spring-boot|@SpringBootApplication注解是如何进行装配的？

从spring-boot的@SpringBootApplication注解来重新了解sring的注解。

我们知道每一个spring-boot的初始是的程序入口都有一个@SpringBootApplication注解。点开@SpringBootApplication的源码可以看到如下

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
      @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
 // 略...   
}

可以看到SpringBootApplication注解类中带着@ComponentScan注解，这也是为什么可以spring-boot可以自动装配bean。当我们进一步查看注解的注解的源码时可以在最低处看到@Component注解。

正是因为注解省去了很多配置文件。

下面我们谈谈什么是spring 的模式注解。

何为模式注解

在spring的官方文档上是这样解释的

Stereotype Annotations
A *stereotype annotation* is an annotation that is used to declare the role that a component plays within the application. For example, the @Repository annotation in the Spring Framework is a marker for any class that fulfills the role or stereotype of a repository (also known as Data Access Object or DAO).

@Component is a generic stereotype for any Spring-managed component. Any component annotated with @Component is a candidate for component scanning. Similarly, any component annotated with an annotation that is itself meta-annotated with @Component is also a candidate for component scanning. For example, @Service is meta-annotated with @Component.

Core Spring provides several stereotype annotations out of the box, including but not limited to: @Component, @Service, @Repository, @Controller, @RestController, and @Configuration. @Repository, @Service, etc. are specializations of @Component.

构造型注释是一种注释，用于声明组件在应用程序中扮演的角色。例如，Spring Framework中的@Repository注释是任何满足存储库角色（也称为数据访问对象或DAO）的类的标记。

模式注解举例

spirng 注解	使用说明	起始版本
@Repository	数据仓库注解	2.0
@Component	通用组件模式注解	2.5
@Service	服务注解模式	2.5
@Controller	Web控制器模式注解	2.5
@Configuration	配置模式注解	3.0

spring的两种装配方式

配置文件扫描

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:tx="http://www.springframework.org/schema/tx" xmlns:jdbc="http://www.springframework.org/schema/jdbc"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:task="http://www.springframework.org/schema/task"
       xsi:schemaLocation="http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
     http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
     http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd
     http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task.xsd">

    <!-- 寻找被@Component 或者其派生的注解标记的类，将其注册为spring Bean -->
    <context:component-scan base-package="com.*" annotation-config="true">
    </context:component-scan>

</beans>

@ComponentScan方式

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@ComponentScan(basePackages = "com.dsdj.springboot")
public class SpringConfiguration {
}

spring自定义注解的使用

初始项目https://start.spring.io/

自定义repository注解

package com.dsdj.springbootdemo1.annotation;

import org.springframework.stereotype.Repository;

import java.lang.annotation.*;

/**
 * @ClassName MyFirstRepository
 * @Description 我的自定义注解
 * @Author dsdj
 * @Date 2018/12/15 下午4:36
 * @Version 1.0
 **/

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Repository
public @interface MyFirstRepository {
    String value() default "";
}

自定义了MyFirstRepository注解，继承了@Repository注解，即具有了@Repository注解的功能。

测试：

定义一个类

package com.dsdj.springbootdemo1.repository;

import com.dsdj.springbootdemo1.annotation.MyFirstRepository;

/**
 * @ClassName TestRepository
 * @Description TODO
 * @Author dsdj
 * @Date 2018/12/15 下午4:44
 * @Version 1.0
 **/
@MyFirstRepository(value = "testRespository")
public class TestRepository {

}

入口函数

package com.dsdj.springbootdemo1.bootstrap;

import com.dsdj.springbootdemo1.repository.TestRepository;
import org.springframework.boot.WebApplicationType;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.stereotype.Component;

/**
 * @ClassName RepositoryBootstrap
 * @Description 仓储的引导类
 * @Author dsdj
 * @Date 2018/12/15 下午4:46
 * @Version 1.0
 **/
@ComponentScan(basePackages = "com.dsdj.springbootdemo1.*")
public class RepositoryBootstrap {

    public static void main(String[] args) {

        // spring boot的启动入口，获取spring的上下文
        ConfigurableApplicationContext context = new SpringApplicationBuilder(RepositoryBootstrap.class)
                // spring boot2.0写法,说明不是web类型项目
                .web(WebApplicationType.NONE)
                .run(args);

        TestRepository testRespository = context.getBean("testRespository", TestRepository.class);
        System.out.println("bean是否扫描到："+testRespository.toString());

        context.close();
    }
}

输出

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2018-12-15 16:58:37.991  INFO 21446 --- [           main] c.d.s.bootstrap.RepositoryBootstrap      : No active profile set, falling back to default profiles: default
2018-12-15 16:58:38.219  INFO 21446 --- [           main] c.d.s.bootstrap.RepositoryBootstrap      : Started RepositoryBootstrap in 0.521 seconds (JVM running for 0.958)
bean是否扫描到：com.dsdj.springbootdemo1.repository.TestRepository@2f465398

可以看到spring-boot 成功装配了bean

通过上述的例子可以看到注解的特性

注解的特性

继承性

@MyFirstRepository注解类中有@Repostory注解，使之可以实现@Repostory的功能，所以spring可以对@MyFirstRepository注解的类进行装配。

同理，当我们继续查看@Repository的源码时，可以看到@Component元注解，也就是说，@ComponentScan对@Component注解类进行装配，而@Repository、@Controller、@Service“继承”了@Component所有可以进行装配。

层次性

可以发现@Component–>@Repostory—>@MyFirstRepository一种层次的关系。