如何理解Spring中的Bean？

Question

I am a beginner of Spring and I still can't understand clearly what Bean is. From its definition, it seems a object that is determined by some pre-set configuration files or using annotation on a class. Once the spring starts up, the bean has been created. But can Spring use DI to create some instances of which attributes are not pre-determined?(Like, a user posts a json from website to Spring. And this json contains some data that are used to new a instance. Can Spring use this json to create the instance by using DI?)

Answer 1

Bean is just the object created by your spring application. As you know any spring application has several interacting objects working together to give rise to the desired programmed behavior.

A Bean is basically a managed object ie at run time the IOC container creates the bean object based on its definition supplied by the coder or as configured in the apllicationContext.xml file under beans tag, and injects it to other classes as required.

Any Spring application is basically a conglomeration of various objects interacting with each other, these objects or beans collaborate to create the application.

A Bean's lifecycle is managed by the Spring IOC container.

The JSON consumed by the Spring Application is taken care of by the HttpMessageConverter. When recieving a new request, the Spring framework will use the content-type header to determine the media type of the request. It will then try to find the corresponding converter, available in the classpath of the application, to convert the Request body.

Thus its clear that the incoming request body object is not managed by the Spring IOC container and hence is not a Bean.

But these deserialized instances are used as Data Transfer Objects in a Spring application's various layers like, service, DAO, controller.

Answer 2

Spring beans are the objects that comprise your application and are managed by the Spring framework. Comparing them to the concepts of JavaBeans and POJOs provides some explanatory context , and the Spring reference documentation contains extensive documentation of Spring beans, including this summary:

A bean definition essentially is a recipe for creating one or more objects. The container looks at the recipe for a named bean when asked, and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.

Also included in the reference documentation are descriptions of various ways to instantiate beans via an xml-based or annotation-based configuration approach as well as the Java Config approach (which also uses annotations). This is managed by the Spring BeanFactory interface (API here ; source here ).

The @Bean annotation is used to indicate that a method instantiates, configures and initializes a new object to be managed by the Spring IoC container. For those familiar with Spring's XML configuration the @Bean annotation plays the same role as the element. You can use @Bean annotated methods with any Spring @Component, however, they are most often used with @Configuration beans.

You refer in the question to Dependency Injection (DI), a design pattern based on the Inversion of Control principle, which is a critical part of the Spring Framework, particularly for bean instantiation. DI allows values to be passed into the object from outside. The Spring documentation describes both the constructor-based and setter-based approaches to DI provided by the Spring IoC container for instantiating objects (beans).