spring boot 核心实现源码分析--开启一个web应用-天翼云

通过分析demo程序源码来掌握技术的本质

@SpringBootApplication
@RestController
public class DemoApplication {
    @RequestMapping("/")
    String home(){
        return "hello spring-boot and spring cloud";
    }
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

这里有几个地方是值得我们关注的，首先是三个注解，然后是SpringApplication这个类。首先来看看SpringApplication这个类的实现方式

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
        return run(new Class[]{primarySource}, args);
    }

这个方法返回的是一个ConfigurableApplicationContext类型的上下文
继续看run方法

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
        return (new SpringApplication(primarySources)).run(args);
    }

继续看

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
        this.configureHeadlessProperty();
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        listeners.starting();

        Collection exceptionReporters;
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
            this.configureIgnoreBeanInfo(environment);
            Banner printedBanner = this.printBanner(environment);
            context = this.createApplicationContext();
            exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
            this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            this.refreshContext(context);
            this.afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }

            listeners.started(context);
            this.callRunners(context, applicationArguments);
        } catch (Throwable var10) {
            this.handleRunFailure(context, var10, exceptionReporters, listeners);
            throw new IllegalStateException(var10);
        }

        try {
            listeners.running(context);
            return context;
        } catch (Throwable var9) {
            this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);
            throw new IllegalStateException(var9);
        }
    }

这是调用的主run函数，首先开启SpringApplicationRunListeners监听器

class SpringApplicationRunListeners {
        private final Log log;
        private final List<SpringApplicationRunListener> listeners;

        SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) {
            this.log = log;
            this.listeners = new ArrayList(listeners);
        }
        public void starting() {
            Iterator var1 = this.listeners.iterator();

            while(var1.hasNext()) {
                SpringApplicationRunListener listener = (SpringApplicationRunListener)var1.next();
                listener.starting();
            }

        }
    }

然后创建了一个应用上下文ConfigurableApplicationContext

public interface ConfigurableApplicationContext extends ApplicationContext, Lifecycle, Closeable {
    String CONFIG_LOCATION_DELIMITERS = ",; \t\n";
    String CONVERSION_SERVICE_BEAN_NAME = "conversionService";
    String LOAD_TIME_WEAVER_BEAN_NAME = "loadTimeWeaver";
    String ENVIRONMENT_BEAN_NAME = "environment";
    String SYSTEM_PROPERTIES_BEAN_NAME = "systemProperties";
    String SYSTEM_ENVIRONMENT_BEAN_NAME = "systemEnvironment";

    void setId(String var1);

    void setParent(@Nullable ApplicationContext var1);

    void setEnvironment(ConfigurableEnvironment var1);

    ConfigurableEnvironment getEnvironment();

    void addBeanFactoryPostProcessor(BeanFactoryPostProcessor var1);

    void addApplicationListener(ApplicationListener<?> var1);

    void addProtocolResolver(ProtocolResolver var1);

    void refresh() throws BeansException, IllegalStateException;

    void registerShutdownHook();

    void close();

    boolean isActive();

    ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;
}

通过这个上下文加载应用所需的类和各种环境配置等，最后启动一个实例。也就是说，springboot在创建应用的时候，将springmvc中原本需要程序猿自己配的环境设置，交给了程序来完成。现在来看看程序是如何进行配置的以及我们如何去修改这些配置。

context = this.createApplicationContext();

根据这句代码，找到ConfigurableApplicationContext这个方法

protected ConfigurableApplicationContext createApplicationContext() {
        Class<?> contextClass = this.applicationContextClass;
        if (contextClass == null) {
            try {
                switch(this.webApplicationType) {
                case SERVLET:
                    contextClass = Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext");
                    break;
                case REACTIVE:
                    contextClass = Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext");
                    break;
                default:
                    contextClass = Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext");
                }
            } catch (ClassNotFoundException var3) {
                throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3);
            }
        }

        return (ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass);
    }

Class.forName是通过反射的方式来调用需要的class，根据this.webApplicationType进行判断，

switch(this.webApplicationType) {
                case SERVLET:
                    contextClass = Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext");
                    break;
                case REACTIVE:
                    contextClass = Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext");
                    break;
                default:
                    contextClass = Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext");
                }

