springboot的pom文件：

1. 附上springboot的完成pom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>com.ljj</groupId>
   <artifactId>Ljjboot-01</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <packaging>jar</packaging>

   <name>Ljjboot-01</name>
   <description>Demo project for Spring Boot</description>

   <parent>
   	<groupId>org.springframework.boot</groupId>
   	<artifactId>spring-boot-starter-parent</artifactId>
   	<version>2.1.0.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</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>

2. 可以看到他的父类项目为spring-boot-starter-parent

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.1.0.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>

我们点击进去可以看到：

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-dependencies</artifactId>
        <version>2.1.0.RELEASE</version>
        <relativePath>../../spring-boot-dependencies</relativePath>
    </parent>
    <artifactId>spring-boot-starter-parent</artifactId>

spring-boot-starter-parent的父类为：spring-boot-dependencies
点击进去：

<properties>
     <activemq.version>5.15.7</activemq.version>
     <antlr2.version>2.7.7</antlr2.version>
     <appengine-sdk.version>1.9.67</appengine-sdk.version>
     <artemis.version>2.6.3</artemis.version>
     <aspectj.version>1.9.2</aspectj.version>
     <assertj.version>3.11.1</assertj.version>
     <atomikos.version>4.0.6</atomikos.version>
     <bitronix.version>2.1.4</bitronix.version>
     <build-helper-maven-plugin.version>3.0.0</build-helper-maven-plugin.version>
     <byte-buddy.version>1.9.3</byte-buddy.version>
     <caffeine.version>2.6.2</caffeine.version>
     <cassandra-driver.version>3.6.0</cassandra-driver.version>
     <classmate.version>1.4.0</classmate.version>
     <commons-codec.version>1.11</commons-codec.version>
     <commons-dbcp2.version>2.5.0</commons-dbcp2.version>
     <commons-lang3.version>3.8.1</commons-lang3.version>
     <commons-pool.version>1.6</commons-pool.version>
     <commons-pool2.version>2.6.0</commons-pool2.version>
     <couchbase-cache-client.version>2.1.0</couchbase-cache-client.version>
     <couchbase-client.version>2.7.0</couchbase-client.version>
     <derby.version>10.14.2.0</derby.version>
     <dom4j.version>1.6.1</dom4j.version>
     <dropwizard-metrics.version>4.0.3</dropwizard-metrics.version>
     <ehcache.version>2.10.6</ehcache.version>
     <ehcache3.version>3.6.1</ehcache3.version>
     <elasticsearch.version>6.4.2</elasticsearch.version>
     <embedded-mongo.version>2.1.1</embedded-mongo.version>
     <exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
     <flatten-maven-plugin.version>1.0.1</flatten-maven-plugin.version>
     <flyway.version>5.2.1</flyway.version>
     <freemarker.version>2.3.28</freemarker.version>
     <hamcrest.version>1.3</hamcrest.version>
     <hazelcast.version>3.11</hazelcast.version>
     <hazelcast-hibernate5.version>1.2.3</hazelcast-hibernate5.version>
     <hibernate.version>5.3.7.Final</hibernate.version>
     ....
     </properties>

这个spring-boot-denpendencies来真正管理Spring Boot应用里面的所有依赖版本；
所以我们可以知道：
Spring Boot的版本仲裁中心；
以后我们导入依赖默认是不需要写版本；（没有在dependencies里面管理的依赖自然需要声明版本号）

springboot场景启动器

1. 我们通过spring-boot-starts-web启动器为例：

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

我们点击进去:

  <dependencies>
   <dependency>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter</artifactId>
     <version>2.1.0.RELEASE</version>
     <scope>compile</scope>
   </dependency>
   <dependency>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter-json</artifactId>
     <version>2.1.0.RELEASE</version>
     <scope>compile</scope>
   </dependency>
   <dependency>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter-tomcat</artifactId>
     <version>2.1.0.RELEASE</version>
     <scope>compile</scope>
   </dependency>
   <dependency>
     <groupId>org.hibernate.validator</groupId>
     <artifactId>hibernate-validator</artifactId>
     <version>6.0.13.Final</version>
     <scope>compile</scope>
   </dependency>
   <dependency>
     <groupId>org.springframework</groupId>
     <artifactId>spring-web</artifactId>
     <version>5.1.2.RELEASE</version>
     <scope>compile</scope>
   </dependency>
   <dependency>
     <groupId>org.springframework</groupId>
     <artifactId>spring-webmvc</artifactId>
     <version>5.1.2.RELEASE</version>
     <scope>compile</scope>
   </dependency>
 </dependencies>

可以看出：spring-boot-starter-web：
spring-boot-starter：spring-boot场景启动器；帮我们导入了web模块正常运行所依赖的组件；
所以我们可以这么理解：
Spring Boot将所有的功能场景都抽取出来，做成一个个的starters（启动器），只需要在项目里面引入这些starter 相关场景的所有依赖都会导入进来。要用什么功能就导入什么场景的启动器