java框架之SpringBoot(10)-启动流程及自定义starter

时间 2019-12-06

标签 java 框架 springboot 启动流程自定义 starter 栏目 Java 繁體版

原文原文链接

启动流程

直接从 SpringBoot 程序入口的 run 方法看起：java

1 public static ConfigurableApplicationContext run(Object source, String... args) {
2     return run(new Object[] { source }, args);
3 }

org.springframework.boot.SpringApplication#run(java.lang.Object, java.lang.String...)

执行第 2 行的 run(new Object[] { source }, args) 方法：react

1 public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
2     return new SpringApplication(sources).run(args);
3 }

org.springframework.boot.SpringApplication#run(java.lang.Object[], java.lang.String[])

能够看到，这里启动其实是分为两步，先是建立 SpringApplication 对象，而后执行该对象的 run 方法。下面根据这两步进行深刻：web

建立SpringApplication对象

下一步会进入 SpringApplication 类的构造方法：redis

1 public SpringApplication(Object... sources) {
2     initialize(sources);
3 }

org.springframework.boot.SpringApplication#SpringApplication(java.lang.Object...)

接着进入 initialize 方法：spring

 1 private void initialize(Object[] sources) {
 2     if (sources != null && sources.length > 0) {
 3         // 保存配置类，包含入口类
 4         this.sources.addAll(Arrays.asList(sources));
 5     }
 6     // 判断当前是不是一个 web 应用
 7     this.webEnvironment = deduceWebEnvironment();
 8     // 从类路径下找到 META-INF/spring.factories 配置的全部 ApplicationContextInitializer 类并反射建立实例并保存到当前 SpringApplication 实例的 initializers 属性
 9     setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
10     // 从类路径下找到 META-INF/spring.factories 配置的全部 ApplicationListener 类并反射建立实例保存到当前 SpringApplication 实例的 listeners 属性
11     setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
12     // 从调用栈中获取到执行了 main 方法的配置类即入口类保存到当前 SpringApplication 实例的 mainApplicationClass 属性
13     this.mainApplicationClass = deduceMainApplicationClass();
14 }

org.springframework.boot.SpringApplication#initialize

在建立 SpringApplication 对象的过程当中实例化了类路径下 META-INF/spring.factories 中 ApplicationContextInitializer 节对应的应用上下文初始化器类和 ApplicationListener 节对应的应用监听器类并保存到了 SpringApplication 对象中。apache

能够看到在不少包中都有这个文件，下面为自动配置包中的 spring.factories 文件：springboot

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

# 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.OnClassCondition

# 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.CloudAutoConfiguration,\
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.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
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.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
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.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.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.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.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
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.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
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.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

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

# 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.JspTemplateAvailabilityProvider

spring-boot-autoconfigure-1.5.19.RELEASE.jar!/META-INF/spring.factories

执行SpringApplication.run()

执行 org.springframework.boot.SpringApplication#run(java.lang.String...) 方法：websocket

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    // 声明一个 IoC 容器
    ConfigurableApplicationContext context = null;
    FailureAnalyzers analyzers = null;
    // AWT 相关
    configureHeadlessProperty();
    // 经过 getSpringFactoriesInstances 从类路径下找到 META-INF/spring.factories 配置的全部 SpringApplicationRunListener 类并反射建立实例返回
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 回调执行全部 SpringApplicationRunListener 实例的 starting 方法
    listeners.starting();
    try {
        // 封装命令行参数
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                args);
        // 准备环境，建立环境完成后在该方法中又回调执行了全部 SpringApplicationRunListener 实例的 environmentPrepared 方法，表示环境准备完成
        ConfigurableEnvironment environment = prepareEnvironment(listeners,
                applicationArguments);
        // 控制台打印 SpringBoot 图标
        Banner printedBanner = printBanner(environment);
        // 根据当前环境的不一样建立不一样类型的 IoC 容器
        context = createApplicationContext();
        // 初始化异常分析器
        analyzers = new FailureAnalyzers(context);
        // 准备上下文环境：
        // 一、将 environment 保存到了 context 即 IoC 容器中
        // 二、经过 applyInitializers(context) 方法，执行了全部 ApplicationContextInitializer 实例的 initialize(context) 方法（这些 ApplicationContextInitializer 实例是在初始化 SpringApplication 实例时建立保存的）
        // 三、还会回调执行全部 SpringApplicationRunListener 实例的 contextPrepared(context) 方法
        // 四、在 prepareContext 方法最后一行还会回调执行全部 SpringApplicationRunListener 实例的 contextLoaded(context) 方法
        prepareContext(context, environment, listeners, applicationArguments,
                printedBanner);
        // 刷新 IoC 容器，IoC 容器初始化，扫描、建立、加载全部组件（配置类、自动配置功能），若是是 web 应用，在这一步还会建立嵌入式 Servlet 容器
        refreshContext(context);
        // 刷新后处理，会经过 callRunners(context, args) 方法从 IoC 容器中获取全部 ApplicationRunner 和 CommandLineRunner 的 bean，并回调它们的 run 方法
        afterRefresh(context, applicationArguments);
        // 回调执行全部的 SpringApplicationRunListener 实例的 finished 方法，表示应用启动完成
        listeners.finished(context, null);
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass)
                    .logStarted(getApplicationLog(), stopWatch);
        }
        // 整个 SpringBoot 应用启动完成后返回 IoC 容器
        return context;
    }
    catch (Throwable ex) {
        handleRunFailure(context, listeners, analyzers, ex);
        throw new IllegalStateException(ex);
    }
}

