Spring Boot的自动配置原理源码分析

Spring Boot的主方法会有一个注解

@SpringBootApplication

点进去看以后

@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}
)}
)

会有这些注释
点进@SpringBootConfiguration
会有这些注释

@Target({
    
    ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration

再点进@Configuration

@Target({
    
    ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
    
    
    @AliasFor(
        annotation = Component.class
    )
    String value() default "";

    boolean proxyBeanMethods() default true;
}

有@Component
所以它是个组件
所以说@SpringBootApplication注释的这个类本身就是spring的一个组件
其实spring boot项目的核心是pom.xml
他有一个父项目spring-boot-starter-parent

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.7.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
</parent>

点进去spring-boot-starter-parent看
配置了很多的依赖和插件，以我现在使用的2.2.7版本的spring-boot-starter-parent为例

<?xml version="1.0" encoding="utf-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.2.7.RELEASE</version>
    <relativePath>../../spring-boot-dependencies</relativePath>
  </parent>
  <artifactId>spring-boot-starter-parent</artifactId>
  <packaging>pom</packaging>
  <name>Spring Boot Starter Parent</name>
  <description>Parent pom providing dependency and plugin management for applications
		built with Maven</description>
  <url>https://projects.spring.io/spring-boot/#/spring-boot-starter-parent</url>
  <properties>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <java.version>1.8</java.version>
    <resource.delimiter>@</resource.delimiter>
    <maven.compiler.source>${java.version}</maven.compiler.source>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.target>${java.version}</maven.compiler.target>
  </properties>
</project>

看到spring-boot-dependencies了，我们的核心依赖就在父工程中
我们在写或者引入一些SpringBoot依赖的时候，不需要指定版本，因为就有这些版本仓库
再点进去spring-boot-dependencies看

<?xml version="1.0" encoding="utf-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-dependencies</artifactId>
  <version>2.2.7.RELEASE</version>
  <packaging>pom</packaging>
  <name>Spring Boot Dependencies</name>
  <description>Spring Boot Dependencies</description>
  <url>https://projects.spring.io/spring-boot/#</url>
  <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>[email protected]</email>
      <organization>Pivotal Software, Inc.</organization>
      <organizationUrl>https://www.spring.io</organizationUrl>
    </developer>
  </developers>
  <scm>
    <url>https://github.com/spring-projects/spring-boot</url>
  </scm>
<properties>
    <activemq.version>5.15.12</activemq.version>
    <antlr2.version>2.7.7</antlr2.version>
    <appengine-sdk.version>1.9.80</appengine-sdk.version>
    <artemis.version>2.10.1</artemis.version>
    <aspectj.version>1.9.5</aspectj.version>
    <assertj.version>3.13.2</assertj.version>
    <atomikos.version>4.0.6</atomikos.version>
    <awaitility.version>4.0.2</awaitility.version>
    <bitronix.version>2.1.4</bitronix.version>
</properties>

然后我们举几个例子看他导入了哪些依赖
消息队列：

 <activemq.version>5.15.12</activemq.version>

织入包：

<aspectj.version>1.9.5</aspectj.version>

然后看启动器

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
</dependency>

启动器：说白了就是SpringBoot的启动场景；
比如spring-boot-starter-web，它就会帮我们自动导入web环境所有的依赖
springboot会将所有的功能场景，都变成一个个的启动器
我们要使用什么功能，只需要找到对应的启动器starter就可以了

然后重点看依赖

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
</dependency>

这是我们刚刚引入的一个web依赖，这个依赖帮我们集成了tomcat，帮我们做了配置（dispatcherServlet），帮我们配了xml
然后是单元测试

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
</dependency>

后面是打jar包的插件

<plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
</plugins>

SpringBoot所有的依赖都是用spring-boot-starter开头的
主要有四个部分：
1、项目元数据信息：创建时候输入的Project Metadata部分，也就是Maven项目的基本元素，包括groupId、artifactId、name、description等
2、parent：继承spring-boot-starter-parent的依赖管理，控制版本与打包等内容
3、dependencies：项目具体依赖，这里包含了spring-boot-starter-web用于实现HTTP接口（该依赖中包含Spring MVC），官网对它的描述是：使用Spring MVC构建Web（包括RESTful）应用程序的入门者，使用Tomcat作为默认嵌入式容器。；spring-boot-starter-test用于编写单元测试的依赖包
4、build：构造配置部分。默认使用了spring-boot-maven-plugin，配合spring-boot-starter-parent就可以直接把Spring Boot应用打包成JAR来直接运行

