Spring 5.0.0框架介绍_中英文对照_3.9

文章作者:Tyan
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3.9 Annotation-based container configuration

Are annotations better than XML for configuring Spring?

The introduction of annotation-based configurations raised the question of whether this approach is ‘better’ than XML. The short answer is it depends. The long answer is that each approach has its pros and cons, and usually it is up to the developer to decide which strategy suits them better. Due to the way they are defined, annotations provide a lot of context in their declaration, leading to shorter and more concise configuration. However, XML excels at wiring up components without touching their source code or recompiling them. Some developers prefer having the wiring close to the source while others argue that annotated classes are no longer POJOs and, furthermore, that the configuration becomes decentralized and harder to control.

No matter the choice, Spring can accommodate both styles and even mix them together. It’s worth pointing out that through its JavaConfig option, Spring allows annotations to be used in a non-invasive way, without touching the target components source code and that in terms of tooling, all configuration styles are supported by the Spring Tool Suite.

 

在配置Spring时注解是否比XML更好?

基于注解配置的引入引出了一个问题——这种方式是否比基于XML的配置更好。简短的回答是视情况而定。长一点的回答是每种方法都有它的优点和缺点,通常是由开发者决定哪一种策略更适合他们。由于注解的定义方式,注解在它们的声明中提供了许多上下文,导致配置更简短更简洁。然而,XML擅长连接组件而不必接触源代码或重新编译它们。一些开发者更喜欢接近源代码,而另一些人则认为基于注解的类不再是POJOs,此外,配置变的去中心化,而且更难控制。

无论选择是什么,Spring都能容纳这两种风格,甚至可以将它们混合在一起。值得指出的是,通过它的Java配置选项,Spring允许注解以一种非入侵的方式使用,不触碰目标组件源码和那些工具,所有的配置风格由Spring工具套件支持。

An alternative to XML setups is provided by annotation-based configuration which rely on the bytecode metadata for wiring up components instead of angle-bracket declarations. Instead of using XML to describe a bean wiring, the developer moves the configuration into the component class itself by using annotations on the relevant class, method, or field declaration. As mentioned in the section called “Example: The RequiredAnnotationBeanPostProcessor”, using a BeanPostProcessor in conjunction with annotations is a common means of extending the Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing required properties with the @Required annotation. Spring 2.5 made it possible to follow that same general approach to drive Spring’s dependency injection. Essentially, the @Autowired annotation provides the same capabilities as described in Section 3.4.5, “Autowiring collaborators” but with more fine-grained control and wider applicability. 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. Details about those annotations can be found in the relevant section.

基于注解的配置提供了一种XML设置的可替代方式,它依赖于字节码元数据来组装组件,而不是用尖括号声明的方式。代替使用XML来描述bean组装,开发者通过将注解使用在相关的类,方法或字段声明中,将配置移动到了组件类本身的内部。正如在“Example: The RequiredAnnotationBeanPostProcessor”那节提到的那样,使用BeanPostProcessor与注解结合是扩展Spring IoC容器的的常见方法。例如,Spring 2.0引入了@Required注解来执行需要的属性的可能性。Spring 2.5使以同样地通用方法来驱动Spring的依赖注入变为可能。本质上来说,@Autowired提供了如3.4.5小节描述的同样的能力。“Autowiring collaborators”但更细粒度的控制和更广的应用性。Spring 2.5也添加对JSR-250注解的支持,例如,@PostConstruct@PreDestroy
。Spring 3.0添加了对JSR-330,包含在javax.inject包内的注解(Java的依赖注入)的支持,例如@Inject@Named。关于这些注解的细节可以在相关的小节找到。

Annotation injection is performed before XML injection, thus the latter configuration will override the former for properties wired through both approaches.

 

注解注入在XML注入之前进行,因此对于通过两种方法进行组装的属性后者的配置会覆盖前者。

As always, you can register them as individual bean definitions, but they can also be implicitly registered by including the following tag in an XML-based Spring configuration (notice the inclusion of the context namespace):

跟以前一样,你可以作为单独的bean定义来注册它们,但也可以通过在一个基于XML的Spring配置(注入包含上下文命名空间)中包含下面的标签来隐式的注册它们:

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<?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>

(The implicitly registered post-processors include AutowiredAnnotationBeanPostProcessor, CommonAnnotationBeanPostProcessor, PersistenceAnnotationBeanPostProcessor, as well as the aforementioned RequiredAnnotationBeanPostProcessor.)