也就是说，是按照这三个类进行配置的

AnnotationConfigServletWebServerApplicationContext
AnnotationConfigReactiveWebServerApplicationContext
AnnotationConfigApplicationContext

最后的结果通过BeanUtils.instantiateClass(contextClass);返回

public static <T> T instantiateClass(Class<T> clazz) throws BeanInstantiationException {
        Assert.notNull(clazz, "Class must not be null");
        if (clazz.isInterface()) {
            throw new BeanInstantiationException(clazz, "Specified class is an interface");
        } else {
            try {
                Constructor<T> ctor = KotlinDetector.isKotlinType(clazz) ? BeanUtils.KotlinDelegate.getPrimaryConstructor(clazz) : clazz.getDeclaredConstructor();
                return instantiateClass(ctor);
            } catch (NoSuchMethodException var2) {
                throw new BeanInstantiationException(clazz, "No default constructor found", var2);
            } catch (LinkageError var3) {
                throw new BeanInstantiationException(clazz, "Unresolvable class definition", var3);
            }
        }
    }

核心代码如下：

Constructor<T> ctor = KotlinDetector.isKotlinType(clazz) ? BeanUtils.KotlinDelegate.getPrimaryConstructor(clazz) : clazz.getDeclaredConstructor();
return instantiateClass(ctor);

这里似乎用到了kotlin，触及到了我的知识盲区，可以盲目分析一波，应该是对某些特定的情况进行排除,然后再来看看return instantiateClass(ctor);中的这个方法：ctor是一个构造器

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
        Assert.notNull(ctor, "Constructor must not be null");

        try {
            ReflectionUtils.makeAccessible(ctor);
            return KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ? BeanUtils.KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args);
        } catch (InstantiationException var3) {
            throw new BeanInstantiationException(ctor, "Is it an abstract class?", var3);
        } catch (IllegalAccessException var4) {
            throw new BeanInstantiationException(ctor, "Is the constructor accessible?", var4);
        } catch (IllegalArgumentException var5) {
            throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", var5);
        } catch (InvocationTargetException var6) {
            throw new BeanInstantiationException(ctor, "Constructor threw exception", var6.getTargetException());
        }
    }

看看核心代码：

ReflectionUtils.makeAccessible(ctor);
return KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ? BeanUtils.KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args);

public static void makeAccessible(Constructor<?> ctor) {
        if ((!Modifier.isPublic(ctor.getModifiers()) || !Modifier.isPublic(ctor.getDeclaringClass().getModifiers())) && !ctor.isAccessible()) {
            ctor.setAccessible(true);
        }

    }

感觉上是创建应用的构造器，并使构造器可用。来看看run函数中下面三句代码：

this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
this.refreshContext(context);
this.afterRefresh(context, applicationArguments);

第一句代码中传入了构造器，监听器，环境

ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);

来看看这三个函数

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment, SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);
        this.postProcessApplicationContext(context);
        this.applyInitializers(context);
        listeners.contextPrepared(context);
        if (this.logStartupInfo) {
            this.logStartupInfo(context.getParent() == null);
            this.logStartupProfileInfo(context);
        }

        context.getBeanFactory().registerSingleton("springApplicationArguments", applicationArguments);
        if (printedBanner != null) {
            context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
        }
        Set<Object> sources = this.getAllSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        this.load(context, sources.toArray(new Object[0]));
        listeners.contextLoaded(context);
    }

其中this.load(context, sources.toArray(new Object[0]));将调用

protected void load(ApplicationContext context, Object[] sources) {
        if (logger.isDebugEnabled()) {
            logger.debug("Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
        }

        BeanDefinitionLoader loader = this.createBeanDefinitionLoader(this.getBeanDefinitionRegistry(context), sources);
        if (this.beanNameGenerator != null) {
            loader.setBeanNameGenerator(this.beanNameGenerator);
        }

        if (this.resourceLoader != null) {
            loader.setResourceLoader(this.resourceLoader);
        }

        if (this.environment != null) {
            loader.setEnvironment(this.environment);
        }

        loader.load();
    }
    //BeanDefinitionLoader 加载应用定义的和需要的类及各种资源
    public int load() {
        int count = 0;
        Object[] var2 = this.sources;
        int var3 = var2.length;

