关于@Autowired @Qualifier @Resource的使用与区别

朱雀 2020-10-25 14:13 834阅读 0赞

# 前言： #

Spring注入三兄弟一如奶茶三兄弟，三者各有不同却总因为神似混淆。

**@Autowired @Qualifier 属于 Spring 定义的注解**

**@Resource 属于 JSR-250 规范定义的注解。**

### @Autowired  ###

**@Autowired 注解，根据bean类型进行对象注入。始于spring2.5**

**以下是@Autowired 的源码：**

package org.springframework.beans.factory.annotation;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Autowired {
        boolean required() default true;
    }

从源码中可以看出：

首先，@Autowired可修饰的对象包含：构造函数、方法、参数、字段、注解类型。

其次，它包含一个required属性，用来声明使用该注解的对象是否必须，默认值为true。

最后，通过@Retention(RetentionPolicy.RUNTIME)元注解，可以得知@Autowired可以在JVM运行时被其他代码读取和使用。

### @Qualifier ###

**@Qualifier注解，根据bean名称进行对象注入。始于spring2.5.**

**以下是****@Qualifier源码：**

package org.springframework.beans.factory.annotation;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Inherited;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.TYPE, ElementType.ANNOTATION_TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Inherited
    @Documented
    public @interface Qualifier {
        String value() default "";
    }

从源码中可以看出：

首先，@Qualifier可修饰的对象包含：字段、方法、参数、类型、注解类型，注意它与@Autowired两者修饰范围的不同。

其次，它包含一个value属性，用来指定注入的bean的名称，默认值为空。

最后，通过@Retention(RetentionPolicy.RUNTIME)元注解，可以得知@Qualifier可以在JVM运行时被其他代码读取和使用。这点与@Autowired相同。

### @Resource 源码： ###

@Resource属于JSR-250规范定义的注解，支持根据类型和名称进行对象注入。

@Resource注解始于jdk1.6。

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 "";
    }

从源码中可以看出：

首先，@Resource可修饰的对象包含：类型、字段、方法，它修饰的范围与以上二者都不同。

其次，它包含的属性较多，如下：

name:用来指定bean名称

type:用来指定bean类型

lookup:不知

shareable:不知

mappedName:不知

description:资源的描述

authenticationType:不知

最后，通过@Retention(RetentionPolicy.RUNTIME)元注解，可以得知@Resource可以在JVM运行时被其他代码读取和使用,与以上二者相同。

注意：