(隐式注册的后处理器包括 AutowiredAnnotationBeanPostProcessorCommonAnnotationBeanPostProcessorPersistenceAnnotationBeanPostProcessor和前面提到的RequiredAnnotationBeanPostProcessor。)

<context:annotation-config/> only looks for annotations on beans in the same application context in which it is defined. This means that, if you put <context:annotation-config/> in a WebApplicationContext for a DispatcherServlet, it only checks for @Autowired beans in your controllers, and not your services. See Section 18.2, “The DispatcherServlet” for more information.

 

<context:annotation-config/>仅在定义它的同样的应用上下文中寻找注解的beans。这意味着,如果你在一个为DispatcherServlet服务的WebApplicationContext中放置了<context:annotation-config/>,它只能在你的控制器中寻找@Autowired注解的beans,而不是在你的服务层中。更多信息请看18.2小节,“The DispatcherServlet”。

3.9.1 @Required

The @Required annotation applies to bean property setter methods, as in the following example:

@Required注解应用到bean属性的setter方法上,例子如下:

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

This annotation simply indicates that the affected bean property must be populated at configuration time, through an explicit property value in a bean definition or through autowiring. The container throws an exception if the affected bean property has not been populated; this allows for eager and explicit failure, avoiding NullPointerExceptions or the like later on. It is still recommended that you put assertions into the bean class itself, for example, into an init method. Doing so enforces those required references and values even when you use the class outside of a container.

这个注解仅仅是表明受影响的bean属性必须在配置时通过显式的bean定义或自动组装填充。如果受影响的bean属性没有填充,容器会抛出一个异常,这允许及早明确的失败,避免NullPointerExceptions或后面出现类似的情况。仍然建议你在bean类本身加入断言,例如,加入到初始化方法中。这样做可以强制这些需要的引用和值,甚至是你在容器外部使用这个类的时候。

3.9.2 @Autowired

JSR 330’s @Inject annotation can be used in place of Spring’s @Autowired annotation in the examples below. See here for more details.

 

在下面的例子中JSR 330的@Inject注解可以用来代替Spring的@Autowired注解。

You can apply the @Autowired annotation to constructors:

你可以将@Autowired注解应用到构造函数上。

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

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.

 

从Spring框架4.3起,如果目标bena仅定义了一个构造函数,那么@Autowired注解的构造函数不再是必要的。如果一些构造函数是可获得的,至少有一个必须要加上注解,以便于告诉容器使用哪一个。

As expected, you can also apply the @Autowired annotation to “traditional” setter methods:

正如预料的那样,你也可以将@Autowired注解应用到“传统的”setter方法上:

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

You can also apply the annotation to methods with arbitrary names and/or multiple arguments:

你也可以应用注解到具有任何名字和/或多个参数的方法上:

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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:

你也可以应用@Autowired到字段上,甚至可以与构造函数混合用:

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

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:

通过给带有数组的字段或方法添加@Autowired注解,也可以从ApplicationContext中提供一组特定类型的bean:

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

The same applies for typed collections:

同样也可以应用到具有同一类型的集合上:

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

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.

 

如果你希望数组或列表中的项按指定顺序排序,你的bean可以实现org.springframework.core.Ordered接口,或使用@Order或标准@Priority注解。

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:

只要期望的key是String,那么类型化的Maps就可以自动组装。Map的值将包含所有期望类型的beans,key将包含对应的bean名字:

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

By default, the autowiring fails whenever zero candidate beans are available; the default behavior is to treat annotated methods, constructors, and fields as indicating required dependencies. This behavior can be changed as demonstrated below.

默认情况下,当没有候选beans可获得时,自动组装会失败;默认的行为是将注解的方法,构造函数和字段看作指明了需要的依赖。这个行为也可以通过下面的方式去改变。

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

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.

@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.

 

每个类只有一个构造函数可以标记为必需的,但可以注解多个非必需的构造函数。在这种情况下,会考虑这些候选者中的每一个,Spring使用最贪婪的构造函数,即依赖最满足的构造函数,具有最大数目的参数。

建议在@Required注解之上使用@Autowiredrequired特性。required特性表明这个属性自动装配是不需要的,如果这个属性不能被自动装配,它会被忽略。另一方面@Required是更强大的,在它强制这个属性被任何容器支持的bean设置。如果没有值注入,会抛出对应的异常。

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.

