve as containers for compiling multiple files into a single compressed archive. This format facilitates easy distribution and deployment as well as aids in managing dependencies and resources more efficiently within Spring Boot applications, simplifying development and maintenance processes significantly.

  • POM dependency management

POM is Project – Object-Model, it contains the information about the project in the XML file. The POM dependency management is the centralised process to manage the dependency information. The Springboot dependencies allow the developers to manage dependencies without relying on the parent POM or XML file.

The POM dependency management system ensures consistency and coherence throughout the project. This standardized way ensures everyone working on the project uses the same dependencies. It helps prevent issues and keeps everything working together smoothly.

  • XML configuration is not required

The developers can avoid the XML configuration in Java Spring Boot, which appeals to the developers as it allows them to skip extra steps.

This streamlining enhances efficiency, allowing developers to focus on essential tasks without requiring intricate setup procedures. 

  • Boilerplate code is decreased

The boilerplate code can be used by using the Spring Boot embedded server, as it decreases the boilerplate code. The absence of boilerplate code gives the developers a scope to lessen the time to develop applications and increase their productivity. 

Also, Spring boot starter is another feature, they are dependency descriptors. They can be added under the dependency section in pom. or xml.

Spring Boot Annotations Everyone Should Know

Understanding the use of annotations is crucial for enhancing the efficiency and functionality of applications. A Spring Boot Annotations list with explanations can help you better comprehend the use case of these annotations.

1. @Bean

The @Bean annotations are used at the method level and indicate that a method produces a bean that is to be managed by the Spring container. It is an alternative to the XML<bean> tag. 

Example:

@Bean

Public BeanExample beanExample ()

{

return new BeanExample (),

}

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In the spring boot annotation, beans are the objects that are the backbone of the application and are managed by the Spring IoC container. The Spring Bean annotation is declared in the configuration classes method.

This annotation is important for configuring the Spring IoC (Inversion of Control) container. By applying @Bean to a method within a configuration class, developers explicitly declare that the method produces a bean instance managed by the Spring container. 

This compact programmatic approach eliminates the need for extensive XML configurations, providing a more streamlined and maintainable way to define beans.

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2. @Service

It is used at the class level. It shows that the annotated class is a service class, such as business basic logic, and call external APIs.

Example:

@Service

public class TestService

{

public void service1()

{

// business code

}

}

The @service annotation is used where the classes provide some business functionalities. The spring context autodetects these classes as the annotation is used with those classes where the business functionalities are to be used.

This annotation serves as a marker and allows injecting service components into other Spring-managed beans. It fosters a modular and organized approach to developing applications, where the @Service annotation contributes to the effective structuring and autodetection of business logic components within the Spring context.

Read: Spring Boot Projects & Topics

3. @Repository

It is a Data Access Object (DAO) that accesses the database directly. It indicates that the annotated class is a repository. 

Example:

@Repository

public class TestRepository

{

public void delete()

{

// persistence code

}

}

The repository annotation indicates the class has the capability of storage, retrieval, updating, deletion, and search.

It is a key component in the data access layer. Using this annotation, developers communicate to the Spring framework that the annotated class serves as a data storage and retrieval repository.  

This annotation is particularly useful for classes that interact directly with databases, handling tasks such as data manipulation, querying, and providing a structured approach to database operations.

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4. @Configuration

It is used as a source of bean definitions. It is a class-level annotation.

Example:

@Configuration

public class Bus

{

@BeanBus engine()

{

return new Bus();

}

}

The configuration annotation represents the class having one or more @Bean methods. The spring container could go ahead and generate the Spring Beans to be used.

@Configuration is regarded as one of the basic annotations in Spring Boot. It is fundamental for defining bean configurations in a concise and programmatic manner. When a class is annotated with @Configuration, it signifies that this class serves as a source of bean definitions, particularly by including one or more @Bean methods within the class.

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5. @Controller

Among all Spring Boot annotations in Java, @Controller is regarded as one of the most important annotations in Spring Boot. 

The annotation is used to indicate that the class is a web request handler. It is often used to present web pages. It is most commonly used with @RequestMapping annotation. 

The @Controller annotation indicates that the class serves the controller role, playing a pivotal role in handling web requests. By marking a class with @Controller, developers explicitly state that this class is responsible for processing HTTP requests and often involves the presentation of web pages. 

This annotation is frequently paired with the @RequestMapping annotation to define the URL patterns that map to specific methods within the controller.

Example:

@Controller

@RequestMapping(“cars”)

public class CarsController

{

@RequestMapping(value= “/{name}”, method= RequestMethod.GET)

public Employee getCarsByName()

{

Return carsTemplate;

}

}

The controller annotation indicates the class serves the role of a controller. The controller annotation is a specialisation of the component annotation where it is auto-detected through the classpath scanning.

