初学Springboot2（二）HelloWorld 探究_spring boot 2 flatten-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_42672204/article/details/104673050

1.pom文件

在helloworld的pom文件中，导入了一个父项目

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.5.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

这个父项目的父项目是

  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.2.5.RELEASE</version>
    <relativePath>../../spring-boot-dependencies</relativePath>
  </parent>

再往上，它的父项目是 spring-boot-dependencies，它来真正管理spring boot里的所有依赖的版本，它就是Spring Boot的版本仲裁中心

<?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>org.springframework.boot</groupId>
  <artifactId>spring-boot-dependencies</artifactId>
  <version>2.2.5.RELEASE</version>
  <packaging>pom</packaging>
  <name>Spring Boot Dependencies</name>
  <description>Spring Boot Dependencies</description>
  <url>https://projects.spring.io/spring-boot/#</url>

在这个xml文件中，往下翻，有properties标签，里面定义了要用到的依赖的版本

我只复制了一部分

<properties>
    <activemq.version>5.15.11</activemq.version>
    <antlr2.version>2.7.7</antlr2.version>
    <appengine-sdk.version>1.9.78</appengine-sdk.version>
    <artemis.version>2.10.1</artemis.version>
    <aspectj.version>1.9.5</aspectj.version>
    <assertj.version>3.13.2</assertj.version>
    <atomikos.version>4.0.6</atomikos.version>
    <awaitility.version>4.0.2</awaitility.version>
    <bitronix.version>2.1.4</bitronix.version>
    <build-helper-maven-plugin.version>3.0.0</build-helper-maven-plugin.version>
    <byte-buddy.version>1.10.8</byte-buddy.version>
    <caffeine.version>2.8.1</caffeine.version>
    <cassandra-driver.version>3.7.2</cassandra-driver.version>
    <classmate.version>1.5.1</classmate.version>
    <commons-codec.version>1.13</commons-codec.version>
    <commons-dbcp2.version>2.7.0</commons-dbcp2.version>
    <commons-lang3.version>3.9</commons-lang3.version>
    <commons-pool.version>1.6</commons-pool.version>
    <commons-pool2.version>2.7.0</commons-pool2.version>
    <couchbase-cache-client.version>2.1.0</couchbase-cache-client.version>
    <couchbase-client.version>2.7.12</couchbase-client.version>
    <db2-jdbc.version>11.5.0.0</db2-jdbc.version>
    <dependency-management-plugin.version>1.0.9.RELEASE</dependency-management-plugin.version>
    <derby.version>10.14.2.0</derby.version>
    <dropwizard-metrics.version>4.1.3</dropwizard-metrics.version>
    <ehcache.version>2.10.6</ehcache.version>
    <ehcache3.version>3.8.1</ehcache3.version>
    <elasticsearch.version>6.8.6</elasticsearch.version>
    <embedded-mongo.version>2.2.0</embedded-mongo.version>
    <exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
    <flatten-maven-plugin.version>1.1.0</flatten-maven-plugin.version>
    <flyway.version>6.0.8</flyway.version>
    <freemarker.version>2.3.29</freemarker.version>
    <git-commit-id-plugin.version>3.0.1</git-commit-id-plugin.version>
    <glassfish-el.version>3.0.3</glassfish-el.version>
    <glassfish-jaxb.version>2.3.2</glassfish-jaxb.version>
    <groovy.version>2.5.9</groovy.version>
    <gson.version>2.8.6</gson.version>
    <h2.version>1.4.200</h2.version>
    <hamcrest.version>2.1</hamcrest.version>
.................

以后我们导入依赖默认是不需要写版本的；

没有在dependencies里面管理的依赖，自然是要声明版本号的

2.启动器

helloworld的pom中有下面这个依赖

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

进入spring-boot starter-web，可以看到里面导入了我们web开发需要的依赖

<dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
      <version>2.2.5.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.2.5.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.2.5.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-validation</artifactId>
      <version>2.2.5.RELEASE</version>
      <scope>compile</scope>
      <exclusions>
        <exclusion>
          <artifactId>tomcat-embed-el</artifactId>
          <groupId>org.apache.tomcat.embed</groupId>
        </exclusion>
      </exclusions>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-web</artifactId>
      <version>5.2.4.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.2.4.RELEASE</version>
      <scope>compile</scope>
    </dependency>

Spring Boot将所有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

3.主程序类，主入口类

/**
 * @SpringBootApplication 来标注一个主程序类，说明这是一个Spring Boot应用
 */
@SpringBootApplication
public class Helloworld01Application {

    public static void main(String[] args) {
        
        SpringApplication.run(Helloworld01Application.class, args);
    }

}

@SpringBootApplication： Spring Boot应用---标注在某个类上，说明这个类是SpringBoot的主配置类，SpringBoot就应该运行这个类的main方法来启动SpringBoot应用

进入这个注解，可以看到这是一个组合注解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {

@SpringBootConfiguration： Spring Boot的配置类，标注在某个类上，表明这是一个SpringBoot的配置类

它的底层其实是Spring的 @Configuration 注解

@EnableAutoConfiguration：开启自动配置功能

以前我们需要配置的东西，SpringBoot帮我们自动配置。它也是一个组合注解

@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {

@AutoConfigurationPackage：自动配置包

@Import({AutoConfigurationImportSelector.class})

Spring的底层注解@Import，给容器导入一个组件；导入的组件由{AutoConfigurationImportSelector.class决定，最终实现的是 将主配置类（@SpringBootApplication标注的类）所在的包以及下面所有子包的所有组件扫描到Spring容器

具体的实现，好底层，已经听懵了，先留着，等会用后再回头学习下底层，

重点是spring-boot-autoconfigure-2.2.5.RELEASE.jar

是下面这个文件

看下Spring注解版---谷粒学院