The IoC Container 1.9.3-1.9.8

1.9.3. Fine-tuning Annotation-based Autowiring with @Primary

在有多个装配候选人的情况下，如果是基于类型进行装配，会出现歧义，可以用@Primary指定相应的bean定义，表示优先使用该候选人进行自动装配。
如下，使用@Primary注解firstMovieCatalog，表示优先使用该bean定义：

@Configuration
public class MovieConfiguration {

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

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

    // ...
}

下面代码在上面配置的作用下，movieCatalog会使用firstMovieCatalog进行自动装配：

public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;

    // ...
}

以上注解配置对应的xml配置如下：

1.9.4. Fine-tuning Annotation-based Autowiring with Qualifiers

@Qualifier注解可以更精确的控制自动装配的对象。下面的示例表示movieCatalog会自动装配Qualifier注解值为main的对象：

public class MovieRecommender {

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

    // ...
}

如果构造函数实例化可以像下面这样定义：

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;
    }

    // ...
}

被装配的对象在xml中这样定义：

相当于在被装配对象和自动装配处都指定qualifier的value，如果value值可以两方可以对应上，那么就选择这个bean进行装配，上述例子中，movieCatalog自动装配的就是SimpleMovieCatalog1。

基于注解的配置方式如下：

package examples;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component
@Qualifier("main")
public class SimpleMovieCatalog1 implements MovieCatalog {
}

如果在自动装配字段上使用@Qualifier注解，但是不提供@Qualifier注解的value值，Spring会默认使用bean name作为value的值（然而，Spring默认就是按照bean name进行匹配的，所以如果用户想按照bean name匹配，则不需要用到@Qualifier注解）。实际上，@Qualifier注解在寻找装配对象的候选人时，依然是按照类型来匹配，也就意味着，可以多个bean拥有相同的Qualifier的value，这种情况下，可以用来装配集合类型，比如Set，所有具有相同Qualifier值的MovieCatalog对象都会被放入Set，进行对象的自动装配。
自动装配注解配置：

    @Autowired
    @Qualifier("action")
    private Set movieCatalogSet;

候选人的注解配置：

package examples;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;

@Configuration
public class MovieConfiguration {

    @Bean
    @Primary
    @Qualifier("action")
    public MovieCatalog firstMovieCatalog() {
        return new MovieCatalog() {
            @Override
            public int hashCode() {
                return super.hashCode();
            }
        };
    }

    @Bean
    @Qualifier("action")
    public MovieCatalog secondMovieCatalog() {
        return new MovieCatalog() {
            @Override
            public int hashCode() {
                return super.hashCode();
            }
        };
    }

    @Bean
    @Qualifier("action")
    public MovieCatalog qualiferMovieCatalog() {
        return new MovieCatalog() {
            @Override
            public int hashCode() {
                return super.hashCode();
            }
        };
    }
}

在自动装配完成后，movieCatalogSet中将包含上面三个@Qualifier("action")注解的MovieCatalog对象。

@Autowired和@Resource注解的使用场景：

@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. By contrast, @Resource is supported only for fields and bean property setter methods with a single argument. As a consequence, you should stick with qualifiers if your injection target is a constructor or a multi-argument method.

Spring还支持自定义@Qualifier注解：

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Genre {

    String value();
}

用法跟@Qualifier注解相同：

public class MovieRecommender {

    @Autowired
    @Genre("Action")
    private MovieCatalog actionCatalog;

    private MovieCatalog comedyCatalog;

    @Autowired
    public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) {
        this.comedyCatalog = comedyCatalog;
    }

    // ...
}

xml配置中将候选人的qualifier type设置成Genre或者类的完全限定名，value还跟之前的意义相同：

在某些情况下，不带value值的自定义注解就足够用了，那么直接如下定义即可：

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Offline {

}

public class MovieRecommender {

    @Autowired
    @Offline 
    private MovieCatalog offlineCatalog;

    // ...
}

Spring还支持自定义@Qualifier注解时新增属性：

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MovieQualifier {

    String genre();

    Format format();
}

public enum Format {
    VHS, DVD, BLURAY
}

这样可以做更加细粒度的控制：

public class MovieRecommender {

    @Autowired
    @MovieQualifier(format=Format.VHS, genre="Action")
    private MovieCatalog actionVhsCatalog;

    @Autowired
    @MovieQualifier(format=Format.VHS, genre="Comedy")
    private MovieCatalog comedyVhsCatalog;

    @Autowired
    @MovieQualifier(format=Format.DVD, genre="Action")
    private MovieCatalog actionDvdCatalog;

    @Autowired
    @MovieQualifier(format=Format.BLURAY, genre="Comedy")
    private MovieCatalog comedyBluRayCatalog;

    // ...
}

xml配置候选人时，有两种配置方式（qualifier和meta两种）：

1.9.5. Using Generics as Autowiring Qualifiers

使用泛型作为自动装配候选人：

public interface Store {

}

@Configuration
public class MyConfiguration {

    @Bean
    public StringStore stringStore() {
        return new StringStore();
    }

    @Bean
    public IntegerStore integerStore() {
        return new IntegerStore();
    }
}

@Component
public class StringStore implements Store {
    
}

@Component
public class IntegerStore implements Store {
    
}

通过@Autowire实现泛型的自动装配：

    @Autowired
    private Store s1; //  qualifier, injects the stringStore bean

    @Autowired
    private Store s2; //  qualifier, injects the integerStore bean

1.9.6. Using CustomAutowireConfigurer

之前通过继承@Qualifier注解实现自定义@Qualifier注解，本节提供一种方式，在不继承@Qualifier注解的情况下，同样可以自定义qualifier annotation type。通过以下方式注册任意注解后，该注解就可以支持qualifier annotation type：

    
        
            examples.CustomQualifier
        
    

The AutowireCandidateResolver determines autowire candidates by（AutowireCandidateResolver决定装配对象的流程）:

• The autowire-candidate value of each bean definition
• Any default-autowire-candidates patterns available on the element
• The presence of @Qualifier annotations and any custom annotations registered with the CustomAutowireConfigurer

1.9.7. Injection with @Resource

@Resource是JSR-250支持的注解，可以用来注解字段或者setter方法。Spring也支持@Resource注解。

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Resource(name="myMovieFinder") 
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}

@Resource注解的属性是name，name指定bean的名称，所以@Resource是autowire by name的，如果不指定name，默认从setter方法或者字段推导出相应的name。
推导规则如下：

If no name is explicitly specified, the default name is derived from the field name or setter method. In case of a field, it takes the field name. In case of a setter method, it takes the bean property name.

大多数情况下，@Resource和@Autowired功能类似：

public class MovieRecommender {

    @Resource
    private CustomerPreferenceDao customerPreferenceDao;

    @Resource
    private ApplicationContext context; 

    public MovieRecommender() {
    }

    // ...
}

1.9.8. Using @PostConstruct and @PreDestroy

javax.annotation.PostConstruct 和javax.annotation.PreDestroy也是JSR-250的标准。Spring同样支持这两个注解。

public class CachingMovieLister {

    @PostConstruct
    public void populateMovieCache() {
        // populates the movie cache upon initialization...
    }

    @PreDestroy
    public void clearMovieCache() {
        // clears the movie cache upon destruction...
    }
}

@PostConstruct，@PreDestroy和@Resource都是由CommonAnnotationBeanPostProcessor处理的

@PostConstruct，@PreDestroy和@Resource注解在JAVA9之后已经删除，如果需要引用，需要到maven下载第三方包