Difference between Spring annotations

Question

Questions:

1) Difference between @Component and @Configuration?

I have read that both remove the necessity of wiring code to be put in XML, but did not get the difference between these.

2) What are the differences between @Autowired, @Inject and @Resource?
- Which one to use when?
- What are the pros/cons of each?

Answer 1

@Component and @Configuration are indeed very different types of annotations.

@Component and similar annotations (@Service, @Repository, etc. )and its JSR-330 counterpart @Named allow you to declare beans that are to be picked up by autoscanning with <context:component-scan/> or @ComponentScan they register the bean definition for the classes, so they are roughly equivalent to declaring the specified beans with the <bean ... /> tag in XML. This bean types will adhere to the standard proxy creation policies.

@Configuration annotation was designed as the replacement of the XML configuration file. To create @Configuration annotated beans, Spring will always use CGLIB to subclass the @Configuration annotated class, overriding its @Bean annotated method to replace it with the bean lookup method to make singleton beans to be created only once. (Spring does not use CGLIB to intercept internal method calls of normal Spring beans, it creates a separate instance of proxy instead(same way like JDK proxy does). Doing so allows to use proxies to avoid cardinality mismatch - for example a proxy singleton can fetch current session bean, which is not possible with class inheritance only. ). Despite that, @Configuration annotated classes are still able to use annotated(@Autowired, @Inject etc.) fields and properties to request beans (and even other @Configuration annotated beans too) from the container.

Example from 4.12.5 section of the documentation

@Configuration
public class AppConfig {

  @Bean
  public ClientService clientService1() {
    ClientServiceImpl clientService = new ClientServiceImpl();
    clientService.setClientDao(clientDao());
    return clientService;
  }
  @Bean
  public ClientService clientService2() {
    ClientServiceImpl clientService = new ClientServiceImpl();
    clientService.setClientDao(clientDao());
    return clientService;
  }

  @Bean
  public ClientDao clientDao() {
    return new ClientDaoImpl();
  }
}

in the example above only one ClientDao instance will be created.

@Autowired is Spring annotation, while @Inject is a JSR-330 annotation. @Inject is equivalent to @Autowired or @Autowired(required=true), but you can't get @Autowired(required=false) behavior with the JSR-330 @Injectannotation. This annotation always uses by-type autowiring.

Spring implements JSR-250 @Resource annotation in a rather special way. @Resource was originally designed for locating JNDI resources in Java EE, but Spring widens it applicability making it possible to wire to any bean in the container(JNDI resources are available as beans with the help of SimpleJndiBeanFactory). The name of the corresponding bean can be specified as name attribute of @Resource annotation, if no name was specified, then the name of the annotated field or property will be used. Another strange feature is that if no bean with the property name was found spring will fallback to by-type wiring.

Example Imagine that we have an AlphaClass bean named beanAlpha and a BetaClass bean beanBeta in the container.

@Resource 
BetaClass something;  // Wires to beanBeta - by-type

@Resource 
BetaClass beanAlpha;  // Will throw exception, because "beanAlpha" is not BetaClass -> it's a bad idea to use @Resource as a replacement of @Autowired

@Resource 
Object beanAlpha;  //Wires to beanAlpha - by-name

So it's a good practice to always specify resource name explicitly when using @Resource annotation.

Documentation

Spring annotations

Bean standard annotations

update fixed JSR references as shevchik has pointed out. DI specific annotations are provided by JSR-330, which was developed by Google (Guice Framework) and SpringSource(Spring Framework) engineers. @Resource is JNDI based and provided by JSR-250.