This means that Spring Boot, during its component scanning process, identifies classes annotated with @Controller and registers them as components, making them accessible for handling web requests in the application.

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6. @RequestMapping

RequestMapping is used to map the HTTP request. It is used with the class as well as the method. It has many other optional elements like consumes, name, method, request, path, etc.

Example:

@Controller

public class FlowersController

{

@RequestMapping (“/red-colour/flowers”)

public String getAllFlowers(Model model)

{

//application code

return “flowerlist”;

}

It is one of the most used annotations in Spring Boot and can be applied at both the class and method levels, allowing developers to define URL patterns for handling incoming requests.

In addition to its basic functionality, @RequestMapping offers a range of optional elements that enhance its flexibility and customization. 

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7. @Autowired

This annotation is used to auto-wire spring bean on setter methods, constructor and instance variable. It injects object dependency implicitly. When we use this annotation, the spring container auto-wires the bean by its matching data type.

Example:

@Component

public class Employee

private Person person;

@Autowired

public Employee(Person person)

{

this.person=person

}

}

The Autowired annotation is used to inject the dependency in the bean. The Autowired provides more control to the developers over where the Autowire should be used.

It offers developers ample control and flexibility to dictate where and how it should be applied within their application. Additionally, it enhances code readability and maintainability. This makes it easier for them to manage and understand the dependency injection process in a Spring Boot application.

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8. @Component

@Component is also regarded as one of the most used annotations in Spring Boot. 

It is a class-level annotation that turns the class into Spring bean at the auto-scan time.

Example:

@Component

Public class Teachers

{

……

}

The component annotation can automatically detect custom beans. It represents that the framework could autodetect these classes for dependency injection.

This annotation serves as a key component in Spring’s component scanning mechanism. This autodetection capability significantly simplifies the configuration process, enabling developers to focus on the core functionalities of their classes rather than explicitly registering each bean. 

It promotes a convention-over-configuration approach, where the framework autonomously identifies and manages custom beans. This contributes to Spring Boot applications’ overall modularity and maintainability, aligning with the framework’s emphasis on simplicity and ease of development.

9. @SpringBootApplication

It consists of @Configuration, @ComponentScan, and @EnabeAutoConfiguration. The class annotated with @SpringBootApplication is kept in the base package. This annotation does the component scan. However, only the sub-packages are scanned. 

The SpringBoot Application mark a configuration class that declares Bean methods, either one or more than that. The Spring Boot Application contains-

  1. Auto-configuration –The @EnableAutoConfiguration annotation is a core component of @SpringBootApplication. It triggers Spring Boot’s auto-configuration mechanism, which automatically configures the application based on the dependencies present in the classpath. Auto-configuration aims to reduce the need for explicit configuration by providing sensible defaults, making it easier for developers to get started with Spring Boot projects.
  2. Spring Boot Configuration –The @SpringBootConfiguration annotation is synonymous with @Configuration and indicates that the annotated class contains configuration methods. Configuration methods, marked with @Bean, define and configure beans in the Spring application context. This annotation is essential for organizing and centralizing the configuration of the application.
  3. Component Scan –The @ComponentScan annotation automatically discovers and registers Spring components within specified packages, such as controllers, services, and repositories. By default, it scans the class package annotated with @SpringBootApplication, making all components in that package and its sub-packages available for dependency injection. Component scanning enhances modularity and allows for the seamless integration of Spring components without explicit configuration.

10. @EnableAutoConfiguration

It’s one of the most important annotations in Spring Boot that simplifies application context initialization and setup by automating configuration based on classpath settings and added dependencies.

It is placed on the main application class. Based on classpath settings, other beans, and various property settings, this annotation instructs SpringBoot to start adding beans.

The Enable Auto Configuration allows the spring boot to auto-figure the application context. The applications are auto figured basis the added jar dependencies.

This annotation eliminates the need for extensive manual configuration, enabling developers to leverage sensible defaults and conventions. The auto-configuration mechanism is a key feature of Spring Boot, facilitating rapid development and reducing the burden of explicit bean definitions.

11. @ComponetScan

It is used to scan a package of beans. It is used with the annotation @Configuration to allow Spring to know the packages to be scanned for annotated components. This annotation is also used to specify base packages.

Example:

@ComponentScan(basePackages = “com.xyz”)

@Configuration

Public class ScanComponent

{

//…

}

The component scan annotation specifies the packages the developers want to scan. The component scan annotations specify the packages and sub-packages that are supposed to be scanned.

The @ComponentScan annotation becomes particularly useful when developers want to pinpoint specific packages or sub-packages for scanning. This specificity ensures that only the intended portions of the application are scanned for components, contributing to a more organized and efficient application structure.