你也可以对那些已知的具有可解析依赖的接口使用@AutowiredBeanFactoryApplicationContextEnvironment, ResourceLoaderApplicationEventPublisherMessageSource。这些接口和它们的扩展接口,例如ConfigurableApplicationContextResourcePatternResolver,可以自动解析,不需要特别的设置。

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public class MovieRecommender {
@Autowired
private ApplicationContext context;
public MovieRecommender() {
}
// ...
}

@Autowired, @Inject, @Resource, and @Value annotations are handled by Spring BeanPostProcessor implementations which in turn means that you cannot apply these annotations within your own BeanPostProcessor or BeanFactoryPostProcessor types (if any). These types must be ‘wired up’ explicitly via XML or using a Spring @Bean method.

 

@Autowired@Inject@Resource@Value注解是通过Spring BeanPostProcessor实现处理,这反过来意味着你不能在你自己的BeanPostProcessorBeanFactoryPostProcessor中应用这些注解(如果有的话)。这些类型必须显式的通过XML或使用Spring的@Bean方法来’wired up’。

3.9.3 Fine-tuning annotation-based autowiring with @Primary

Because autowiring by type may lead to multiple candidates, it is often necessary to have more control over the selection process. One way to accomplish this is with Spring’s @Primary annotation. @Primary indicates that a particular bean should be given preference when multiple beans are candidates to be autowired to a single-valued dependency. If exactly one ‘primary’ bean exists among the candidates, it will be the autowired value.

因为根据类型的自动装配可能会导致多个候选目标,所以在选择过程中进行更多的控制经常是有必要的。一种方式通过Spring的@Primary注解来完成。当有个多个候选bean要组装到一个单值的依赖时,@Primary表明指定的bean应该具有更高的优先级。如果确定一个’primary’ bean位于候选目标中间,它将是那个自动装配的值。

Let’s assume we have the following configuration that defines firstMovieCatalog as the primary MovieCatalog.

假设我们具有如下配置,将firstMovieCatalog定义为主要的MovieCatalog

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@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.

根据这样的配置,下面的MovieRecommender将用firstMovieCatalog进行自动装配。

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public class MovieRecommender {
@Autowired
private MovieCatalog movieCatalog;
// ...
}

The corresponding bean definitions appear as follows.

对应的bean定义如下:

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<?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>

3.9.4 Fine-tuning annotation-based autowiring with qualifiers

@Primary is an effective way to use autowiring by type with several instances when one primary candidate can be determined. When more control over the selection process is required, Spring’s @Qualifier annotation can be used. You can associate qualifier values with specific arguments, narrowing the set of type matches so that a specific bean is chosen for each argument. In the simplest case, this can be a plain descriptive value:

当有多个实例需要确定一个主要的候选对象时,@Primary是一种按类型自动装配的有效方式。当需要在选择过程中进行更多的控制时,可以使用Spring的@Qualifier注解。为了给每个选择一个特定的bean,你可以将限定符的值与特定的参数联系在一起,减少类型匹配集合。在最简单的情况下,这是一个纯描述性值:

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public class MovieRecommender {
@Autowired
@Qualifier("main")
private MovieCatalog movieCatalog;
// ...
}

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

@Qualifier注解也可以指定单个构造函数参数或方法参数:

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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;
}
// ...
}

The corresponding bean definitions appear as follows. The bean with qualifier value “main” is wired with the constructor argument that is qualified with the same value.

对应的bean定义如下。限定符值为”main”的bean被组装到有相同值的构造函数参数中。

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<?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>

For a fallback match, the bean name is considered a default qualifier value. Thus you can define the bean with an id “main” instead of the nested qualifier element, leading to the same matching result. However, although you can use this convention to refer to specific beans by name, @Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even with the bean name fallback, always have narrowing semantics within the set of type matches; they do not semantically express a reference to a unique bean id. Good qualifier values are “main” or “EMEA” or “persistent”, expressing characteristics of a specific component that are independent from the bean id, which may be auto-generated in case of an anonymous bean definition like the one in the preceding example.

