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 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.

Spring IoC Container

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Overview

At the heart of the Spring Framework is the core container, which comes with a configuration model and a dependency injection mechanism. Support for different application architectures, including messaging, transactions and persistence is built in top of the core container.

Inversion of Control 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.

Prefer constructor injection to ensure that objects can be instantiate directly. Constructor injection used instead of field injection makes test easier to write.

Spring Dependency Injection Framework

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 that potentially modifies the behavior of the application, is addressed in the "Application Configuration Concepts" section.

Configuration Metadata

XML-based Configuration

Representative example of an XML configuration file.

Examples: XML-bases Spring Security Configuration.

Java-based Configuration

Java-based configuration can achieve the same results as XML-based configuration, and it is an alternative to it.

Annotation injection is performed before XML injection, thus the XML configuration overrides the annotations for properties wired through both approaches.

Examples: Java-bases Spring Security Configuration.

Configuration time. Define what that is.

Configuration Class

@Configuration

It is possible to extend the application bootstrap configuration to add more configuration. However, it is good practice to create a new configuration class for each kind of configuration (web, data, security, etc.), keeping the application bootstrap configuration clean and simple.

Autoconfiguration

Both XML-based configuration and Java-based configuration are often unnecessary, as Spring is capable of automatically configuring its components. Automatic configuration employs Spring techniques such as autowiring and component scanning. Clarify the relationship with Spring Boot autoconfiguration.

Component Scanning

Spring runtime capability to automatically discover components from an application's classpath and create them as beans in the application context. This lets you declare classes with annotations like @Component, @Controller, @Service, @Repository etc. to have Spring automatically discover them and register them as components in the application context. Component scanning is enabled by @ComponentScan.

Autowiring

Spring runtime capability to automatically inject components within other beans that they depend on. @Autowired

If the class has just one constructor, it will be considered automatically autowiring target, even without @Autowired.

To Distribute

Always use constructor based dependency injection in your beans. Always use assertions for mandatory dependencies.

Application Context

https://docs.spring.io/spring-framework/docs/5.1.0.RELEASE/javadoc-api/org/springframework/context/ApplicationContext.html

The application context provides:

• Bean factory functionality, inherited from ListableBeanFactory.
• Ability to load file resources, inherited from ResourceLoader.
• Ability to publish events to registered listeners, inherited from ApplicationEventPublisher.
• Ability to resolve messages, inherited from MessageSource.
• Inheritance from a parent context.

The application context is created during the bootstrapping phase of a Spring application.

Is the Spring container and application context one and the same thing? Some texts claim so.

Beans

Any non-trivial application consists of many components, each responsible for its own piece of the overall application functionality. These components coordinate with each other. Spring names these these components beans. The Spring container creates and introduces the beans to each other.

Bean Name

Bean ID

Bean alias

Collaborator beans

Describe bean initialization process. Who does it?

Stereotypes

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.

@Bean What is the fundamental difference between @Bean and the rest (@Component, etc.)?

Bean Scopes

Singleton

@Component
@Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
public class MySingletonComponent {
...
}

@Configuration
public class MyConfiguration {
  @Bean("mySingletonComponent")
  @Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
  public MySingletonComponent getMySingletonComponent() {
    return new MySingletonComponent();
 }
}