Springboot基本原理（二）

1. 简化依赖原理分析

1.1 spring-boot-starter-parent

　　点击pom.xml文件中的springboot-starter-parent，跳转到了springboot-starter-parent的pom.xml，xml配置如下（这里只截了部分重点配置）

  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.2.1.RELEASE</version>
    <relativePath>../../spring-boot-dependencies</relativePath>
  </parent>

　　点击pom.xml中的spring-boot-starter-dependencies，跳转到了spring-boot-starter-dependencies的pom.xml，配置如下（部分重要配置）：

<properties>
    <activemq.version>5.15.10</activemq.version>
    <antlr2.version>2.7.7</antlr2.version>
    <appengine-sdk.version>1.9.76</appengine-sdk.version>
    <artemis.version>2.10.1</artemis.version>
    <aspectj.version>1.9.4</aspectj.version>
    <assertj.version>3.13.2</assertj.version>
    <atomikos.version>4.0.6</atomikos.version>
    <awaitility.version>4.0.1</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>1.10.2</byte-buddy.version>
    <caffeine.version>2.8.0</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>
    ....................
  </properties>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot</artifactId>
        <version>2.2.1.RELEASE</version>
      </dependency>
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-test</artifactId>
        <version>2.2.1.RELEASE</version>
      </dependency>
      ....................
  </dependencyManagement>
  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <groupId>org.apache.johnzon</groupId>
          <artifactId>johnzon-maven-plugin</artifactId>
          <version>${johnzon.version}</version>
        </plugin>
        <plugin>
          <groupId>org.jetbrains.kotlin</groupId>
          <artifactId>kotlin-maven-plugin</artifactId>
          <version>${kotlin.version}</version>
        </plugin>
       ....................
      </plugins>
    </pluginManagement>
  </build>

　　从上面的spring-boot-starter-dependencies的pom.xml中我们可以发现，一部分坐标的版本、依赖管理、插件已经定义好，所以我们的springboot工程继承springboot-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。

1.2 spring-boot-starter-web

　　点击pom.xml中的spring-boot-starter-web，跳转到spring-boot-starter-web的pom.xml，配置如下：

<?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>2.2.1.RELEASE</version>
  </parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
  <version>2.2.1.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>https://www.apache.org/licenses/LICENSE-2.0</url>
    </license>
  </licenses>
  <developers>
    <developer>
      <name>Pivotal</name>
      <email>[e-mail]</email>
      <organization>Pivotal Software, Inc.</organization>
      <organizationUrl>https://www.spring.io</organizationUrl>
    </developer>
  </developers>
  <scm>
    <connection>scm:git:git://github.com/spring-projects/spring-boot.git</connection>
    <developerConnection>scm:git:ssh://[e-mail]/spring-projects/spring-boot.git</developerConnection>
    <url>https://github.com/spring-projects/spring-boot</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>2.2.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.2.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.2.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-validation</artifactId>
      <version>2.2.1.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>5.2.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.2.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>
</project>

　　从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依赖了spring-boot-starter-web，就可以进行web开发，同样体现了依赖传递的作用。

2. 自动配置原理解析

　　点击启动类上的@SpringBootApplication注解，@SpringBootApplication注解的源码如下（部分内容）：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration // 等同于@Configuration,即标注该类是Spring的一个配置类
@EnableAutoConfiguration // SpringBoot自动配置功能开启
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )
    Class<?>[] exclude() default {};
    ... ... ...
}

　　点击查看注解@EnableAutoConfiguration

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

　　其中，@Import(AutoConfigurationImportSelector.class) 导入了AutoConfigurationImportSelector类，点击查看AutoConfigurationImportSelector源码

   public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
            AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }
    ... ... ...
     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方法的作用就是去加载META-INF/spring.factories，这个外部文件中有很多自动配置的类。

 　　spring.properties文件中有关自动配置的配置信息如下（只截取部分用来说明）：

 　　上面的配置文件中存在大量的以Configuration为结尾的类名称，这些都是存有自动配置信息的类。SpringApplication在获取这些类名后再加载。

　　以ServletWebServerFactoryAutoConfiguration为例分析：

@Configuration(
    proxyBeanMethods = false
)
@AutoConfigureOrder(-2147483648)
@ConditionalOnClass({ServletRequest.class})
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class}) // 加载ServerProperties服务器配置属性类
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, EmbeddedTomcat.class, EmbeddedJetty.class, EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
    ... ... ...
}

　　进入ServerProperties.class源码如下：

@ConfigurationProperties(
    prefix = "server",
    ignoreUnknownFields = true
)
public class ServerProperties {
    private Integer port;
    private InetAddress address;
    ... ... ...
}

　prefix = "server" 表示SpringBoot配置文件中的前缀，SpringBoot会将配置文件中以server开头的属性映射到该类的字段中。映射关系如下：