Introduction to Spring Framework Annotations

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In this post, we will get a quick introduction to Spring Framework Annotations. We will explore different annotations along with a few of the internal details for these Spring Framework Annotations.

Introduction

Java 5.0 introduced support for the annotations and quickly it became one of the preferred ways, Spring Framework was quick to adopt this and with 2.5 Spring Framework started support for annotations.

Though we can still use XML based configurations to control/manage our Spring application, Spring Framework Annotations are now becoming a de facto standard for Spring-based enterprise applications.

1. Spring Core Annotation

In this section, we will cover Spring Framework Core Annotations.

1.1 @Required

@Required annotation applied to the bean setter method to enforce require property. It can be used when we want to make sure that a specific bean is required to be injected by Spring, in case required Spring bean cannot inject the required bean, BeanInitializationException is thrown.

@Required public void setClient(OkHttpClient client) { this.client = client; }

1.2 @Autowired

@Autowired annotation used to inject object implicitly, We can apply this annotation to fields, setter methods, and constructors.

You can use @Autowired annotation on the constructor, Spring will inject dependency at the time of object creation. Please note that you can only use this annotation to the maximum of 1 constructor for a class.

@Autowired public WebConfiguration(RequestResult result){ this.requestResult=result }

For @Autowire annotation, Spring does not have the requirement for the constructor to be public.

We can also use @Autowire annotation on the class property

@Autowired private OkHttpClient client; @Autowired private ObjectMapper jacksonObjectMapper;

The third option is to use this annotation on the setter method, Spring internally will try to perform injection by Type.

@Autowired public void setClient(OkHttpClient client) { this.client = client; }

With Spring 4.3, @Autowire has become optional on classes with a single constructor, so for the below code, Spring will try to inject an instance of the Customer (even we do not have @Autowire annotation)

@Component public class Customer { private Order order; public Customer(Order order) { this.order = order; } }

1.3 @Qualifier

When you need more control over the dependency injection process, consider using @Qualifier annotation. The qualifier annotation helps disambiguate bean references when Spring otherwise could not do so this annotation is used to avoid confusion in the dependency injection.

To understand it more, let’s take a simple example of HelloWorldService , let’s assume that we have 2 implementations of our HelloWorldService

public interface HelloWorldService { void sayHello(); } public class HelloWorldService1 implements HelloWorldService { @Override public void sayHello() { } } public class HelloWorldService2 implements HelloWorldService { @Override public void sayHello() { } }

In case we want to Inject our HelloWorldService1 using @Autowire annotation, Spring will not know which one of the two implementations to inject.

public class HelloWordTest { @Autowired private HelloWorldService helloWorldService; }

To handle all similar issues where we want to let Spring know about which dependency to pick and inject, we can use @Qualifier annotation.

@Autowired @Resource(name = "helloService1") private HelloWorldService helloWorldService;

1.4 @Bean

@Bean annotation is used to create a bean in the Spring Framework, we use this annotation at the method level

@Bean public Customer customer() { // instantiate and configure customer obj return customer; }

By default, the strategy for determining the name of a bean is to use the name of the @Bean method, however, if we want to explicitly name our bean, we can always use “name” or “alias” value to do that.

@Bean({"user", "customer"}) public Customer customer() { // instantiate and configure customer obj return customer; }

1.5 @Configuration

We use this annotation at the class level which defines Beans, think of @Configuration annotation as a configuration using Java class (then traditional XML file to define Spring Beans)

@Configuration public class ApplicationConfig { @Bean public OkHttpClient client(){ return new OkHttpClient.Builder() .build(); } @Bean public CommonsRequestLoggingFilter requestLoggingFilter() { CommonsRequestLoggingFilter loggingFilter = new CommonsRequestLoggingFilter(); loggingFilter.setIncludeClientInfo(true); loggingFilter.setIncludeQueryString(true); loggingFilter.setIncludePayload(true); loggingFilter.setIncludeHeaders(false); return loggingFilter; } }

1.6 @ComponentScan

We use this annotation to configures component scanning directives for use with @Configuration classes, we can specify base package from where we want Spring to scan packages. If we do not define specific packages, scanning will occur from the package of the class that declares this annotation.

1.7 @Lazy

Spring by default use eager initialization strategy i.e. all autowired dependencies will be created and configured at startup.If we do not want this to happen, we can use @Lazy annotation to ensure he @Bean or @Component will not be initialized until referenced by another bean or explicitly retrieved from the enclosing BeanFactory.

@Configuration @Lazy public class AppConf { @Bean public MyBean bean() { return new MyBean(); } @Bean public MyBean1 bean1() { return new MyBean1(); } //Bean2 will be eagerly loaded by Spring @Bean @Lazy(value = false) public Bean2 bean2() { return new Bean2(); } }

For more detail, please read Guide to the Spring @Lazy Annotation

1.8 @Value

This is at the field or method/constructor parameter level that shows a default value expression for the affected argument. This is very similar to @Autowire annotation but @Value annotation will inject values from properties.

To understand it more, let’s take an example where we want to get some values from the property file, Let’s define our property file

user.name=Java Dev Journal language=java

Here is our Java code

@Value("user.name") private String username; @Value("language") private String language;

Please read @ConfigurationProperties in Spring Boot to understand how it works for Spring Boot.In this section, we will cover Stereotype Annotations provided by Spring Framework.

1.9 @Service

This is a class level annotation shows that an annotated class is a "Service" , Apart from the fact that it is used to show that it’s holding the business logic, there’s no noticeable specialty that this annotation provides.

@Service("helloService") public class HelloWorldService1 implements HelloWorldService { @Override public void sayHello() { } }

1.10 @Repository

This annotation is used on the classes which are working directly with the DB layer, @Repository annotation shows that an annotated class is a "Repository" .One of the different thing being performed by classes annotated with @Repository annotation is to catch Platform specific exceptions and re-throw them as one of Spring’s unified unchecked exception.

1.11 @Component

We use this annotation to show a Spring-powered component. This is a general-purpose stereotype annotation showing that the class is a spring component. If we check definitions of @Service or @Repository annotations

@Component public @interface Service { // } @Component public @interface Repository { // }

we can safely say that @Service or @Repository annotations are special kinds of @Component annotation.

1.12 @Lookp Annotation

To put it in simple words, lookup method injection is the process to override a Spring bean at the runtime. The constructor and property are the most common used dependency injection methods. Both the options happen during the initialization of the Bean. The @Lookup annotation in Spring is helpful when we like to implement a factory method which returns a new bean on every call without we implementing the method. Here are some of the most common use cases for the @Lookup :

Please read @Lookup Annotation in Spring for more detail.

1.13 @Order Annotation

@Order annotation defines the sort order for an annotated component. This annotation is available from Spring 2.0. This annotation has an optional value argument which represents an order value as defined in the Ordered interface. The default value is Ordered.LOWEST_PRECEDENCE , showing lowest priority, similarly, the value Ordered.HIGHEST_PRECEDENCE for overriding the highest priority.

For more detail, please read @Order in Spring

Summary

In this post, we get an introduction to Spring Framework Annotations. We checked different annotations provided by Spring Framework with information about when to use these Annotations.In the next article, we will cover some important Annotations provided by Spring Boot and Spring MVC framework.