Spring Boot原理分析

一、起步依赖原理分析

1. spring-boot-starter-parent（版本控制）

​ 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）：

<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-starter-dependencies的pom.xml中我们可以发现，一部分版本的坐标、依赖管理、插件管理已经定义好，所以继承spring-boot-starter-parent使得Spring Boot工程具有了版本控制的功能。

​ 起步依赖的作用就是进行依赖的传递。

2. spring-boot-starter-web（web开发）

​ 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、自动配置原理分析

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，即标注该类是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(
				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

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：