Spring注解实现原理

注解顾名思义，就是类似于添加注释，但是又不跟注释完全一样，因此，我们可以将它理解为将类或者方法与特定的信息进行关联。
1、接口使用@interface定义。
2、通过继承以下注解，实现功能：
元注解@Target,@Retention,@Documented,@Inherited

• 元注解@Target,@Retention,@Documented,@Inherited *
• @Target 表示该注解用于什么地方，可能的 ElemenetType 参数包括：
• ElemenetType.CONSTRUCTOR 构造器声明
• ElemenetType.FIELD 域声明（包括 enum 实例）
• ElemenetType.LOCAL_VARIABLE 局部变量声明
• ElemenetType.METHOD 方法声明
• ElemenetType.PACKAGE 包声明
• ElemenetType.PARAMETER 参数声明
• ElemenetType.TYPE 类，接口（包括注解类型）或enum声明
• @Retention 表示在什么级别保存该注解信息。可选的 RetentionPolicy 参数包括：
• RetentionPolicy.SOURCE 注解将被编译器丢弃
• RetentionPolicy.CLASS 注解在class文件中可用，但会被VM丢弃
• RetentionPolicy.RUNTIME VM将在运行期也保留注释，因此可以通过反射机制读取注解的信息。
• @Documented 将此注解包含在 javadoc 中
• @Inherited 允许子类继承父类中的注解

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited

spring的controller来当做示例

Controller类使用继承@Component注解的方法，将其以单例的形式放入spring容器，如果仔细看的话会发现每个注解里面都有一个默认的value()方法，它的作用是为当前的注解声明一个名字，一般默认为类名，然后spring会通过配置文件中的的配置，进行如下操作：
1、使用asm技术扫描.class文件，并将包含@Component及元注解为@Component的注解@Controller、@Service、@Repository或者其他自定义的的bean注册到beanFactory中，
2、然后spring在注册处理器
3、实例化处理器，然后将其放到beanPostFactory中，然后我们就可以在类中进行使用了。
4、创建bean时，会自动调用相应的处理器进行处理。

/*

 * Copyright 2002-2007 the original author or authors.

 *

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 *

 *      http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

 

package org.springframework.stereotype;

 

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;

 

/**
 * Indicates that an annotated class is a "Controller" (e.g. a web controller).
 *
 * 
This annotation serves as a specialization of {@link Component @Component},
 * allowing for implementation classes to be autodetected through classpath scanning.
 * It is typically used in combination with annotated handler methods based on the
 * {@link org.springframework.web.bind.annotation.RequestMapping} annotation.
 *
 * @author Arjen Poutsma
 * @author Juergen Hoeller
 * @since 2.5
 * @see Component
 * @see org.springframework.web.bind.annotation.RequestMapping
 * @see org.springframework.context.annotation.ClassPathBeanDefinitionScanner
 */
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {
    /**
     * The value may indicate a suggestion for a logical component name,
     * to be turned into a Spring bean in case of an autodetected component.
     * @return the suggested component name, if any
     */
    String value() default "";
}

下面是@Component的源码：

/*
 * Copyright 2002-2007 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.stereotype;
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;
/**
 * Indicates that an annotated class is a "component".
 * Such classes are considered as candidates for auto-detection
 * when using annotation-based configuration and classpath scanning.
 *
 * 
Other class-level annotations may be considered as identifying
 * a component as well, typically a special kind of component:
 * e.g. the {@link Repository @Repository} annotation or AspectJ's
 * {@link org.aspectj.lang.annotation.Aspect @Aspect} annotation.
 *
 * @author Mark Fisher
 * @since 2.5
 * @see Repository
 * @see Service
 * @see Controller
 * @see org.springframework.context.annotation.ClassPathBeanDefinitionScanner

 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Component {
    /**
     * The value may indicate a suggestion for a logical component name,
     * to be turned into a Spring bean in case of an autodetected component.
     * @return the suggested component name, if any
     */
    String value() default "";
}