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
* @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
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<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。
@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). * * <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 ""; }
@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 ""; }
原文：https://www.cnblogs.com/jvStarBlog/p/11273908.html