然后来看主程序
里面的注解@SpringBootApplication标注这个类是一个springboot的应用
然后类里面的run方法SpringApplication.run(DemoApplication.class, args);表示将springboot应用启动

//@SpringBootApplication：标注这个类是一个springboot的应用
@SpringBootApplication
public class DemoApplication {
    
    

    public static void main(String[] args) {
    
    
        //将springboot应用启动
        SpringApplication.run(DemoApplication.class, args);
    }

}

看注解
点进去@SpringBootApplication，他是一个组合注解
除了这四个标准注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited

之外，还有这样几个注解，一个叫@SpringBootConfiguration，一个叫@EnableAutoConfiguration，一个是@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })，这个就是扫描这下面的包，可以剔除哪些东西。所以核心还是@SpringBootConfiguration和@EnableAutoConfiguration这两个东西。

见名知意，@SpringBootConfiguration就是springboot的配置
点进去可以看到，这个配置又是由一个@Configuration的配置类配置起来的，而@Configuration其实就是spring配置类，说明@SpringBootConfiguration这个启动类其实也就是一个配置类。然后@Configuration再往里面走，是一个@Component组件，说明这也是也是一个spring的组件。

见名知意，@EnableAutoConfiguration是自动导入配置
点击去可以发现除了四个标准注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited

之外，有一个@AutoConfigurationPackage自动配置包
点进去会发现它导入了一个选择器@Import(AutoConfigurationPackages.Registrar.class)
再点进去

	/**
	 * {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
	 * configuration.
	 */
	static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
    
    

		@Override
		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
    
    
			register(registry, new PackageImport(metadata).getPackageName());
		}

		@Override
		public Set<Object> determineImports(AnnotationMetadata metadata) {
    
    
			return Collections.singleton(new PackageImport(metadata));
		}

	}

这边注册了metadata这个元数据，然后PackageImport(metadata)导入了这些元数据，然后.getPackageName()导入这些包名

然后回到上一级，还导入了一个@Import(AutoConfigurationImportSelector.class)选择器
点进去，可以看到它没有注解了，就是一个单纯的类，往下翻，有个选择组件

@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
    
    
		if (!isEnabled(annotationMetadata)) {
    
    
			return NO_IMPORTS;
		}
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
				annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}

选择组件是选择我们配置的pom.xml里面的组件的
这里面有一个loadMetadata(this.beanClassLoader)加载元数据，点进去

static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
    
    
		return loadMetadata(classLoader, PATH);
	}

static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
    
    
		try {
    
    
			Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
					: ClassLoader.getSystemResources(path);
			Properties properties = new Properties();
			while (urls.hasMoreElements()) {
    
    
				properties.putAll(PropertiesLoaderUtils.loadProperties(new UrlResource(urls.nextElement())));
			}
			return loadMetadata(properties);
		}
		catch (IOException ex) {
    
    
			throw new IllegalArgumentException("Unable to load @ConditionalOnClass location [" + path + "]", ex);
		}
	}

加载里面还有加载
然后回到AutoConfigurationImportSelector类

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {
    
    
		if (!isEnabled(annotationMetadata)) {
    
    
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(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 new AutoConfigurationEntry(configurations, exclusions);
	}

中的List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);获取所有的配置
里面的方法getCandidateConfigurations获取候选的配置，点进去，获取候选的配置就有个方法

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    
    
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
				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;
	}

