SpringBoot autoconfigure

springboot 实现了根据classpath下依赖内容自动配置bean到IOC容器，----》Auto configuration（自动配
置）.Springboot可以完美配置各种Spring子项目，如Spring mvc，security、data。。。

springboot auto configuration会尝试推断哪些bean是用户需要的，并进行初始化

1、通过class标注@configuration，并基于@ConditionalOnClass和OnMissingBean等@Conditional注解的条件过滤，保证springboot满足一定条件下，才会将bean自动注入IOC容器
2、要使用自动配置，首先要添加@EnableAutoConfiguration注解，springboot通过该注解使用给@import注解完成导入配置的功能，配置在META-INFO/spring.factories中定义

@Configuration(proxyBeanMethods = false)
class ServletWebServerFactoryConfiguration {

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedTomcat {

        @Bean
        TomcatServletWebServerFactory tomcatServletWebServerFactory(
                ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
                ObjectProvider<TomcatContextCustomizer> contextCustomizers,
                ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
            TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
            factory.getTomcatConnectorCustomizers()
                    .addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatContextCustomizers()
                    .addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatProtocolHandlerCustomizers()
                    .addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

    }

    /**
     * Nested configuration if Jetty is being used.
     */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedJetty {

        @Bean
        JettyServletWebServerFactory JettyServletWebServerFactory(
                ObjectProvider<JettyServerCustomizer> serverCustomizers) {
            JettyServletWebServerFactory factory = new JettyServletWebServerFactory();
            factory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

    }

    /**
     * Nested configuration if Undertow is being used.
     */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedUndertow {

        @Bean
        UndertowServletWebServerFactory undertowServletWebServerFactory(
                ObjectProvider<UndertowDeploymentInfoCustomizer> deploymentInfoCustomizers,
                ObjectProvider<UndertowBuilderCustomizer> builderCustomizers) {
            UndertowServletWebServerFactory factory = new UndertowServletWebServerFactory();
            factory.getDeploymentInfoCustomizers()
                    .addAll(deploymentInfoCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getBuilderCustomizers().addAll(builderCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

        @Bean
        UndertowServletWebServerFactoryCustomizer undertowServletWebServerFactoryCustomizer(
                ServerProperties serverProperties) {
            return new UndertowServletWebServerFactoryCustomizer(serverProperties);
        }

    }

}

ServletWebServerFactoryAutoConfiguration.java定义：根据Conditional条件注解实现Bean初始化

@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)     // 对应的Properties配置
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {

    @Bean
    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,
            ObjectProvider<WebListenerRegistrar> webListenerRegistrars) {
        return new ServletWebServerFactoryCustomizer(serverProperties,
                webListenerRegistrars.orderedStream().collect(Collectors.toList()));
    }

    @Bean
    @ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
    public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
            ServerProperties serverProperties) {
        return new TomcatServletWebServerFactoryCustomizer(serverProperties);
    }

Spring整合的有：
amqp，AOP，CAP,batch,cache,cassandra数据库，codec,condition注入，couchbase,dao,data,elasticsearch,flyway,freemarker,graphql,groovy,gson,h2,hateosa,hazelcast,http,indlux,info,interation,jackson,jdbc,jersey,jms,jmx,joop,jsonb,kafkamldap,lihibase,logging,mail,mongodb,mustache,neo4j,netty,orm,quarts,r2dbc,rsocket,security,sendgrid,session,sql,webervice,websocket,thymealeaf.

消息队列有:jms，amqp，kafka
数据库： jdbc,couchbase,r2dbc,mongodb:neo4j
模板语言：,mustache ，freemarker，thymealeaf
组件：quarts
网络协议：http,netty

参考文档：
https://www.cnblogs.com/clarino/p/16344670.html