SpringBoot基础24_SpringBoot的原理分析3

一、起步依赖原理分析

　1、 分析spring-boot-starter-parent

　　按住Ctrl点击pom.xml中的spring-boot-starter-parent，跳转到了spring-boot-starter-parent的pom.xml，xml配置如下（只摘抄了部分重点配置）：

<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-dependencies</artifactId>
  <version>2.0.1.RELEASE</version>
  <relativePath>../../spring-boot-dependencies</relativePath>
</parent>

　　按住Ctrl点击pom.xml中的spring-boot-dependencies，跳转到了spring-boot-dependencies的pom.xml，xml配置如下（只摘抄了部分重点配置）：

<properties>
      <activemq.version>5.15.3</activemq.version>
      <antlr2.version>2.7.7</antlr2.version>
      <appengine-sdk.version>1.9.63</appengine-sdk.version>
      <artemis.version>2.4.0</artemis.version>
      <aspectj.version>1.8.13</aspectj.version>
      <assertj.version>3.9.1</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>1.7.11</byte-buddy.version>
      ... ... ...
</properties>
<dependencyManagement>
      <dependencies>
          <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot</artifactId>
            <version>2.0.1.RELEASE</version>
          </dependency>
          <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-test</artifactId>
            <version>2.0.1.RELEASE</version>
          </dependency>
          ... ... ...
    </dependencies>
</dependencyManagement>
<build>
      <pluginManagement>
        <plugins>
              <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
              </plugin>
              <plugin>
                <groupId>org.jooq</groupId>
                <artifactId>jooq-codegen-maven</artifactId>
                <version>${jooq.version}</version>
              </plugin>
              <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <version>2.0.1.RELEASE</version>
              </plugin>
              ... ... ...
        </plugins>
      </pluginManagement>
</build>

　　从上面的spring-boot-dependencies的pom.xml中我们可以发现，一部分坐标的版本、依赖管理、插件管理已经定义好，所以我们的SpringBoot工程继承spring-boot-starter-parent后已经具备版本锁定等配置了。所以起步依赖的作用就是进行依赖的传递。

　2、分析spring-boot-starter-web

　　按住Ctrl点击pom.xml中的spring-boot-starter-web，跳转到了spring-boot-starter-web的pom.xml，xml配置如下（只摘抄了部分重点配置）：

<?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>2.0.1.RELEASE</version>
      </parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
      <version>2.0.1.RELEASE</version>
      <name>Spring Boot Web Starter</name>
  
      <dependencies>
        <dependency>
              <groupId>org.springframework.boot</groupId>
              <artifactId>spring-boot-starter</artifactId>
              <version>2.0.1.RELEASE</version>
              <scope>compile</scope>
        </dependency>
        <dependency>
              <groupId>org.springframework.boot</groupId>
              <artifactId>spring-boot-starter-json</artifactId>
              <version>2.0.1.RELEASE</version>
              <scope>compile</scope>
        </dependency>
        <dependency>
              <groupId>org.springframework.boot</groupId>
              <artifactId>spring-boot-starter-tomcat</artifactId>
              <version>2.0.1.RELEASE</version>
              <scope>compile</scope>
        </dependency>
        <dependency>
              <groupId>org.hibernate.validator</groupId>
              <artifactId>hibernate-validator</artifactId>
              <version>6.0.9.Final</version>
              <scope>compile</scope>
        </dependency>
        <dependency>
              <groupId>org.springframework</groupId>
              <artifactId>spring-web</artifactId>
              <version>5.0.5.RELEASE</version>
              <scope>compile</scope>
        </dependency>
        <dependency>
              <groupId>org.springframework</groupId>
              <artifactId>spring-webmvc</artifactId>
              <version>5.0.5.RELEASE</version>
              <scope>compile</scope>
        </dependency>
      </dependencies>
</project>

　　从上面的spring-boot-starter-web的pom.xml中我们可以发现，spring-boot-starter-web就是将web开发要使用的spring-web、spring-webmvc等坐标进行了“打包”，这样我们的工程只要引入spring-boot-starter-web起步依赖的坐标就可以进行web开发了，同样体现了依赖传递的作用。

二、自动配置原理解析

　按住Ctrl点击查看启动类MySpringBootApplication上的注解@SpringBootApplication

@SpringBootApplication
public class MySpringBootApplication {
    public static void main(String[] args) {
        SpringApplication.run(MySpringBootApplication.class);
    }
}

　注解@SpringBootApplication的源码

@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 {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};

    ... ... ...

}

　其中，

　　@SpringBootConfiguration：等同与@Configuration，既标注该类是Spring的一个配置类

　　@EnableAutoConfiguration：SpringBoot自动配置功能开启

　　　按住Ctrl点击查看注解@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    ... ... ...
}

　　　　其中，@Import(AutoConfigurationImportSelector.class) 导入了AutoConfigurationImportSelector类

　　　　按住Ctrl点击查看AutoConfigurationImportSelector源码

public String[] selectImports(AnnotationMetadata annotationMetadata) {
        ... ... ...
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                                                                   attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return StringUtils.toStringArray(configurations);
}

    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());
        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

　　　　　其中，SpringFactoriesLoader.loadFactoryNames 方法的作用就是从META-INF/spring.factories文件中读取指定类对应的类名称列表

　　　　　　

　　　　　spring.factories 文件中有关自动配置的配置信息如下：

... ... ...

org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\

... ... ...

　　　　上面配置文件存在大量的以AutoConfiguration为结尾的类名称，这些类就是存有自动配置信息的类，而SpringApplication在获取这些类名后再加载

　　　　我们以ServletWebServerFactoryAutoConfiguration为例来分析源码：

@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {
    ... ... ...
}

　　　　　其中，@EnableConfigurationProperties(ServerProperties.class) 代表加载ServerProperties服务器配置属性类

　　　　　进入ServerProperties.class源码如下：

@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
public class ServerProperties {

    /**
     * Server HTTP port.
     */
    private Integer port;

    /**
     * Network address to which the server should bind.
     */
    private InetAddress address;
  
      ... ... ...
  
}

　　　　　　其中，prefix = "server" 表示SpringBoot配置文件中的前缀，所有的以server.开头的配置都在spring-configuration-metadata.json中配好了

　　　　　　　

　　　　　　配好之后通过ServerProperties进行加载，加载完毕再通过@EnableConfigurationProperties(ServerProperties.class)进行引入，最终默认配置就会配好了。　　　　　　　 

　　　　　　可以自定义application.properties配置文件进行默认配置修改，配置以server开始的属性映射到该类的字段中映射关系如下：

　　　　　　

　　更改端口后启动项目，浏览器就需要访问http://localhost:8888/quick2

　　　　

　　更改当前web应用的名称，修改application.properties

spring.application.name=springboot_quick2
#服务器的端口号
server.port=8888
#当前web应用的名称
server.servlet.context-path=/demo

　　启动项目，这时浏览器就需要访问http://localhost:8888/demo/quick2