        for(int var4 = 0; var4 < var3; ++var4) {
            Object source = var2[var4];
            count += this.load(source);
        }

        return count;
    }
    //
    private int load(Object source) {
        Assert.notNull(source, "Source must not be null");
        if (source instanceof Class) {
            return this.load((Class)source);
        } else if (source instanceof Resource) {
            return this.load((Resource)source);
        } else if (source instanceof Package) {
            return this.load((Package)source);
        } else if (source instanceof CharSequence) {
            return this.load((CharSequence)source);
        } else {
            throw new IllegalArgumentException("Invalid source type " + source.getClass());
        }
    }
    //
    private int load(Class<?> source) {
        if (this.isGroovyPresent() && BeanDefinitionLoader.GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
            BeanDefinitionLoader.GroovyBeanDefinitionSource loader = (BeanDefinitionLoader.GroovyBeanDefinitionSource)BeanUtils.instantiateClass(source, BeanDefinitionLoader.GroovyBeanDefinitionSource.class);
            this.load(loader);
        }

        if (this.isComponent(source)) {
            this.annotatedReader.register(new Class[]{source});
            return 1;
        } else {
            return 0;
        }
    }
    //
    private int load(BeanDefinitionLoader.GroovyBeanDefinitionSource source) {
        int before = this.xmlReader.getRegistry().getBeanDefinitionCount();
        ((GroovyBeanDefinitionReader)this.groovyReader).beans(source.getBeans());
        int after = this.xmlReader.getRegistry().getBeanDefinitionCount();
        return after - before;
    }
    //
    private boolean isComponent(Class<?> type) {
        if (AnnotationUtils.findAnnotation(type, Component.class) != null) {
            return true;
        } else {
            return !type.getName().matches(".*\\$_.*closure.*") && !type.isAnonymousClass() && type.getConstructors() != null && type.getConstructors().length != 0;
        }
    }

通过上面的一系列方法，实现了强大的自动配置功能

来看看@SpringBootApplication注解

public @interface SpringBootApplication {
    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )
    Class<?>[] exclude() default {};

    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )
    String[] excludeName() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackages"
    )
    String[] scanBasePackages() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackageClasses"
    )
    Class<?>[] scanBasePackageClasses() default {};
}
//可以看出，包含了额外的三个注解，分别是：
//@EnableAutoConfiguration、@ComponentScan和@SpringBootConfiguration

@EnableAutoConfiguration用于启动自动配置，倒入一些自动配置的类定义

public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}
//包含了AutoConfigurationPackage

@ComponentScan 将扫描和加载应用中的一些自定义的类

public @interface ComponentScan {
    @AliasFor("basePackages")
    String[] value() default {};

    @AliasFor("value")
    String[] basePackages() default {};

    Class<?>[] basePackageClasses() default {};

    Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;

    Class<? extends ScopeMetadataResolver> scopeResolver() default AnnotationScopeMetadataResolver.class;

    ScopedProxyMode scopedProxy() default ScopedProxyMode.DEFAULT;

    String resourcePattern() default "**/*.class";

    boolean useDefaultFilters() default true;

    ComponentScan.Filter[] includeFilters() default {};

    ComponentScan.Filter[] excludeFilters() default {};

    boolean lazyInit() default false;

    @Retention(RetentionPolicy.RUNTIME)
    @Target({})
    public @interface Filter {
        FilterType type() default FilterType.ANNOTATION;

        @AliasFor("classes")
        Class<?>[] value() default {};

        @AliasFor("value")
        Class<?>[] classes() default {};

        String[] pattern() default {};
    }
}

在架包里面

# 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.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.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.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.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.reactor.core.ReactorCoreAutoConfiguration,\
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.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.OAuth2ClientAutoConfiguration,\
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.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.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

在主程序中使用注解@Configuration 注解来代替@SpringBootApplication注解，然后使用@Import指定需要加载的配置类。
类似于这样的方法

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

修改的demo

@Configuration
@Import({
    WebClientAutoConfiguration.class,
    HttpEncodingAutoConfiguration.class
    ...
})
@RestController
public class Application{
    @RequestMapping("/")
    String home(){
        return "hello";
   }
}

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spring boot 核心实现源码分析--开启一个web应用

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