依赖和自动配置

以最常用的SpringMVC为例，其在maven中对应的为spring-boot-starter-web,打开文件夹，查看.pom依赖文件

代码清单如下：

<?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>2.1.3.RELEASE</version>
  </parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
  <version>2.1.3.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>[email protected]</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://[email protected]/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>
  <!-- Spring boot 依赖-->
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
      <version>2.1.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
      <!-- jsont 依赖-->
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.1.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  <!-- tomcat依赖 依赖-->
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.1.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
	<!-- hibernate-validator 依赖 -->
    <dependency>
      <groupId>org.hibernate.validator</groupId>
      <artifactId>hibernate-validator</artifactId>
      <version>6.0.14.Final</version>
      <scope>compile</scope>
    </dependency>
·	<!--  Spring web 依赖-->
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-web</artifactId>
      <version>5.1.5.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <-- Spring mvc依赖-->
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.1.5.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>
</project>

当boot项目加入spring-boot-starter-web之后，会通过maven将相应的资源加载到工程中，但是这样还不够，这样还无法运行Springnmvc项目，要运行Springmvc项目需要对其进行配置，项目还是跑不起来。此处就使用了自动配置。
自动配置在spring-boot-autoconfigure中，解压出来看，其中自动配置下的web、servlet，DispatcherServletAutoConfiguration就是实现其自动配置的。

看其源码，着重分析其重要的内部类

@Configuration
	@Conditional(DefaultDispatcherServletCondition.class)
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties({ HttpProperties.class, WebMvcProperties.class })
	protected static class DispatcherServletConfiguration {

		private final HttpProperties httpProperties;

		private final WebMvcProperties webMvcProperties;

		public DispatcherServletConfiguration(HttpProperties httpProperties,
				WebMvcProperties webMvcProperties) {
			this.httpProperties = httpProperties;
			this.webMvcProperties = webMvcProperties;
		}

		@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
		public DispatcherServlet dispatcherServlet() {
			DispatcherServlet dispatcherServlet = new DispatcherServlet();
			dispatcherServlet.setDispatchOptionsRequest(
					this.webMvcProperties.isDispatchOptionsRequest());
			dispatcherServlet.setDispatchTraceRequest(
					this.webMvcProperties.isDispatchTraceRequest());
			dispatcherServlet.setThrowExceptionIfNoHandlerFound(
					this.webMvcProperties.isThrowExceptionIfNoHandlerFound());
			dispatcherServlet.setEnableLoggingRequestDetails(
					this.httpProperties.isLogRequestDetails());
			return dispatcherServlet;
		}

		@Bean
		@ConditionalOnBean(MultipartResolver.class)
		@ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
		public MultipartResolver multipartResolver(MultipartResolver resolver) {
			// Detect if the user has created a MultipartResolver but named it incorrectly
			return resolver;
		}

	}

------- THE END ----------后期补充源码分析----