对于回退匹配,bean名字被认为是默认的限定符值。因此你可以定义一个id为main的bean来代替内嵌的限定符元素,会有同样的匹配结果。然而,尽管你可以使用这个约定根据名字引用特定的beans,但是@Autowired从根本上来讲是使用可选的语义限定符来进行类型驱动注入的。这意味着限定符的值,即使回退到bean名称,总是缩小语义类型匹配的集合;它们没有从语义上将一个引用表达为一个唯一的bean id。好的限定符值是”main”或”EMEA”或”persistent”,表达一个特定组件的性质,这个组件是独立于bean id的,即使前面例子中像这个bean一样的匿名bean会自动生成id。

Qualifiers also apply to typed collections, as discussed above, for example, to Set<MovieCatalog>. In this case, all matching beans according to the declared qualifiers are injected as a collection. This implies that qualifiers do not have to be unique; they rather simply constitute filtering criteria. For example, you can define multiple MovieCatalog beans with the same qualifier value “action”, all of which would be injected into a Set<MovieCatalog> annotated with @Qualifier("action").

正如前面讨论的那样,限定符也可以应用到类型结合上,例如,Set<MovieCatalog>。在这个例子中,根据声明的限定符匹配的所有beans作为一个集合进行注入。这意味着限定符不必是唯一的;它们只是构成过滤标准。例如,你可以定义多个具有同样限定符值”action”的MovieCatalog,所有的这些都将注入到带有注解@Qualifier("action")Set<MovieCatalog>中。

If you intend to express annotation-driven injection by name, do not primarily use @Autowired, even if is technically capable of referring to a bean name through @Qualifier values. Instead, use the JSR-250 @Resource annotation, which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process. @Autowired has rather different semantics: After selecting candidate beans by type, the specified String qualifier value will be considered within those type-selected candidates only, e.g. matching an “account” qualifier against beans marked with the same qualifier label.

For beans that are themselves defined as a collection/map or array type, @Resource is a fine solution, referring to the specific collection or array bean by unique name. That said, as of 4.3, collection/map and array types can be matched through Spring’s @Autowired type matching algorithm as well, as long as the element type information is preserved in @Bean return type signatures or collection inheritance hierarchies. In this case, qualifier values can be used to select among same-typed collections, as outlined in the previous paragraph.

As of 4.3, @Autowired also considers self references for injection, i.e. references back to the bean that is currently injected. Note that self injection is a fallback; regular dependencies on other components always have precedence. In that sense, self references do not participate in regular candidate selection and are therefore in particular never primary; on the contrary, they always end up as lowest precedence. In practice, use self references as a last resort only, e.g. for calling other methods on the same instance through the bean’s transactional proxy: Consider factoring out the affected methods to a separate delegate bean in such a scenario. Alternatively, use @Resource which may obtain a proxy back to the current bean by its unique name.

@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, stick with qualifiers if your injection target is a constructor or a multi-argument method.

@Autowired可以应用到字段,构造函数和多参数方法上,允许通过限定符注解在参数层面上缩减候选目标。相比之下,@Resource仅支持字段和bean属性的带有单个参数的setter方法。因此,如果你的注入目标是一个构造函数或一个多参数的方法,坚持使用限定符。

 

如果你想通过名字表达注解驱动的注入,不要主要使用@Autowired,虽然在技术上能通过@Qualifier值引用一个bean名字。作为可替代产品,可以使用JSR-250 @Resource注解,它在语义上被定义为通过组件唯一的名字来识别特定的目标组件,声明的类型与匹配过程无关。@Autowired有不同的语义:通过类型选择候选beans,特定的String限定符值被认为只在类型选择的候选目标中,例如,在那些标记为具有相同限定符标签的beans中匹配一个”account”限定符。

对于那些本身定义在集合/映射或数组类型中的beans来说,@Resource是一个很好的解决方案,适用于特定的集合或通过唯一名字区分的数组bean。也就是说,自Spring 4.3起,集合/映射和数组类型中也可以通过Spring的@Autowired类型匹配算法进行匹配,只要元素类型信息在@Bean中保留,返回类型签名或集合继承体系。在这种情况下,限定符值可以用来在相同类型的集合中选择,正如在前一段中概括的那样。