它通过一个加载去加载所有的，再通过getBeanClassLoader()
一步一步看，首先它去加载了一个包，那他加载了什么呢？是不是就有getSpringFactoriesLoaderFactoryClass()和getBeanClassLoader()
我们一个一个进，首先进入getBeanClassLoader()

protected ClassLoader getBeanClassLoader() {
    
    
		return this.beanClassLoader;
	}

点beanClassLoader是private ClassLoader beanClassLoader;
这是一个加载器，就是当前类的
再点进去getSpringFactoriesLoaderFactoryClass()

protected Class<?> getSpringFactoriesLoaderFactoryClass() {
    
    
		return EnableAutoConfiguration.class;
	}

这个获取的配置要加在哪个地方他有一个方法叫getSpringFactoriesLoaderFactoryClass()，加载那个类的class
这个地方就只返回了一个东西，叫EnableAutoConfiguration.class
就标注了这个类的所有包
我们回过头来看一下，@SpringBootApplication里面就标注了他

@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) })

所以它去加载了配置是从这个注解来的，而这个注解被@SpringBootApplication继承了，所以说兜兜转转就为了做一件事情，启动类下的所有资源被导入
除此以外他还做了一件事情

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.");

断言Assert非空.notEmpty
就是保证他不为空，然后No auto configuration classes found in META-INF/spring.factories.可能没有自动加载出一个文件
META-INF/spring.factories：自动配置的核心文件
在这里插入图片描述
在下载的jar包里面可以找到，点开spring.factories，可以发现它里面有很多配置

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveRestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\
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,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

Initializers初始化的
Application Listeners监听的
Auto Configuration Import Listeners和Auto Configuration Import Filters自动选择导入的包
Auto Configure自动配置，没有的需要我们手动配，有的就自动配置了
里面有一个org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\，点进去看
有一个@Configuration(proxyBeanMethods = false)说明他是一个java配置类
总的来看，相当于我们导入了一个WebMvc，它导入了这么多自动配置，这个自动配置从哪来，就是先读了这个配置文件，读了这个配置文件才找到这些配置类，所以这个文件很重要
刚刚我们是从这里的断言进来的

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    
    
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
				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;
	}

现在我们从loadFactoryNames方法找，点进去，这个方法就在另外一个类了，叫SpringFactoriesLoader，spring工厂的加载
然后看代码

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    
    
        //获取类的名字
        String factoryTypeName = factoryType.getName();
        //获取默认的包名，断言不为空
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    
    
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
    
    
            return result;
        } else {
    
    
            try {
    
    
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

                while(urls.hasMoreElements()) {
    
    
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
    
    
                        Entry<?, ?> entry = (Entry)var6.next();
                        String factoryTypeName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;

                        for(int var11 = 0; var11 < var10; ++var11) {
    
    
                            String factoryImplementationName = var9[var11];
                            result.add(factoryTypeName, factoryImplementationName.trim());
                        }
                    }
                }

                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {
    
    
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

首先会去加载loadSpringFactories这个classLoader，这个classLoader就标注了application那个主启动类里面有EnableAutoConfiguration，然后这个类调了下面的这个方法
做了枚举去遍历它，有个urls，urls是从类加载器中去获得所有的资源classLoader.getResources("META-INF/spring.factories")，获取所有的系统资源ClassLoader.getSystemResources("META-INF/spring.factories")
然后是一个while循环，while后面括号里面的urls.hasMoreElements()是判断有没有更多的元素
如果有的话，就把它放在url里面，然后把url封装加载到配置类里面去了
用思维导图梳理一下整个流程吧
在这里插入图片描述
结论：
1、 springboot在启动的时候，从类路径下META-INF/spring.factories获取指定的值；
2、将这些自动配置的类导入容器，自动配置类就会生效，帮我们进行自动配置；
3、以前我们需要自动配置的东西，现在springboot帮我们做了；
4、整合javaEE，解决方案和自动配置的东西都在spring-boot-autoconfigure-2.2.7.RELEASE.jar这个包下；
5、它会把所有需要导入的组件，以类名的方式返回，这些组件就会被添加到容器；
6、容器中也会存在非常多的XXXAutoConfiguration（@Bean）的文件，就是这些类给容器中导入了这个场景需要的所有组件，并自动配置，@Configuration,JavaConfig；
7、有了自动配置类，免去了我们手动编写配置文件的工作

