Spring Framework------>version4.3.5.RELAESE----->Reference Documentation学习心得----->使用spring framework的IoC容器功能----->方法二：使用annotion配置beans之间的依赖关系

annotation-based configurations

1.概述：

- spring framework支持三种形式的configuration，xml-based configuration、annotion-based configuration、Java-based configuration。这三种方法各有优缺点，开发者可以根据实际需要选取某种方式编写配置代码来配置beans之间的依赖关系，也可以混合使用多种方式来编写配置代码。
- spring framework除了拥有自己定义的annotion之外，还支持其他标准所定义的annotion，如
  - - Spring 2.5 also added support for JSR-250 annotations such as @PostConstruct, and @PreDestroy.
    - Spring 3.0 added support for JSR-330 (Dependency Injection for Java) annotations contained in the javax.inject package such as @Inject and @Named.
- 如果程序中同时使用了annotion-based configuration和xml-based configuration，那么xml中的配置信息会覆盖annotion中的配置信息。

2.spring framework所支持的注释

spring framework自定义的annotion：

@Required，指示某个属性必须被赋初始值，若没被赋初始值则抛出异常

该注解可以放在setter()或者init method上，使得实例化相应bean时，对应的属性必须被赋初始值
example,

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Required
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...

}

@Autowired自动初始化，给参数或者属性赋初始值

该注解可以放置在某个构造函数上或者被放置在setter函数上，也可以被放置在某个含参数的成员函数上，也可以直接修饰一个属性
@Autowired是根据type匹配，并注册dependencies,有时候按照type匹配会匹配上多个bean实例，这时就需要@Autowired和@Primary、@Qulifier注解搭配使用
@Autowired方式和@Resource注解都可以配置beans之间的依赖关系，前者按照type进行匹配，后者按照name进行匹配，通常我们建议使用@Resource注解配置依赖关系

public class MovieRecommender {

    private final CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...

}

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...

}

public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog,
            CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...

}

You can apply @Autowired to fields as well and even mix it with constructors:

public class MovieRecommender {

    private final CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    private MovieCatalog movieCatalog;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...

}

public class MovieRecommender {

    @Autowired
    private MovieCatalog[] movieCatalogs;

    // ...

}

public class MovieRecommender {

    private Set<MovieCatalog> movieCatalogs;

    @Autowired
    public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...

}

public class MovieRecommender {

    private Map<String, MovieCatalog> movieCatalogs;

    @Autowired
    public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...

}

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired(required=false)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...

}

@Order or standard @Priority annotation
- Your beans can implement the org.springframework.core.Ordered interface or either use the @Order or standard @Priority annotation if you want items in the array or list to be sorted into a specific order.

@Primary

使用该注解微调@autowired注解，@Autowired注解默认情况下会使IOC容器按照type类型注入dependencies,但是按照type进行匹配的话有可能会有多个候选方案都满足匹配条件，这会抛出异常，所以使用@Primary对@Autowired进行更精确的控制，使得有多个bean实例都满足要求时，只选用使用@Primary 标注的实例被用于注册到其他的bean实例中

@Configuration
public class MovieConfiguration {

    @Bean
    @Primary
    public MovieCatalog firstMovieCatalog() { ... }

    @Bean
    public MovieCatalog secondMovieCatalog() { ... }

    // ...

}

With such configuration, the following MovieRecommender will be autowired with the firstMovieCatalog.

public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;

    // ...

}

上述配置和下面的xml-based configuration等效：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog" primary="true">
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>

@Qualifier

和@Primary注解一样，这个注解也是用于有多个bean实例符合备选条件的情况，当有多个bean实例符合备选条件时，IOC容器会不知道应该选择哪个实例作为依赖注入其他bean实例，这时就需要使用@Qulifier注解来告诉IOC容器应该选择哪个具体的bean实例作为被注入的bean实例

public class MovieRecommender {

    @Autowired
    @Qualifier("main")
    private MovieCatalog movieCatalog;

    // ...

}

The @Qualifier annotation can also be specified on individual constructor arguments or method parameters:

public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(@Qualifier("main")MovieCatalog movieCatalog,
            CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...

}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier value="main"/>

        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier value="action"/>

        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>

@Resource

3.如何使用spring framework中的annotion-based configuration

- step1，首先弄懂spring framework中支持哪些注解，如@required、@autowired。。。
- step2,在spring 的IOC容器中声明相应的bean,以使得IOC容器可以识别这些annotion
  - 概述，可以有三种方法声明annotion相关的bean,从而使得IOC容器可以识别这些annotion
  - 方法一，直接声明相应注解对应的bean,如若要使用 @Required注解，就必须声明RequiredAnnotationBeanPostProcessor的Bean
    - - AutowiredAnnotationBeanPostProcessor,@Autowired
      - CommonAnnotationBeanPostProcessor,@Resource、@PostConstruct、@PreDestroy
      - PersistenceAnnotationBeanPostProcessor,@PersistenceContext
      - RequiredAnnotationBeanPostProcessor，@Required
      - 。。。　　　　
  - 方法二，直接通过<context:annotation-config/>来将上述所有注解相关的XxxAnnotionBeanPostProcessor都包含进对应的IOC容器，使得该容器可以识别所有的注解
  - 方法三，可以直接通过<context:component-scan base-package="pack.pack"/>使得IOC容器可以识别上述所有注解
  - 更多解释参见博客：如何使得spring framework相关的注解annotion生效
  - 具体操作：spring-configuration.xml
```
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
 
    
    <context:annotation-config/>

</beans>
```
- step3,明确上述代码虽然配置IOC容器使得该IOC容器能够识别相关注解，但是这个识别是有一定范围的：即该配置使得该IOC容器只会扫描该容器范围之内的bean的annotion信息，范围之外的annotion信息它是不会理会的。

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