自Spring 4.3起,@Autowired也考虑自引用注入,例如,引用返回当前注入的bean。注意自注入是备用;普通对其它组件的依赖关系总是优先的。在这个意义上,自引用不参与普通的候选目标选择,因此尤其是从不是主要的;恰恰相反,它们最终总是最低的优先级。在实践中,自引用只是作为最后的手段,例如,通过bean的事务代理调用同一实例的其它方法:在考虑抽出受影响的方法来分隔代理bean的场景中。或者,使用@Resource通过它的唯一名字可能得到一个返回当前bean的代理。

@Autowired可以应用到字段,构造函数和多参数方法上,允许通过限定符注解在参数层面上缩减候选目标。相比之下,@Resource仅支持字段和bean属性的带有单个参数的setter方法。因此,如果你的注入目标是一个构造函数或一个多参数的方法,坚持使用限定符。

You can create your own custom qualifier annotations. Simply define an annotation and provide the @Qualifier annotation within your definition:

你可以创建自己的定制限定符注解。简单定义一个注解,在你自己的定义中提供@Qualifier注解:

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

Then you can provide the custom qualifier on autowired fields and parameters:

然后你可以在自动装配的字段和参数上提供定制的限定符:

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public class MovieRecommender {
@Autowired
@Genre("Action")
private MovieCatalog actionCatalog;
private MovieCatalog comedyCatalog;
@Autowired
public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) {
this.comedyCatalog = comedyCatalog;
}
// ...
}

Next, provide the information for the candidate bean definitions. You can add <qualifier/> tags as sub-elements of the <bean/> tag and then specify the type and value to match your custom qualifier annotations. The type is matched against the fully-qualified class name of the annotation. Or, as a convenience if no risk of conflicting names exists, you can use the short class name. Both approaches are demonstrated in the following example.

接下来,提供候选bean定义的信息。你可以添加<qualifier/>标记作为<bean/>标记的子元素,然后指定匹配你的定制限定符注解的类型和值。类型用来匹配注解的全限定类名称。或者,如果没有名称冲突的风险,为了方便,你可以使用简写的类名称。下面的例子证实了这些方法。

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<?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 type="Genre" value="Action"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<qualifier type="example.Genre" value="Comedy"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean id="movieRecommender" class="example.MovieRecommender"/>
</beans>

In Section 3.10, “Classpath scanning and managed components”, you will see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see Section 3.10.8, “Providing qualifier metadata with annotations”.

在3.10小节,“类路径扫描和管理组件”中,你将看到一个基于注解的替代方法,在XML中提供限定符元数据。特别地,看3.10.8小节,“用注解提供限定符元数据”。

In some cases, it may be sufficient to use an annotation without a value. This may be useful when the annotation serves a more generic purpose and can be applied across several different types of dependencies. For example, you may provide an offline catalog that would be searched when no Internet connection is available. First define the simple annotation:

在某些情况下,使用没有值的注解就是足够的。当注解为了通用的目的时,这是非常有用的,可以应用到跨几个不同类型的依赖上。例如,当网络不可用时,你可以提供一个要搜索的离线目录。首先定义一个简单的注解:

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

Then add the annotation to the field or property to be autowired:

然后将注解添加到要自动装配的字段或属性上:

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public class MovieRecommender {
@Autowired
@Offline
private MovieCatalog offlineCatalog;
// ...
}

Now the bean definition only needs a qualifier type:

现在bean定义只需要一个限定符类型:

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

You can also define custom qualifier annotations that accept named attributes in addition to or instead of the simple value attribute. If multiple attribute values are then specified on a field or parameter to be autowired, a bean definition must match all such attribute values to be considered an autowire candidate. As an example, consider the following annotation definition:

你也可以定义接收命名属性之外的定制限定符注解或代替简单的值属性。如果要注入的字段或参数指定了多个属性值,bean定义必须匹配所有的属性值才会被认为是一个可自动装配的候选目标。作为一个例子,考虑下面的注解定义:

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@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MovieQualifier {
String genre();
Format format();
}

In this case Format is an enum:

这种情况下Format是枚举类型:

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public enum Format {
VHS, DVD, BLURAY
}

The fields to be autowired are annotated with the custom qualifier and include values for both attributes: genre and format.

要自动装配的字段使用定制限定符进行注解,并且包含了两个属性值:genreformat

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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;
// ...
}

Finally, the bean definitions should contain matching qualifier values. This example also demonstrates that bean meta attributes may be used instead of the <qualifier/> sub-elements. If available, the <qualifier/> and its attributes take precedence, but the autowiring mechanism falls back on the values provided within the <meta/> tags if no such qualifier is present, as in the last two bean definitions in the following example.

