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【Spring】Autowired原理及与Resource注解区别

程序员文章站 2022-03-13 17:16:54
Autowired注解 Autowired顾名思义,表示自动注入,如下是Autowired注解的源代码: 从Autowired的实现可以看到,Autowired可以用于类的构造方法,类的字段,类的方法以及注解类型上,但是Autowired不能用于类上面。 关于Autowired注解,有如下问题需要解 ......

Autowired注解

Autowired顾名思义,表示自动注入,如下是Autowired注解的源代码:

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {

    /**
     * Declares whether the annotated dependency is required.
     * <p>Defaults to {@code true}.
     */
    boolean required() default true;

}

从Autowired的实现可以看到,Autowired可以用于类的构造方法,类的字段,类的方法以及注解类型上,但是Autowired不能用于类上面。

关于Autowired注解,有如下问题需要解决:

 1. Autowired作用在不同的范围上(构造方法,字段、方法)上,它的装配策略如何,按名称还是类型?

 2. 为构造方法,字段和方法添加Autowired注解之后,谁来解析这个Autowired注解,完成装配

 3. 装配的bean从何处而来,是在Spring的xml文件中定义的bean吗?

从Autowired的javadoc开始

 从Autowired的javadoc中得到如下信息

 1. AutowiredAnnotationBeanPostProcessor负责扫描Autowired注解,然后完成自动注入

 2. 可以对私有的字段使用Autowired进行自动装配,而无需为私有字段定义getter/setter来read/write这个字段

 3. 使用Autowired注解的类方法,可以是任意的方法名,任意的参数,Spring会从容器中找到合适的bean进行装配,setter自动注入跟对字段自动注入效果一样

Autowired注解的解析

当项目中使用了Autowired注解时,需要明确的告诉Spring,配置中引用了自动注入的功能,在Spring的配置文件,做法有两种

1. 配置AutowiredAnnotationBeanPostProcessor

2. 使用<context:annotation-config/>。<context:annotationconfig/> 将隐式地向 Spring 容器注册AutowiredAnnotationBeanPostProcessorCommonAnnotationBeanPostProcessorPersistenceAnnotationBeanPostProcessor以及equiredAnnotationBeanPostProcessor 这 4 个 BeanPostProcessor。

 

实例

1. 实例一:

  • UserSerice依赖的UserDao使用Autowired注解,

  • UserDao没有在Spring配置文件中定义

    结果:UserDao为null

 

2. 实例二:

  • UserSerice依赖的UserDao使用Autowired注解

  • UserDao在Spring配置文件中有定义

   结果:UserDao为null

 

3. 实例三:

  • UserSerice依赖的UserDao使用Autowired注解

  • UserDao在Spring配置文件中有定义

  • Spring中使用<context:annotation-config/>

   结果:UserDao正确注入,在Spring中配置的UserDao的实现,而在UserService中的是UserDao的接口,也就是说,虽然它们类型没有完全匹配,但是由于是实现

关系,Spring仍然能够完成自动注入

 

4. 实例四:

  • UserSerice依赖的UserDao使用Autowired注解

  • UserDao在Spring配置文件中有定义

  • Spring中配置AutowiredAnnotationBeanPostProcessor

结果:UserDao正确注入,同实例三

 

5. 实例五:

  • UserSerice依赖的UserDao使用Autowired注解

  • UserDao在Spring配置文件中有两份定义(id不同)

  • Spring中使用<context:annotation-config/>

结果:

1. 如果UserDao的属性名与某个bean的id相同,那么按照属性名和id名称匹配原则,自动装配

2. 如果UserService中定义的UserDao的属性名,与Spring配置文件中的两个id都不同,那么注入失败,异常抛出,提示,无法完整自动装配

 

结论:

1. 使用Autowired自动装配,必须在Spring的配置文件中使用<context:annotation-config/>来告诉Spring需要进行自动装配扫描(AutowiredAnnotationBeanPostProcessor不推荐使用)

2. Autowired默认按类型进行匹配,当匹配到多个满足条件的bean时,再按照属性名和bean的id进行匹配,如果仍然有多个匹配上或者没有一个匹配上,则抛出异常,提示自动装配失败

3. 在使用Autowired时,可以使用Qualifier注解,显式的指定,当冲突发生时,使用那个id对应的bean

4. Autowired注解自动装配功能完成的是依赖的自动注入,因此,在一个bean中,它依赖的bean可以通过自动注入的方式完成而不需要显式的为它的属性进行注入。但是这些依赖的bean仍然不能省略,还是要在Spring中进行配置,省略的仅仅是bean属性的注入配置代码

 

 Resource注解

 Resource注解在功能和目的上,等效于Autowried+Qualifier注解,Resource注解是JSR-250规范的一部分,它定义在JDK的javax.annoation包中,如下是它的定义:

package javax.annotation;

import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;

