[Java知识库]【Spring Boot】spring-boot-starter-web 如何完成自动装配

1. 前言

参考官网：理解能够自动装配的bean

• 装配了web能力的Spring Boot 项目启动 —— 监听8080端口

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

• 一个最简单的 Spring Boot 项目启动 —— 自然退出，“啥也不干”

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter</artifactId>
		</dependency>

• 本文探究 spring-boot-starter-web 依赖做了什么事情，让项目开始监听8080端口，成为了一个web 服务。

2. 环境准备

• 开启logback支持，设置日志级别为DEBUG

logging:
  config: classpath:logback.xml

<?xml version='1.0' encoding='UTF-8'?>
<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d [%thread] %-5level %logger{50} -[%file:%line]- %msg%n</pattern>
            <charset>UTF-8</charset>
        </encoder>
    </appender>
    <root level="debug">
        <appender-ref ref="STDOUT"/>
    </root>
</configuration>

• 每次运行项目都执行 mvn clean 避免class path 有旧依赖引入的类

3. 找到 XXXAutoConfiger 的类

• pom文件声明为web项目

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

3.1. 怎么找?

spring-boot-starter-web 那么就找到autoconfigure 模块下的web 包。找一个servlet的子包下的任意一个类。现在取WebMvcAutoConfiguration

• 条件装配
  • @ConditionalOnWebApplication 表达当前是一个web应用
  • @ConditionalOnClass 表达在classpath中需要有的class才能装配
  • @ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class }) 表示需要引入Servlet DispatcherServlet WebMvcConfigurer 到classpath中。

当pom.xml引入以下依赖

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

• 装配
  启动时WebMvcAutoConfiguration 上的条件注解都满足，所以执行装配。通过引入依赖就让项目拥有监听端口的能力（变成web）项目，这个特性就是自动装配。而这个加入到classpath的操作可以由maven替我们完成，而不用写配置文件。

4. 后记

Under the hood, auto-configuration is implemented with standard @Configuration classes. Additional @Conditional annotations are used to constrain when the auto-configuration should apply. Usually, auto-configuration classes use @ConditionalOnClass and @ConditionalOnMissingBean annotations. This ensures that auto-configuration applies only when relevant classes are found and when you have not declared your own @Configuration.
You can browse the source code of spring-boot-autoconfigure to see the @Configuration classes that Spring provides (see the META-INF/spring.factories file).

Spring Boot checks for the presence of a META-INF/spring.factories file within your published jar. The file should list your configuration classes under the EnableAutoConfiguration key, as shown in the following example:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.mycorp.libx.autoconfigure.LibXAutoConfiguration,\
com.mycorp.libx.autoconfigure.LibXWebAutoConfiguration

借官网的片段，总结下其他要注意的规范：

. Usually, auto-configuration classes use @ConditionalOnClass and @ConditionalOnMissingBean annotations. This ensures that auto-configuration applies only when relevant classes are found and when you have not declared your own @Configuration.

1. 在自己的工程中没有额外声明@Configuration，@ConditionalOnXXX 注解能确保在相关的类在classpath被找到才触发自动装配。

You can browse the source code of spring-boot-autoconfigure to see the @Configuration classes that Spring provides (see the META-INF/spring.factories file).

2. META-INF/spring.factories 提供了Spring 内置的自动装配类声明。

Spring Boot checks for the presence of a META-INF/spring.factories file within your published jar. The file should list your configuration classes under the EnableAutoConfiguration key

3. 自己要实现有自动装配的配置类，同样需要在 META-INF/spring.factories 中声明。并且作为一个 published jar被引入。