@Autowired注解总结

@Autowired使用注解总结

1.使用方法

1.1 作用在构造器上

public class MovieRecommender {
 private final CustomerPreferenceDao customerPreferenceDao;
 @Autowired
 public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
 this.customerPreferenceDao = customerPreferenceDao;
 }
 // ...
}

注意：

As of Spring Framework 4.3, the @Autowired constructor is no longer necessary if the target
bean only defines one constructor. If several constructors are available, at least one must be
annotated to teach the container which one it has to use.

如果使用该注解的类有一个构造器，则 is no longer necessary ，没有必要再单独放置一个@Autowired注解了，Spring框架会自动帮助我们完成注入的功能。但是如果该该有多个注解 several constructors are available，这个时候我们就需要选择一个构造函数，并标注上该注解@Autowired了。

1.2 使用在setter方法上

public class SimpleMovieLister {
 private MovieFinder movieFinder;
 @Autowired
 public void setMovieFinder(MovieFinder movieFinder) {
 this.movieFinder = movieFinder;
 }
 // ...
}

1.3 使用在域属性上

public class MovieRecommender {
 private final CustomerPreferenceDao customerPreferenceDao;
 @Autowired
 private MovieCatalog movieCatalog;
 @Autowired
 public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
 this.customerPreferenceDao = customerPreferenceDao;
 }
 // ...
}

1.4 使用在任意方法名和参数的普通方法上

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

1.5 使用在域属性数组上

It is also possible to provide all beans of a particular type from the ApplicationContext by adding
the annotation to a field or method that expects an array of that type:

public class MovieRecommender {
 @Autowired
 private MovieCatalog[] movieCatalogs;
 // ...
}

1.6 使用在java集合类型上

public class MovieRecommender {
 private Set movieCatalogs;
 @Autowired
 public void setMovieCatalogs(Set movieCatalogs) {
 this.movieCatalogs = movieCatalogs;
 }
 // ...
}

小提示：如果当初定义的bean上有@Order注解或者标准注解@Priority的话，那么注入的集合就是有序的，这个元素顺序是当初注解定义的优先顺序。

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.

使用在Map上

Even typed Maps can be autowired as long as the expected key type is String. The Map values will
contain all beans of the expected type, and the keys will contain the corresponding bean names:

如果注入的Map中Key为String类型，并且代表每个bean的name,value为指定的bean类型，则也可以直接注入.

public class MovieRecommender {
 private Map movieCatalogs;
 @Autowired
 public void setMovieCatalogs(Map movieCatalogs) {
 this.movieCatalogs = movieCatalogs;
 }
 // ...
}

1.9 使用在Spring内部的接口上

You can also use @Autowired for interfaces that are well-known resolvable
dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader,
ApplicationEventPublisher, and MessageSource. These interfaces and their extended
interfaces, such as ConfigurableApplicationContext or ResourcePatternResolver, are
automatically resolved, with no special setup necessary.

该注解同样可以使用在BeanFactory, ApplicationContext, Environment, ResourceLoader,
ApplicationEventPublisher 这些我们都知道而且很重要的接口上。

1.8 required属性

@Autowired’s required attribute is recommended over the `@Required
annotation. The required attribute indicates that the property is not required for autowiring purposes, the property is ignored if it cannot be autowired. @Required, on the other hand, is
stronger in that it enforces the property that was set by any means supported by the container. If
no value is injected, a corresponding exception is raised.

@Autowired注解的required属性值可以为true和false。如果为true的话，则在进行注入的时候，如果找不到要注入的类，则会抛错。如果为false，则表示不是强制必须能够找到相应的类，无论是否注入成功，都不会抛错。

另外，当利用该注解注入相应的对象（其实就是类初始化为一个对象的过程），会调用该对象的构造方法，如果该对象有多个构造方法，则Spring就会“贪心”地调用参数最多的那个构造方法。相关的官方说明如下：

Only one annotated constructor per-class can be marked as required, but multiple non-required
constructors can be annotated. In that case, each is considered among the candidates and Spring
uses the greediest constructor whose dependencies can be satisfied, that is the constructor that
has the largest number of arguments.

2.其他注入注解

• @Inject (javax.inject JSR330 (Dependency Injection for Java))
• @Resource (javax.annotation JSR250 (Common Annotations for Java))

这里引用参考文章的区别总结：

@Autowired和@Resource

• autowired by type
• 可以 通过@Qualifier 显式指定 autowired by qualifier name（非集合类。注意：不是autowired by bean name！）
• 如果 autowired by type 失败（找不到或者找到多个实现），则退化为autowired by field name（非集合类）

@Resource

• 默认 autowired by field name
• 如果 autowired by field name失败，会退化为 autowired by type
• 可以 通过@Qualifier 显式指定 autowired by qualifier name
• 如果 autowired by qualifier name失败，会退化为 autowired by field name。但是这时候如果 autowired by field name失败，就不会再退化为autowired by type了。

更多具体细节参考这篇文章: Spring各种依赖注入注解的区别

说明

1. 这篇总结中所引用的英文描述和代码片段全部来自与Spring的官方文档：Spring Framework Reference Documentation 4.3.3 RELEASE
2. 参考文章:Spring各种依赖注入注解的区别