SpringBoot入门（二）——起步依赖

在前一篇我们通过简单几步操作就生成了一个可以直接运行的Web程序，这是因为SpringBoot代替我们做了许多工作，概括来讲可以分为起步依赖和自动配置。这一篇先来看看起步依赖。

项目构建过程解析

前面提到，Spring Boot构建出来的也是一个Maven项目，可以看下自动生成的pom.xml文件：

<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>top.godtm</groupId>
    <artifactId>blog-demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>blog-demo</name>
    <description>Demo project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.0.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

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

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

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build></project>

去掉一些必要的配置，可以看到只引入了3个依赖。其中spring-boot-starter-thymeleaf是我自己额外引入的可以忽略，那么剩下的就只有spring-boot-starter-web和spring-boot-starter-test了。spring-boot-starter-test是用于编写测试使用的，可以认为跟项目功能没有直接关系。

结果就是：我们为了编写一个简单的Hello World Web项目，只需要引入一个依赖即可，就这么easy！

起步依赖

这里看到的spring-boot-starter-xxx就是SpringBoot的起步依赖。SpringBoot通过提供众多起步依赖降低项目依赖的复杂度。起步依赖本质上是一个Maven项目对象模型，定义了对其他库的传递依赖，这些东西加在一起即支持某项功能。很多起步依赖的命名都暗示了他们提供的某种或某类功能。

以spring-boot-starter-web为例，追踪它的pom文件可以看到熟悉的东西：

<?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>2.0.0.RELEASE</version>
  </parent>
  
  
   
    <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
  <version>2.0.0.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>2.0.0.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.0.0.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.0.0.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.hibernate.validator</groupId>
      <artifactId>hibernate-validator</artifactId>
      <version>6.0.7.Final</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-web</artifactId>
      <version>5.0.4.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.0.4.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies></project>

在这一层已经能看到它为我们传递了spring-web和spring-webmvc。