最后,bean定义应该包含匹配的限定符值。这个例子也证实了bean元属性可以用来代替<qualifier/>子元素。如果可获得<qualifier/>,它和它的属性优先级更高,如果当前没有限定符,自动装配机制会将<meta/>内的值作为备用,正如下面的例子中的最后两个bean定义。

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<?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 type="MovieQualifier">
<attribute key="format" value="VHS"/>
<attribute key="genre" value="Action"/>
</qualifier>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<qualifier type="MovieQualifier">
<attribute key="format" value="VHS"/>
<attribute key="genre" value="Comedy"/>
</qualifier>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<meta key="format" value="DVD"/>
<meta key="genre" value="Action"/>
<!-- inject any dependencies required by this bean -->
</bean>
<bean class="example.SimpleMovieCatalog">
<meta key="format" value="BLURAY"/>
<meta key="genre" value="Comedy"/>
<!-- inject any dependencies required by this bean -->
</bean>
</beans>

3.9.5 Using generics as autowiring qualifiers

In addition to the @Qualifier annotation, it is also possible to use Java generic types as an implicit form of qualification. For example, suppose you have the following configuration:

除了@Qualifier注解外,也可以使用Java的泛型类型作为限定符的一种隐式方式。例如,假设你有如下配置:

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@Configuration
public class MyConfiguration {
@Bean
public StringStore stringStore() {
return new StringStore();
}
@Bean
public IntegerStore integerStore() {
return new IntegerStore();
}
}

Assuming that beans above implement a generic interface, i.e. Store<String> and Store<Integer>, you can @Autowire the Store interface and the generic will be used as a qualifier:

假设上面的beans实现了一个泛型接口,例如,Store<String>Store<Integer>,你可以@Autowire Store接口,泛型将作为限定符使用:

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@Autowired
private Store<String> s1; // <String> qualifier, injects the stringStore bean
@Autowired
private Store<Integer> s2; // <Integer> qualifier, injects the integerStore bean

Generic qualifiers also apply when autowiring Lists, Maps and Arrays:

当自动装配ListsMapsArrays时,也会应用泛型限定符:

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// Inject all Store beans as long as they have an <Integer> generic
// Store<String> beans will not appear in this list
@Autowired
private List<Store<Integer>> s;

3.9.6 CustomAutowireConfigurer

The CustomAutowireConfigurer is a BeanFactoryPostProcessor that enables you to register your own custom qualifier annotation types even if they are not annotated with Spring’s @Qualifier annotation.

CustomAutowireConfigurer是一个能使你注册自己的定制限定符注解类型的BeanFactoryPostProcessor,即使它们不能使用Spring的@Qualifier注解进行注解。

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<bean id="customAutowireConfigurer"
class="org.springframework.beans.factory.annotation.CustomAutowireConfigurer">
<property name="customQualifierTypes">
<set>
<value>example.CustomQualifier</value>
</set>
</property>
</bean>

The AutowireCandidateResolver determines autowire candidates by:

  • the autowire-candidate value of each bean definition

  • any default-autowire-candidates pattern(s) available on the <beans/> element

  • the presence of @Qualifier annotations and any custom annotations registered with the CustomAutowireConfigurer

AutowireCandidateResolver通过下面的方式决定自动装配的候选目标:

  • 每个bean定义的autowire-candidate

  • <beans/>元素可获得的任何default-autowire-candidates模式

  • 存在@Qualifier注解和任何在CustomAutowireConfigurer中注册的定制注解

When multiple beans qualify as autowire candidates, the determination of a “primary” is the following: if exactly one bean definition among the candidates has a primary attribute set to true, it will be selected.

当多个beans符合条件成为自动装配的候选目标时,”primary” bean的决定如下:如果在候选目标中某个确定的bean中的primary特性被设为true,它将被选为目标bean。

3.9.7 @Resource

Spring also supports injection using the JSR-250 @Resource annotation on fields or bean property setter methods. This is a common pattern in Java EE 5 and 6, for example in JSF 1.2 managed beans or JAX-WS 2.0 endpoints. Spring supports this pattern for Spring-managed objects as well.

