Spring Dependency Injection and Inversion of Control Container Concepts

External

• www.tutorialspoint.com/spring/index.htm

Internal

• Spring Framework

Overview

Spring is a dependency injection framework. Its main responsibilities are to provide a configuration model that specifies how components should be defined and what are the dependencies between defined components, detect conforming components in the libraries available on the classpath, instantiate the component graph while transitively honoring the dependency relationships - by instantiating the dependency before the dependent and injecting the appropriate dependency references into dependents - and finally expose the components to the application. Instantiation, dependency injection and component life cycle management take place within the Spring IoC container. Support for different application architectures, including messaging, transactions and persistence is built in top of the core container.

Dependency Injection

Rather than have individual components create and manage the lifecycle of their dependency components, a dependency-injected application relies on container to first create all components, then to inject them into other components that need them. Injection is typically done through constructor arguments or property setters.

Configuration Model

This section refers to component configuration. Configuration as in external data that is provided to the application in form of properties or environment variables, and which potentially modifies the behavior of the application, is addressed in the "Application Configuration Concepts" section.

Spring runtime expects to receive information about what components it should create and how those components should be wired together either in form of XML files or, since Spring 2.5, as annotation-based metadata embedded within the bytecode of the component classes themselves and referred to as Java-based configuration. This information that tells Spring how to build its components and what other components they depend on is called configuration metadata. Configuration metadata expressed as XML is equivalent with configuration metadata provided in form of Java annotations. Either can be used to the same effect.

XML-based configuration and Java-based configuration can also be combined and used together. However, when they are used together, annotation injection is performed before XML injection, thus the XML configuration overrides the annotations for properties wired through both approaches.

Configuration metadata includes the definitions of what constitutes a component - the Java fully qualified classname of the component class, the component's dependencies on other components, the component lifecycle details and where to find component classes. Depending on the metadata format, this information is provided in different formats. For example, in case of XML-based configuration, the fully qualified class name is provided in the "class" attribute of the <bean> element. However, for Java-based configuration, a component class is annotated as a @Component or a similar annotation, so Spring runtime has already the class information when it detects the annotation in the class' bytecode.

XML-based Configuration Metadata

The XML-based configuration historically precedes Java-based configuration. First Spring releases came with only XML configuration support.

This is an example of a simple Spring XML configuration file:

beans.xml

Java-based Configuration Metadata

Java-based configuration is semantically equivalent with XML-based configuration and it is an alternative to it, or it can be used together with it.

Spring Components

Component Definition

When XML-based metadata is used, Spring components are defined using a <bean> element:

<beans ...>
    <bean name="blue" class="playground.Blue"/>
</beans>

For Java-based configuration, @Configuration/@Bean. Components can also be declared using stereotype annotations. A stereotype annotation denotes the role of the annotated bean in the overall architecture at a conceptual, rather than implementation, level:

• @Component designates the annotated class as a generic component. Components are singletons.
• @Controller designates the annotated class as a controller.
• @Indexed designates the annotated class as a stereotype for the index.
• @Repository designates the annotated class as a persistence repository.
• @Service designates the annotated class as a "service", as defined by Domain-Driven Design service.

Spring IoC Container

The Spring IoC container is also referred to as the core container.