/**
 * The Resource annotation marks a resource that is needed
 * by the application.  This annotation may be applied to an
 * application component class, or to fields or methods of the
 * component class.  When the annotation is applied to a
 * field or method, the container will inject an instance
 * of the requested resource into the application component
 * when the component is initialized.  If the annotation is
 * applied to the component class, the annotation declares a
 * resource that the application will look up at runtime. <p>
 *
 * Even though this annotation is not marked Inherited, deployment
 * tools are required to examine all superclasses of any component
 * class to discover all uses of this annotation in all superclasses.
 * All such annotation instances specify resources that are needed
 * by the application component.  Note that this annotation may
 * appear on private fields and methods of superclasses; the container
 * is required to perform injection in these cases as well.
 *
 * @since Common Annotations 1.0
 */
@Target({TYPE, FIELD, METHOD})
@Retention(RUNTIME)
public @interface Resource {
    /**
     * The JNDI name of the resource.  For field annotations,
     * the default is the field name.  For method annotations,
     * the default is the JavaBeans property name corresponding
     * to the method.  For class annotations, there is no default
     * and this must be specified.
     */
    String name() default "";

    /**
     * The name of the resource that the reference points to. It can
     * link to any compatible resource using the global JNDI names.
     *
     * @since Common Annotations 1.1
     */

    String lookup() default "";

    /**
     * The Java type of the resource.  For field annotations,
     * the default is the type of the field.  For method annotations,
     * the default is the type of the JavaBeans property.
     * For class annotations, there is no default and this must be
     * specified.
     */
    Class<?> type() default java.lang.Object.class;

    /**
     * The two possible authentication types for a resource.
     */
    enum AuthenticationType {
            CONTAINER,
            APPLICATION
    }

    /**
     * The authentication type to use for this resource.
     * This may be specified for resources representing a
     * connection factory of any supported type, and must
     * not be specified for resources of other types.
     */
    AuthenticationType authenticationType() default AuthenticationType.CONTAINER;

    /**
     * Indicates whether this resource can be shared between
     * this component and other components.
     * This may be specified for resources representing a
     * connection factory of any supported type, and must
     * not be specified for resources of other types.
     */
    boolean shareable() default true;

    /**
     * A product specific name that this resource should be mapped to.
     * The name of this resource, as defined by the <code>name</code>
     * element or defaulted, is a name that is local to the application
     * component using the resource.  (It's a name in the JNDI
     * <code>java:comp/env</code> namespace.)  Many application servers
     * provide a way to map these local names to names of resources
     * known to the application server.  This mapped name is often a
     * <i>global</i> JNDI name, but may be a name of any form. <p>
     *
     * Application servers are not required to support any particular
     * form or type of mapped name, nor the ability to use mapped names.
     * The mapped name is product-dependent and often installation-dependent.
     * No use of a mapped name is portable.
     */
    String mappedName() default "";

    /**
     * Description of this resource.  The description is expected
     * to be in the default language of the system on which the
     * application is deployed.  The description can be presented
     * to the Deployer to help in choosing the correct resource.
     */
    String description() default "";
}

Autowried注解,首先根据类型匹配,如果类型匹配到多个,那么在根据属性名和bean的id进行匹配(可以由Qualifier注解强制匹配指定的bean id)。Resource注解则顺序不同,它有如下几种可能的情况:

 

  • Resource注解指定了name属性和type属性

    策略:首先进行按名称匹配策略: 匹配name属性和bean的id,如果匹配,则判断查找到的bean是否是type属性指定的类型,如果是type属性指定的类型,则匹配成功。如果不是type属性指定的类型,则抛出异常,提示匹配失败;如果name属性跟bean的id不匹配,则抛出异常提示没有bean的id匹配name属性

 

  • Resource注解指定了name属性,未指定type属性

    策略:查找bean的id为name属性的bean,查找到,不关心类型为什么,都是匹配成功;如果找不到name属性指定的bean id,则匹配失败,抛出异常

 

  • Resource注解指定了type属性,未指定name属性

    策略:首先进行按名称匹配策略: 匹配属性名和bean的id,如果匹配,则判断查找到的bean是否是type属性指定的类型,如果是type属性指定的类型,则匹配成功。如果不是type属性指定的类型,则抛出异常,提示匹配失败;其次进行按类型匹配策略: 如果属性名跟bean的id不匹配,则查找类型为type的bean,如果仅仅找到一个,自动装配成功,其它情况失败。

 

  • Resource注解未指定type属性和name属性

    策略:首先进行按属性名匹配策略,匹配则注入成功;如果属性名不匹配,则进行类型匹配策略,只有为一个类型匹配才成功,其他情况都失败