What in the world are Spring beans?
I am yet to find a high-level definition of Spring beans that I can understand. I see them referenced often in Grails documentation and books, but I think that understanding what they are would be beneficial. So what are Spring beans? How can they be used? Do they have something to do with Dependency Injection?
The objects that form the backbone of your application and that are managed by the Spring IoC* container are called beans. 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, for example, in the form of XML definitions.
More to learn about beans and scope from SpringSource:
When you create a bean definition what you are actually creating is a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, just like a class, you can potentially have many object instances created from a single recipe.
You can control not only the various dependencies and configuration values that are to be plugged into an object that is created from a particular bean definition, but also the scope of the objects created from a particular bean definition. This approach is very powerful and gives you the flexibility to choose the scope of the objects you create through configuration instead of having to 'bake in' the scope of an object at the Java class level. Beans can be defined to be deployed in one of a number of scopes
*IoC: Inversion of Control
Spring beans are just object instances that are managed by the Spring container, namely, they are created and wired by the framework and put into a "bag of objects" (the container) from where you can get them later.
The "wiring" part there is what dependency injection is all about, what it means is that you can just say "I will need this thing" and the framework will follow some rules to get you the proper instance.
For someone who isn't used to Spring, I think Wikipedia Spring's article has a nice description:
Central to the Spring Framework is its inversion of control container, which provides a consistent means of configuring and managing Java objects using reflection. The container is responsible for managing object lifecycles of specific objects: creating these objects, calling their initialization methods, and configuring these objects by wiring them together.
Objects created by the container are also called managed objects or beans . The container can be configured by loading XML files or detecting specific Java annotations on configuration classes. These data sources contain the bean definitions which provide the information required to create the beans.
Objects can be obtained by means of either dependency lookup or dependency injection. Dependency lookup is a pattern where a caller asks the container object for an object with a specific name or of a specific type. Dependency injection is a pattern where the container passes objects by name to other objects, via either constructors, properties, or factory methods.
Well you understood it partially. You have to tailor the beans according to your need and inform Spring container to manage it when required, by using a methodology populalrly known as IoC (Inversion of Control) coined by Martin Fowler, also known as Dependency Injection (DI).
You wire the beans in a way, so that you do not have to take care of the instantiating or evaluate any dependency on the bean. This is popularly known as Hollywood Principle.
Google is the best tool to explore more on this in addition to the links you would get flooded with here in this question. :)
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