Spring注解实现原理

先来说说注解是什么：

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。

那么注解如何实现？

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 

 下面拿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。

这里附上参考文章https://blog.csdn.net/jack_wang001/article/details/78781588

@Controller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 "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 "";

}