In essence, @ComponentScan empowers developers to dictate the packages to be scanned, providing fine-grained control over component discovery and registration within the Spring context. This annotation aligns with Spring Boot’s convention-over-configuration approach, offering a straightforward means of configuring component scanning based on developers’ preferences and application architecture.

12. @Required

This annotation is applied to bean setter methods. It indicates that the required property must be filled at the configuration time in the affected bean, or else it throws an exception: BeanInitializationException.

This annotation, used on bean setter methods, mandates that a required property must be filled during configuration. The BeanInitializationException emphasizes the essential nature of the property for proper bean initialization.

13. @Qualifier

It is used along with @Autowired annotation. It is used when more control is required over the dependency injection process. Individual constructor arguments or method parameters can be specified by using this annotation. Confusion arises when more than one bean of the same type is created, and only one of them is to be wired with a property, @Qualifier is used to get rid of the confusion.

The @Qualifier annotation, used with @Autowired, provides precise control over dependency injection. By applying it to constructor arguments or method parameters, developers can explicitly specify the bean to be injected.

This is crucial when multiple beans of the same type exist, and clarity is needed to identify the specific bean to be wired with a property. @Qualifier resolves this ambiguity, ensuring the correct bean is used for the intended purpose.

14. @CookieValue

It is used at the method parameter level as an argument of the request mapping method. For a given cookie name, the HTTP cookie is bound to a @CookieValue parameter.

The cookie value annotation is used to get the value of any HTTP cookie. Cookies in spring boot are used to show the personalised content to the users and they can be retrieved using the cookie value annotation.

This annotation simplifies the extraction of specific cookie values from incoming HTTP requests. When applied to a method parameter, it allows developers to easily access the value of a particular cookie by specifying its name. This is especially useful when cookies customize user experiences, enabling developers to retrieve and use cookie values for personalized content or functionality in a Spring Boot application.

15. @Lazy

It is used in the component class. At startup, all auto-wired dependencies are created and configured. But a @Lazy annotation can be created if a bean is to be initialized lazily. This means that only if it is requested for a bean will be created. It can also be used on @Configuartion classes. It’s an indication that all @Bean methods within that @Configuration should be lazily initialized.

When applied at the class level in a configuration class, it signals that all @Bean methods within that configuration should follow a lazy initialization strategy. This flexibility allows developers to fine-tune the initialization behavior in a Spring Boot application based on specific use cases and resource considerations.

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Conclusion

Spring Boot has made it easy for Java developers to create Spring applications with ease. As a Java enthusiast or a professional, you should be aware of the basic Spring boot annotations. We hope that this blog helped you understand them. You can take the first step towards a successful Java career by enrolling for upGrad and IIIT-B’s Executive PG Program in Full Stack Development.

These annotations serve as powerful tools for streamlining the development process, providing a concise and expressive way to configure, manage dependencies, and handle various aspects of your application. By mastering these annotations, you enhance your proficiency in Spring Boot and unlock the full potential of the framework, enabling you to build robust and efficient Java applications. 

Profile

Rohan Vats

Blog Author
Software Engineering Manager @ upGrad. Passionate about building large scale web apps with delightful experiences. In pursuit of transforming engineers into leaders.

Frequently Asked Questions (FAQs)

1Why are annotations used in Java?

Java annotations are the metadata, i.e. data about data, of the source code of the program. Annotations are used to supply extra information to the compiler about a Java program. However, these are not a part of the Java program and do not, in any way, influence the program execution. Java annotations start with @. For example, the @Override annotation is used to signal the compiler that the specified method marked with that annotation should override the method contained in the superclass with the same name, parameter list and return type. There are different kinds of formats for annotations used in Java.

2What is the difference between Spring Boot and Spring Cloud?

Spring Boot is an open-source Java framework that is used to create microservices. On the other hand, Spring Cloud is a technology for configuration server that is used to centralize and manage configuration. Spring Cloud is also implemented to ensure the security of applications running on Spring Boot in many cases. By default, Spring Boot implements Spring features and can quickly execute standalone Spring web-based applications. Spring Cloud offers the platform to develop cloud services; its architecture is based on the Spring Boot model. Spring Boot is adopted for unit testing and reducing the time needed for integration testing.

3What is Spring in Java?

Spring in Java is an open-source framework that offers the platform to develop Java applications. Programs written using Java are generally very complex and come with various components that make them heavyweight. Spring is a lightweight, low-cost and secure framework that offers the flexibility needed to develop complex programs and enhance the overall efficiency of execution. Spring also optimizes the overall time taken for developing Java applications and eliminates tedious tasks related to the configuration so that programmers can focus their attention on the business logic of applications. The core logic of the Spring framework is dependency injection that allows developers to create decoupled architecture. This framework facilitates the development of high-performing Java applications.

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