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<context:component-scan>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).
*
* <p>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.
*
* <p>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 "";
}
