SpringBoot系列（一）：第2章SpringBoot 的依赖和自动配置解析

末蓝、 2022-03-15 16:54 183阅读 0赞

# 前言 #

本次主要对spring-boot-starter-web中pom.xml来解析SpringBoot为什么在很少的配置下就能运行起来？

# pom.xml #

<?xml version="1.0" encoding="UTF-8"?>
    <project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
      <modelVersion>4.0.0</modelVersion>
      <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starters</artifactId>
        <version>2.0.3.RELEASE</version>
      </parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
      <version>2.0.3.RELEASE</version>
      <name>Spring Boot Web Starter</name>
      <description>Starter for building web, including RESTful, applications using Spring
    		MVC. Uses Tomcat as the default embedded container</description>
      <url>https://projects.spring.io/spring-boot/#/spring-boot-parent/spring-boot-starters/spring-boot-starter-web</url>
      <organization>
        <name>Pivotal Software, Inc.</name>
        <url>https://spring.io</url>
      </organization>
      <licenses>
        <license>
          <name>Apache License, Version 2.0</name>
          <url>http://www.apache.org/licenses/LICENSE-2.0</url>
        </license>
      </licenses>
      <developers>
        <developer>
          <name>Pivotal</name>
          <email>info@pivotal.io</email>
          <organization>Pivotal Software, Inc.</organization>
          <organizationUrl>http://www.spring.io</organizationUrl>
        </developer>
      </developers>
      <scm>
        <connection>scm:git:git://github.com/spring-projects/spring-boot.git/spring-boot-starters/spring-boot-starter-web</connection>
        <developerConnection>scm:git:ssh://git@github.com/spring-projects/spring-boot.git/spring-boot-starters/spring-boot-starter-web</developerConnection>
        <url>http://github.com/spring-projects/spring-boot/spring-boot-starters/spring-boot-starter-web</url>
      </scm>
      <issueManagement>
        <system>Github</system>
        <url>https://github.com/spring-projects/spring-boot/issues</url>
      </issueManagement>
      <dependencies>
        
        <dependency>
          <groupId>org.springframework.boot</groupId>
          <artifactId>spring-boot-starter</artifactId>
          <version>2.0.3.RELEASE</version>
          <scope>compile</scope>
        </dependency>
        
        
        <dependency>
          <groupId>org.springframework.boot</groupId>
          <artifactId>spring-boot-starter-json</artifactId>
          <version>2.0.3.RELEASE</version>
          <scope>compile</scope>
        </dependency>
    
        
        <dependency>
          <groupId>org.springframework.boot</groupId>
          <artifactId>spring-boot-starter-tomcat</artifactId>
          <version>2.0.3.RELEASE</version>
          <scope>compile</scope>
        </dependency>
    
       
        <dependency>
          <groupId>org.hibernate.validator</groupId>
          <artifactId>hibernate-validator</artifactId>
          <version>6.0.10.Final</version>
          <scope>compile</scope>
        </dependency>
    
       
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-web</artifactId>
          <version>5.0.7.RELEASE</version>
          <scope>compile</scope>
        </dependency>
    
       
        <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-webmvc</artifactId>
          <version>5.0.7.RELEASE</version>
          <scope>compile</scope>
        </dependency>
      </dependencies>
    </project>

从上面的pom.xml可以看出，dang当我们导入web开发场景的时候(spring-boot-starter-web)它会通过Maven对应的资源加载到我们的工程中，这就会形成了依赖。此时我在开发web应用基本资源都有了。