Spring也支持使用JSR-250 @Resource对字段或bean属性setter方法进行注入。这是在Java EE 5和6中的一种通用模式,例如在JSF 1.2管理的beans或JAX-WS 2.0的端点。Spring对它管理的对象也支持这种模式。

@Resource takes a name attribute, and by default Spring interprets that value as the bean name to be injected. In other words, it follows by-name semantics, as demonstrated in this example:

@Resource采用名字属性,默认情况下Spring将名字值作为要注入的bean的名字。换句话说,它遵循by-name语义,下面的例子证实了这一点:

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

If no name is specified explicitly, 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. So the following example is going to have the bean with name “movieFinder” injected into its setter method:

如果没有显式的指定名字,默认名字从字段名或setter方法中取得。在字段情况下,它采用字段名称;在setter方法情况下,它采用bean的属性名。因此下面的例子将名字为movieFinder的bean注入到它的setter方法中:

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

The name provided with the annotation is resolved as a bean name by the ApplicationContext of which the CommonAnnotationBeanPostProcessor is aware. The names can be resolved through JNDI if you configure Spring’s SimpleJndiBeanFactory explicitly. However, it is recommended that you rely on the default behavior and simply use Spring’s JNDI lookup capabilities to preserve the level of indirection.

 

注解提供的名字被CommonAnnotationBeanPostProcessor感知的ApplicationContext解析为bean名字。如果你显式地配置了Spring的SimpleJndiBeanFactory,名字会通过JNDI解析。但是建议你依赖默认行为,简单使用Spring的JNDI查找功能保护间接查找级别。

In the exclusive case of @Resource usage with no explicit name specified, and similar to @Autowired, @Resource finds a primary type match instead of a specific named bean and resolves well-known resolvable dependencies: the BeanFactory, ApplicationContext, ResourceLoader, ApplicationEventPublisher, and MessageSource interfaces.

@Resource特有的没有显式名字指定的情况下,类似于@Autowired@Resource会进行主要的匹配类型来代替指定名字的bean并解析已知的可解析依赖:BeanFactoryApplicationContextResourceLoaderApplicationEventPublisherMessageSource接口。

Thus in the following example, the customerPreferenceDao field first looks for a bean named customerPreferenceDao, then falls back to a primary type match for the type CustomerPreferenceDao. The “context” field is injected based on the known resolvable dependency type ApplicationContext.

因此在下面的例子中,customerPreferenceDao字段首先查找名字为customerPreferenceDao的bean,然后回退到主要的类型为CustomerPreferenceDao的类型匹配。”context”字段会注入基于已知的可解析依赖类型ApplicationContext

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public class MovieRecommender {
@Resource
private CustomerPreferenceDao customerPreferenceDao;
@Resource
private ApplicationContext context;
public MovieRecommender() {
}
// ...
}

3.9.8 @PostConstruct and @PreDestroy

The CommonAnnotationBeanPostProcessor not only recognizes the @Resource annotation but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support for these annotations offers yet another alternative to those described in initialization callbacks and destruction callbacks. Provided that the CommonAnnotationBeanPostProcessor is registered within the Spring ApplicationContext, a method carrying one of these annotations is invoked at the same point in the lifecycle as the corresponding Spring lifecycle interface method or explicitly declared callback method. In the example below, the cache will be pre-populated upon initialization and cleared upon destruction.

CommonAnnotationBeanPostProcessor不仅识别@Resource注解,而且识别JSR-250生命周期注解。在Spring 2.5引入了对这些注解的支持,也提供了在初始化回调函数和销毁回调函数中描述的那些注解的一种可替代方式。假设CommonAnnotationBeanPostProcessor在Spring的ApplicationContext中注册,执行这些注解的方法在生命周期的同一点被调用,作为对应的Spring生命周期接口方法或显式声明的回调方法。在下面的例子中,缓存会预先放置接近初始化之前,并在销毁之前清除。

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public class CachingMovieLister {
@PostConstruct
public void populateMovieCache() {
// populates the movie cache upon initialization...
}
@PreDestroy
public void clearMovieCache() {
// clears the movie cache upon destruction...
}
}

For details about the effects of combining various lifecycle mechanisms, see the section called “Combining lifecycle mechanisms”.

 

关于组合各种生命周期机制的影响的更多细节,请看“组合生命周期机制”小节。

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