SpringBoot起步依赖，自动装配，原理分析

一、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>2.0.3.RELEASE</version>
        <relativePath>../../spring-boot-dependencies</relativePath>
    </parent>

这里他还有个爸爸也就是SpringBoot的爷爷

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

 <dependency>
                <groupId>org.yaml</groupId>
                <artifactId>snakeyaml</artifactId>
                <version>${snakeyaml.version}</version>
            </dependency>
            <dependency>
                <groupId>redis.clients</groupId>
                <artifactId>jedis</artifactId>
                <version>${jedis.version}</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.springframework.boot</groupId>
                    <artifactId>spring-boot-maven-plugin</artifactId>
                    <version>2.0.3.RELEASE</version>
                </plugin>

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

二、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）：

  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starters</artifactId>
    <version>2.0.3.RELEASE</version>
  </parent>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.0.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.hibernate.validator</groupId>
      <artifactId>hibernate-validator</artifactId>
      <version>6.0.10.Final</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-web</artifactId>
      <version>5.0.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.0.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

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

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

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 {
    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )

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
@ComponentScan：组件扫描配置 约定 当前所在的包及子包下的都会被扫描到

详解@EnableAutoConfiguration

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 {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

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

    String[] excludeName() default {};

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

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    private static final String[] NO_IMPORTS = new String[0];
    private static final Log logger = LogFactory.getLog(AutoConfigurationImportSelector.class);
    private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
    private ConfigurableListableBeanFactory beanFactory;
    private Environment environment;
    private ClassLoader beanClassLoader;
    private ResourceLoader resourceLoader;

    public AutoConfigurationImportSelector() {
    }


    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

image.png
spring.factories 文件中有关自动配置的配置信息如下：
image.png
上面配置文件存在大量的以Configuration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
image.png

@Configuration
@AutoConfigureOrder(-2147483648)
@ConditionalOnClass({ServletRequest.class})
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class})
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, EmbeddedTomcat.class, EmbeddedJetty.class, EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
    public ServletWebServerFactoryAutoConfiguration() {
    }

    @Bean
    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties) {
        return new ServletWebServerFactoryCustomizer(serverProperties);
    }

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 {
    private Integer port;
    private InetAddress address;
    @NestedConfigurationProperty
    private final ErrorProperties error = new ErrorProperties();
    private Boolean useForwardHeaders;
    private String serverHeader;
    private int maxHttpHeaderSize = 0;
    private Duration connectionTimeout;
    @NestedConfigurationProperty
    private Ssl ssl;
    @NestedConfigurationProperty
    private final Compression compression = new Compression();
    @NestedConfigurationProperty
    private final Http2 http2 = new Http2();
    private final ServerProperties.Servlet servlet = new ServerProperties.Servlet();
    private final ServerProperties.Tomcat tomcat = new ServerProperties.Tomcat();
    private final ServerProperties.Jetty jetty = new ServerProperties.Jetty();
    private final ServerProperties.Undertow undertow = new ServerProperties.Undertow();

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

image.png