然后以org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration为例再详细分析一下

/**
 * {@link EnableAutoConfiguration Auto-configuration} for configuring the encoding to use
 * in web applications.
 *
 * @author Stephane Nicoll
 * @author Brian Clozel
 * @since 2.0.0
 */
 //表示这是一个配置类
@Configuration(proxyBeanMethods = false)
//自动配置属性：HttpProperties
@EnableConfigurationProperties(HttpProperties.class)
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {
    
    

	private final HttpProperties.Encoding properties;

	public HttpEncodingAutoConfiguration(HttpProperties properties) {
    
    
		this.properties = properties.getEncoding();
	}

	@Bean
	@ConditionalOnMissingBean
	public CharacterEncodingFilter characterEncodingFilter() {
    
    
		CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
		filter.setEncoding(this.properties.getCharset().name());
		filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
		filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
		return filter;
	}

	@Bean
	public LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
    
    
		return new LocaleCharsetMappingsCustomizer(this.properties);
	}

	private static class LocaleCharsetMappingsCustomizer
			implements WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>, Ordered {
    
    

		private final HttpProperties.Encoding properties;

		LocaleCharsetMappingsCustomizer(HttpProperties.Encoding properties) {
    
    
			this.properties = properties;
		}

		@Override
		public void customize(ConfigurableServletWebServerFactory factory) {
    
    
			if (this.properties.getMapping() != null) {
    
    
				factory.setLocaleCharsetMappings(this.properties.getMapping());
			}
		}

		@Override
		public int getOrder() {
    
    
			return 0;
		}

	}

}

首先@Configuration表示这是一个配置类
然后@EnableConfigurationProperties这是自动配置的配置属性，它可以指定我们要配置哪些类的东西，它后面跟的是HttpProperties，而且不仅这个地方装配了HttpProperties，它下面有个私有的属性也是指向HttpProperties，它的构造器也传入了这个HttpProperties。
那它是什么呢？点进去看一下

@ConfigurationProperties(prefix = "spring.http")
public class HttpProperties {
    
    

	/**
	 * Whether logging of (potentially sensitive) request details at DEBUG and TRACE level
	 * is allowed.
	 */
	private boolean logRequestDetails;

	/**
	 * HTTP encoding properties.
	 */
	private final Encoding encoding = new Encoding();

	public boolean isLogRequestDetails() {
    
    
		return this.logRequestDetails;
	}

	public void setLogRequestDetails(boolean logRequestDetails) {
    
    
		this.logRequestDetails = logRequestDetails;
	}

	public Encoding getEncoding() {
    
    
		return this.encoding;
	}

	/**
	 * Configuration properties for http encoding.
	 */
	public static class Encoding {
    
    

		public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

		/**
		 * Charset of HTTP requests and responses. Added to the "Content-Type" header if
		 * not set explicitly.
		 */
		private Charset charset = DEFAULT_CHARSET;

		/**
		 * Whether to force the encoding to the configured charset on HTTP requests and
		 * responses.
		 */
		private Boolean force;

		/**
		 * Whether to force the encoding to the configured charset on HTTP requests.
		 * Defaults to true when "force" has not been specified.
		 */
		private Boolean forceRequest;

		/**
		 * Whether to force the encoding to the configured charset on HTTP responses.
		 */
		private Boolean forceResponse;