# 从一个类看SpringBoot的自动配置 #

以DipatcherServletAutoConfiguration 部分源码来探究，自动配置原理。

@AutoConfigureOrder(-2147483648)
    @Configuration
    @ConditionalOnWebApplication(
        type = Type.SERVLET
    )
    @ConditionalOnClass({DispatcherServlet.class})
    @AutoConfigureAfter({ServletWebServerFactoryAutoConfiguration.class})
    @EnableConfigurationProperties({ServerProperties.class})
    public class DispatcherServletAutoConfiguration {
        public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";
        public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";
    
        public DispatcherServletAutoConfiguration() {
        }
    
        。。。。。。。   
        
        // @Conditional 这个类存在 这个方法就会生效
        // @ConditionalOnClass 没有这个类，这个方法就生效
    
        @Configuration
        @Conditional({DispatcherServletAutoConfiguration.DispatcherServletRegistrationCondition.class})
        @ConditionalOnClass({ServletRegistration.class})
        @EnableConfigurationProperties({WebMvcProperties.class})
        @Import({DispatcherServletAutoConfiguration.DispatcherServletConfiguration.class})
        protected static class DispatcherServletRegistrationConfiguration {
            private final ServerProperties serverProperties;
            private final WebMvcProperties webMvcProperties;
            private final MultipartConfigElement multipartConfig;
    
            public DispatcherServletRegistrationConfiguration(ServerProperties serverProperties, WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfigProvider) {
                this.serverProperties = serverProperties;
                this.webMvcProperties = webMvcProperties;
                this.multipartConfig = (MultipartConfigElement)multipartConfigProvider.getIfAvailable();
            }
    
            @Bean(
                name = {"dispatcherServletRegistration"}
            )
            @ConditionalOnBean(
                value = {DispatcherServlet.class},
                name = {"dispatcherServlet"}
            )
            public ServletRegistrationBean<DispatcherServlet> dispatcherServletRegistration(DispatcherServlet dispatcherServlet) {
                ServletRegistrationBean<DispatcherServlet> registration = new ServletRegistrationBean(dispatcherServlet, new String[]{this.serverProperties.getServlet().getServletMapping()});
                registration.setName("dispatcherServlet");
                registration.setLoadOnStartup(this.webMvcProperties.getServlet().getLoadOnStartup());
                if (this.multipartConfig != null) {
                    registration.setMultipartConfig(this.multipartConfig);
                }
    
                return registration;
            }
        }
    
        @Configuration
        @Conditional({DispatcherServletAutoConfiguration.DefaultDispatcherServletCondition.class})
        @ConditionalOnClass({ServletRegistration.class})
        @EnableConfigurationProperties({WebMvcProperties.class})
        protected static class DispatcherServletConfiguration {
            private final WebMvcProperties webMvcProperties;
            private final ServerProperties serverProperties;
    
            public DispatcherServletConfiguration(WebMvcProperties webMvcProperties, ServerProperties serverProperties) {
                this.webMvcProperties = webMvcProperties;
                this.serverProperties = serverProperties;
            }
    
            @Bean(
                name = {"dispatcherServlet"}
            )
            public DispatcherServlet dispatcherServlet() {
                DispatcherServlet dispatcherServlet = new DispatcherServlet();
                dispatcherServlet.setDispatchOptionsRequest(this.webMvcProperties.isDispatchOptionsRequest());
                dispatcherServlet.setDispatchTraceRequest(this.webMvcProperties.isDispatchTraceRequest());
                dispatcherServlet.setThrowExceptionIfNoHandlerFound(this.webMvcProperties.isThrowExceptionIfNoHandlerFound());
                return dispatcherServlet;
            }
    
            @Bean
            @ConditionalOnBean({MultipartResolver.class})
            @ConditionalOnMissingBean(
                name = {"multipartResolver"}
            )
            public MultipartResolver multipartResolver(MultipartResolver resolver) {
                return resolver;
            }
    
            @Bean
            public DispatcherServletPathProvider mainDispatcherServletPathProvider() {
                return () -> {
                    return this.serverProperties.getServlet().getPath();
                };
            }
        }
    }

通过上述代码，演示可以看出在SpringBoot内部已经为我们做了很多DispatcherServlet的配置，其中@EnableConfigurationProperties 还能够在读取配置内容情况下自动生成SpringMVC所需的类。至此，我们应该清楚了，为什么几乎没怎么配置SpringBoot救恩能够启动SpringMVC的项目，这些都是SpringBoot通过maven依赖找到对应的jar包和嵌入的服务器，然后使用默认的自动配置类，来启动默认的开发环境。