3. SpringBoot原理分析

1. 起步依赖原理分析

1.1 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）：

    org.springframework.boot
    spring-boot-dependencies
    2.0.1.RELEASE
    ../../spring-boot-dependencies

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）：

    
    
        
        junit
        
        
        junit
        
        
        4.9
        
        
        test
    



    
        
            org.apache.maven.plugins
            maven-compiler-plugin
            
                1.8
                1.8
                UTF-8
            
        
    




    5.15.3
    2.7.7
    1.9.63
    2.4.0
    1.8.13
    3.9.1
    4.0.6
    2.1.4
    
    3.0.0
    1.7.11
    ... ... ...



    
        
            org.springframework.boot
            spring-boot
            2.0.1.RELEASE
        
        
            org.springframework.boot
            spring-boot-test
            2.0.1.RELEASE
        
        ... ... ...
    


    
        
            
                org.jetbrains.kotlin
                kotlin-maven-plugin
                ${kotlin.version}
            
            
                org.jooq
                jooq-codegen-maven
                ${jooq.version}
            
            
                org.springframework.boot
                spring-boot-maven-plugin
                2.0.1.RELEASE
            
            ... ... ...
        
    
    

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，从而实现版本的控制。

1.2 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）：

4.0.0
    
        org.springframework.boot
        spring-boot-starters
        2.0.1.RELEASE
    
    org.springframework.boot
    spring-boot-starter-web
    2.0.1.RELEASE
    Spring Boot Web Starter
    
    
        
            org.springframework.boot
            spring-boot-starter
            2.0.1.RELEASE
            compile
        
        
            org.springframework.boot
            spring-boot-starter-json
            2.0.1.RELEASE
            compile
        
        
            org.springframework.boot
            spring-boot-starter-tomcat
            2.0.1.RELEASE
            compile
        
        
            org.hibernate.validator
            hibernate-validator
            6.0.9.Final
            compile
        
        
            org.springframework
            spring-web
            5.0.5.RELEASE
            compile
        
        
            org.springframework
            spring-webmvc
            5.0.5.RELEASE
            compile
        
    

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。

2. 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 configurations = getCandidateConfigurations(annotationMetadata,attributes);
    configurations = removeDuplicates(configurations);
    Set exclusions = getExclusions(annotationMetadata, attributes);
    checkExcludedClasses(configurations, exclusions);
    configurations.removeAll(exclusions);
    configurations = filter(configurations, autoConfigurationMetadata);
    fireAutoConfigurationImportEvents(configurations, exclusions);
    return StringUtils.toStringArray(configurations);
}
protected List getCandidateConfigurations(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
    List configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
    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：

... ... ...
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConf
iguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration
,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfigu
ration,\
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：

@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，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