Core Spring – continued (2) – SpEL, Annotation based bean defs, Java-based configuration

Spring Expression Language (SpEL) introduced in Spring 3.0 introduces EL style language for referencing beans and properties, and gives a standard syntax for doing property replacement into beans.

Annotation based bean definition

• component scan allows you to configure annotated beans – simplest approach, avoids xml:

Enabled with context:component-scan in xml file:

<context:component-scan base-package="your.package"/>

• @Component – declares beans
• @Autowired – bean of a matching type is automatically injected into contructor, method, or field
• Only one constructor can be autowired, but multiple methods and attributes can be autowired
• @Autowired properties don’t need setters
• @Autowired methods don’t need to be setters, and don’t need to be named ‘set…()’
• @Qualifier allows you to specify a specific bean id if matching beans by type is ambiguous

‘The spirit of autowiring is by type’ – use of the @Qualifier should be only if necessary, since it restricts the flexibility and power of the Autowiring.

Use of @Autowired implies @Required (don’t need to explicitly defined @Required if @Autowired)

Stereotype annotations

Also enabled via context:component-scan

• @Service – indicates purpose in layering, but doesn’t add any additional behavior

Following stereotypes do add extra behavior:

• @Repository – data access
• @Controller – MVC controller
• @Configuration – programmatic Spring configuration

Other Spring projects use similar annotation approach for adding additional stereotype based functionality.

JSR 330 CDI Annotation Support

• Spring is a an implementation of JSR330
• provides support for JSR330 standard @Named, @Inject, @Scope, @Singleton
• Spring provides other Spring specific annotations not supported in JSR330 (@Value, @Required, @Lazy)

Bean configuration using Annotations & Java: @Configuration

• Enabled with context:component-scan
• @Configuration marks configuration clas
• Use @Bean on method that provides instance of the bean
• Method name is the id of the bean, eg participantService() – id = “participantService”
• Java code can perform any initialization – you can invoke any other methods necessary to perform initialization
• All @Bean annotated methods produce singletons by default – @Bean methods are proxied to add this behavior – this can be overridden with @Scope(“prototype”)
• @Import allows to import other @Configuration classes
• Alternate configuration: AnnotationConfigApplicationContext – instantiating this context allows a completely xml free configuration approach

 

Annotations vs XML based configuration

• easier to make changes to annotated beans during development
• could always move to xml config when beans become more mature, but no compelling reason to do this, could always just leave the annotations
• xml config always overrides the annotation metadata

Spring JUnit Support
Integration testing support allows testing with Spring Bean dependencies

@ContextConfuguration("example-context.xml")
@RunWith(SpringJUnit4ClassRunner.class)
public class ExampleTest
{
    @Autowired
    private ExampleService exampleService;

    @Test
    public void eampleTest() { ... }
}