		/**
		 * Locale in which to encode mapping.
		 */
		private Map<Locale, Charset> mapping;

		public Charset getCharset() {
    
    
			return this.charset;
		}

		public void setCharset(Charset charset) {
    
    
			this.charset = charset;
		}

		public boolean isForce() {
    
    
			return Boolean.TRUE.equals(this.force);
		}

		public void setForce(boolean force) {
    
    
			this.force = force;
		}

		public boolean isForceRequest() {
    
    
			return Boolean.TRUE.equals(this.forceRequest);
		}

		public void setForceRequest(boolean forceRequest) {
    
    
			this.forceRequest = forceRequest;
		}

		public boolean isForceResponse() {
    
    
			return Boolean.TRUE.equals(this.forceResponse);
		}

		public void setForceResponse(boolean forceResponse) {
    
    
			this.forceResponse = forceResponse;
		}

		public Map<Locale, Charset> getMapping() {
    
    
			return this.mapping;
		}

		public void setMapping(Map<Locale, Charset> mapping) {
    
    
			this.mapping = mapping;
		}

		public boolean shouldForce(Type type) {
    
    
			Boolean force = (type != Type.REQUEST) ? this.forceResponse : this.forceRequest;
			if (force == null) {
    
    
				force = this.force;
			}
			if (force == null) {
    
    
				force = (type == Type.REQUEST);
			}
			return force;
		}

		public enum Type {
    
    

			REQUEST, RESPONSE

		}

	}

}

它也有一个@ConfigurationProperties(prefix = "spring.http")
以string.http为前缀有很多
在这里插入图片描述
在HttpProperties的底下有这些代码

/**
		 * Charset of HTTP requests and responses. Added to the "Content-Type" header if
		 * not set explicitly.
		 */
		private Charset charset = DEFAULT_CHARSET;

		/**
		 * Whether to force the encoding to the configured charset on HTTP requests and
		 * responses.
		 */
		private Boolean force;

		/**
		 * Whether to force the encoding to the configured charset on HTTP requests.
		 * Defaults to true when "force" has not been specified.
		 */
		private Boolean forceRequest;

		/**
		 * Whether to force the encoding to the configured charset on HTTP responses.
		 */
		private Boolean forceResponse;

		/**
		 * Locale in which to encode mapping.
		 */
		private Map<Locale, Charset> mapping;

可以发现和上面是一一对应的
所以也就是说它绑定了配置文件里面所有的string.http.XXXXX
也就是说配置文件能写什么就是去写HttpProperties里面的配置
再往下是spring的底层注解，根据不同的条件，来判断当前配置或者类是否生效
以此为例，@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)，如果他的类型不是一个web应用，它直接就无效了
再往下，@ConditionalOnClass(CharacterEncodingFilter.class)，是否存在CharacterEncodingFilter.class，这就是springMVC的字符编码过滤器
第三个，@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)，如果不存在spring.http.encoding，那就是默认的
下面的是@Conditional的派生注解（Spring注解版原生的@Conditional作用）
作用：必须是@Conditional指定的条件成立，才给容器中添加组件，配置里面的所有内容才生效
在这里插入图片描述
再往下走

public HttpEncodingAutoConfiguration(HttpProperties properties) {
    
    
		this.properties = properties.getEncoding();
	}

它的构造器也指向properties
这个HttpProperties就是它的配置类
再往下看就是一些字符编码的过滤，然后放到Bean中

    @Bean
    //判断容器中是否存在当前组件
	@ConditionalOnMissingBean
	public CharacterEncodingFilter characterEncodingFilter() {
    
    
		CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
		filter.setEncoding(this.properties.getCharset().name());
		//请求过滤
		filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
		//响应过滤
		filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
		return filter;
	}

再往下面就是一些国际化了，就不是很重要了，可以不管他们

最后总结下自动装配的原理
精髓就是：
1、SpringBoot启动会加载大量的自动配置类
2、我们看我们需要的功能有没有在Spring Boot默认写好的自动配置类当中
3、只要我们要用的组件存在在自动配置类配置的组件当中，我们就不需要再手动配置了
4、给容器中自动配置类添加组件的时候，会从properties类中获取某些属性。我们只需要在配置文件中指定这些属性的值即可
核心：
xxxxAutoConfiguration：自动配置类；给容器中添加组件
xxxxProperties：封装配置文件中相关属性

可以在yml里面通过debug=true来查看，哪些自动配置类生效，哪些没有生效

Spring Boot的自动配置原理源码分析

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