Spring Tutorials

The Spring IoC container are called beans managed by the objects that form the backbone of your application. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container. These beans are created with the configuration metadata that you supply to the container, The bean definition contains the information called configuration metadata which is needed for the container to know the followings:

1. How to create a bean
2. Bean's lifecycle details
3. Bean's dependencies

Following the properties of above configuration metadata translates into a set that make up each bean definition.

Spring IOC container is fully decoupled from the format in which this configuration metadata is actually written. There are following three important methods to provide configuration metadata to the Spring Container:

1. XML based configuration file
2. Annotation-based configuration
3. Java-based configuration

XML based configuration file: The XML Schema-based configuration introduced in Spring 2.0 and enhanced and extended in Spring 2.5 and 3.0. The central motivation for moving to XML Schema based configuration files was to make Spring XML configuration easier. The 'classic' <bean/>-based approach is good, but its generic-nature comes with a price in terms of configuration overhead.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans 
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
<bean id="..." class="..."></bean>
<bean id="..." class="..." lazy-init="true"></bean>
<bean id="..." class="..." init-method="..."></bean>
<bean id="..." class="..." destroy-method="..."></bean>
</beans>

Annotation-based configuration: Spring 2.5 introduced support for annotation-based configuration metadata.Starting from Spring 2.5 it became possible to configure the dependency injection using annotations. So instead of using XML to describe a bean wiring, you can move the bean configuration into the component class itself by using annotations on the relevant class, method, or field declaration.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.0.xsd">
<context:annotation-config/>
<!-- bean definitions here -->
</beans>

Some important annotations are as follows:

1. @Required The @Required annotation applies to bean property setter methods.
2. @Autowired The @Autowired annotation can apply to bean property setter methods, non-setter methods, constructor and properties.
3. @Qualifier The @Qualifier annotation along with @Autowired can be used to remove the confusion by specifiying which exact bean will be wired.
4. JSR-250 Annotations Spring supports JSR-250 based annotations which include @Resource, @PostConstruct and @PreDestroy annotations.

Java-based configuration: Starting with Spring 3.0, many features provided by the Spring JavaConfig project became part of the core Spring Framework. Thus you can define beans external to your application classes by using Java rather than XML files. To use these new features, see the @Configuration, @Bean, @Import and @DependsOn annotations.

package com.r4r.snippets.enterprise;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.r4r.snippets.enterprise.services.HelloWorldService;
@Configuration
public class TestConfig {
@Bean(name="helloWorldBean")
public HelloWorldService helloWorldService() {
return new HelloWorldService();
}}

The @Bean annotation over the helloWorld() method indicates that this method produces a bean to be managed by the Spring container.