spring 不同注解的使用场景

spring中各种注解的使用场景。本文摘自stackoverflow.url:http://stackoverflow.com/questions/19414734/understanding-spring-autowired-usage

The 

@Autowired

 annotation
allows you to skip configurations elsewhere of what to inject and just does it for you. Assuming your package is 

com.mycompany.movies

 you
have to put this tag in your XML (application context file):

<context:component-scan base-package="com.mycompany.movies" />

This tag will do an auto-scanning. Assuming each class that has to become a bean is annotated with a correct annotation like 

@Component

 (for
simple bean) or 

@Controller

 (for
a servlet control) or 

@Repository

 (for 

DAO

 classes)
and these classes are somewhere under the package 

com.mycompany.movies

,
Spring will find all of these and create a bean for each one. This is done in 2 scans of the classes - the first time it just searches for classes that need to become a bean and maps the injections it needs to be doing, and on the second scan it injects the
beans. Of course, you can define your beans in the more traditional XML file or with a @Configuration class
(or any combination of the three).

The 

@Autowired

 annotation
tells Spring where an injection needs to occur. If you put it on a method 

setMovieFinder

 it
understands (by the prefix 

set

 +
the 

@Autowired

 annotation)
that a bean needs to be injected. In the second scan, Spring searches for a bean of type 

MovieFinder

,
and if it finds such bean, it inject  

it to this method. If it finds two such beans you will get an 

Exception

.
To avoid the 

Exception

,
you can use the 

@Qualifier

 annotation
and tell it which of the two beans to inject in the following manner:

@Qualifier("redBean")
class Red implements Color {
// Class code here
}

@Qualifier("blueBean")
class Blue implements Color {
// Class code here
}

Or if you prefer to declare the beans in your XML, it would look something like this:

<bean id="redBean" class="com.mycompany.movies.Red"/>

<bean id="blueBean" class="com.mycompany.movies.Blue"/>

In the 

@Autowired

 declaration,
you need to also add the 

@Qualifier

 to
tell which of the two color beans to inject:

@Autowired
@Qualifier("redBean")
public void setColor(Color color) {
this.color = color;
}

If you don't want to use two annotations (the 

@Autowired

 and 

@Qualifier

)
you can use 

@Resource

to
combine these two:

@Resource(name="redBean")
public void setColor(Color color) {
this.color = color;
}

The 

@Resource

 (you
can read some extra data about it in the first comment on this answer) spares you the use of two annotations and instead you only use one.

I'll just add two more comments:
Good practice would be to use 

@Inject

 instead
of 

@Autowired

 because
it is not Spring-specific and is part of the 

JSR-330

 standard.
Another good practice would be to put the 

@Inject

 / 

@Autowired

 on
a constructor instead of a method. If you put it on a constructor, you can validate that the injected beans are not null and fail fast when you try to start the application and avoid a 

NullPointerException

 when
you need to actually use the bean.

So, to sum it up: The 

@Autowired

 annotation
spares you the need to do the wiring by yourself in the XML file (or any other way) and just finds for you what needs to be injected where, and does that for you.