总结：session

在 SpringBoot 应用启动的过程当中会回调一些咱们预配置组件的指定方法，这些组件能够是咱们配置在指定文件中的类，也能够是咱们注册到 IoC 容器的 bean。分类以下：app

配置在 META-INF/spring.factories 中：

org.springframework.context.ApplicationContextInitializer
org.springframework.context.ApplicationListener
org.springframework.boot.SpringApplicationRunListener

注册在 IoC 容器中：

org.springframework.boot.ApplicationRunner
org.springframework.boot.CommandLineRunner

执行的方法按回调顺序排序以下：

SpringApplicationRunListener.starting()
SpringApplicationRunListener.environmentPrepared()
ApplicationContextInitializer.initialize()
SpringApplicationRunListener.contextPrepared()
SpringApplicationRunListener.contextLoaded()
ApplicationRunner.run()
CommandLineRunner.run()
SpringApplicationRunListener.finished()

自定义starter

了解了 SpringBoot 程序的启动流程后，咱们也能够本身编写一个 starter 了，下面为一个小案例，功能是让程序在启动时注册一个 hello 组件，而且能经过配置修改输出内容。

建立自动配置组件

一、使用 maven 建立一个 SpringBoot 程序做为自动配置组件，依赖以下：

<?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-starter-parent</artifactId>
        <version>1.5.19.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.zze.springboot</groupId>
    <artifactId>mystarter_autoconfigure</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>mystarter_autoconfigure</name>
    <description>Demo project for Spring Boot</description>
    <packaging>jar</packaging>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <!--引入 spring-boot-starter-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
    </dependencies>
</project>

pom.xml

二、编写与配置文件属性映射的配置类：

package com.zze.springboot.config;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties(prefix = "com.zze")
public class HelloProperties {
    private String name;
    private String remark;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getRemark() {
        return remark;
    }

    public void setRemark(String remark) {
        this.remark = remark;
    }
}

com.zze.springboot.config.HelloProperties

三、编写 hello 服务：

package com.zze.springboot.service;

import com.zze.springboot.config.HelloProperties;

public class HelloService {

    private HelloProperties helloProperties;

    public void setHelloProperties(HelloProperties helloProperties) {
        this.helloProperties = helloProperties;
    }

    public void sayHello() {
        System.out.println("hello " + helloProperties.getName() + " " + helloProperties.getRemark());
    }
}

com.zze.springboot.service.HelloService

四、编写自动配置类：

package com.zze.springboot.autoconfigure;

import com.zze.springboot.config.HelloProperties;
import com.zze.springboot.service.HelloService;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

/**
 * 自定义自动配置类
 */
@EnableConfigurationProperties(HelloProperties.class) // 启用指定类的配置映射，并将配置类实例注册到 IoC 容器
@Configuration
@ConditionalOnWebApplication // web 应用时才生效
public class MyStarterAutoConfiguration {

    private HelloProperties helloProperties;

    public MyStarterAutoConfiguration(HelloProperties helloProperties){
        this.helloProperties = helloProperties;
    }

    // 注册 hello 服务到 IoC 容器
    @Bean
    public HelloService helloService(){
        HelloService helloService = new HelloService();
        helloService.setHelloProperties(helloProperties);
        return helloService;
    }

}

com.zze.springboot.autoconfigure.MyStarterAutoConfiguration

五、配置自动配置类，让程序启动时加载自动配置类让其生效：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.zze.springboot.autoconfigure.MyStarterAutoConfiguration

META-INF/spring.factories

建立starter工程

一、建立普通的 maven 项目做为 starter 组件，引入上面编写的自动配置组件依赖：

<?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>com.zze.springboot</groupId>
    <artifactId>mystarter</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <!--引入自动配置模块-->
        <dependency>
            <groupId>com.zze.springboot</groupId>
            <artifactId>mystarter_autoconfigure</artifactId>
            <version>0.0.1-SNAPSHOT</version>
        </dependency>
    </dependencies>
</project>

pom.xml

二、将自动配置组件及 starter 组件安装到 maven 仓库。

测试

一、使用 maven 新建立一个 SpringBoot 项目，引入咱们编写的 starter 依赖：

<?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-starter-parent</artifactId>
        <version>1.5.19.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

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

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>com.zze.springboot</groupId>
            <artifactId>mystarter</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

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

</project>

pom.xml

二、配置 hello 服务所须要的相关属性：

com.zze.name=张三
com.zze.remark=会员

application.properties

三、直接注入 hello 服务，测试：

package com.example.demo;

import com.zze.springboot.service.HelloService;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
@SpringBootTest
public class DemoApplicationTests {

    @Autowired
    private HelloService helloService;

    @Test
    public void contextLoads() {
        helloService.sayHello();
        /*
        hello 张三 会员
         */
    }
}

test

启动器模块是一个空 jar 文件，仅提供辅助性依赖管理，这些依赖可能用于自动装配或其它类库。启动器依赖于自动配置，第三方使用只须要引入启动器。
自动配置类能加载的前提是要将其配置在 META-INF/spring.factories 文件中的 org.springframework.boot.autoconfigure.EnableAutoConfiguration 节下。