spring--注解

注解是什么：

Java5.0以后引入了注解的概念。注解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。

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 

@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 注解只在源码阶段保留，在编译器编译时它将被丢弃忽略。 
RetentionPolicy.CLASS 注解只被保留到编译进行的时候，不会被加载到JVM中。 
RetentionPolicy.RUNTIME 注解可以保留到程序运行的时候，它会被加载到JVM中，所以在程序运行的时候可以获取它们。

@Documented 将此注解包含在javadoc中。 

@Inherited 允许子类继承父类中的注解

注解的属性：也叫成员变量，注解只有成员变量，没有方法。

 注解的成员变量在注解中以“无形参的方法”形式来声明，其方法名定义该成员变量的名字，其返回值定义了该成员变量的类型；

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 "";
 
}