我们如何看SpringBoot的源代码

1、快速生成一个简单的SpringBoot项目

进入地址： https://start.spring.io/ ，点击生成代码即可。

2、注解：@SpringBootApplication

一个Web项目，只需要这一行注解。有这么厉害吗？我们一起看看它究竟做了什么？

@SpringBootApplication
public class SpringBootDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootDemoApplication.class, args);
    }

}

The @SpringBootApplication annotation is equivalent to using @Configuration , @EnableAutoConfiguration , and @ComponentScan with their default attributes

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

看代码，明明是 @SpringBootConfiguration 呢，怎么说是 @Configuration 呢？

2.1、注解：@SpringBootConfiguration

@Configuration
public @interface SpringBootConfiguration {  
}

然后再进入到里面去，发现竟然是 Component 注解？是不是非常熟悉呢？

@Component
public @interface Configuration {

Spring provides further stereotype annotations: @Component , @Service , and @Controller . @Component is a generic stereotype for any Spring-managed component. @Repository , @Service , and @Controller are specializations of @Component for more specific use cases (in the persistence, service, and presentation layers, respectively)。

2.2、注解：@ComponentScan

org.springframework.boot.SpringApplication  
// 第1步
public ConfigurableApplicationContext run(String... args) {  
    refreshContext(context);
}
// 第2步
public ConfigurableApplicationContext run(String... args) {  
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
        ConfigurableEnvironment environment = prepareEnvironment(listeners,
                applicationArguments);
        configureIgnoreBeanInfo(environment);
        Banner printedBanner = printBanner(environment);
        context = createApplicationContext();
        exceptionReporters = getSpringFactoriesInstances(
                SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        prepareContext(context, environment, listeners, applicationArguments,
                printedBanner);
        refreshContext(context);
        afterRefresh(context, applicationArguments);
        listeners.started(context);
        // Called after the context has been refreshed.
        callRunners(context, applicationArguments);
    }
    listeners.running(context);

    return context;
}
// 第3步
protected void refresh(ApplicationContext applicationContext) {  
        Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
        ((AbstractApplicationContext) applicationContext).refresh();
}
// 第4步
org.springframework.context.support.AbstractApplicationContext  
public void refresh() throws BeansException, IllegalStateException {  
    // Invoke factory processors registered as beans in the context.
    invokeBeanFactoryPostProcessors(beanFactory);
}
// 第5步
org.springframework.context.support.PostProcessorRegistrationDelegate  
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {  
    if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
        registryProcessor.postProcessBeanDefinitionRegistry(registry);
    }
}
// 第6步
org.springframework.context.annotation.ConfigurationClassPostProcessor  
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {  
    processConfigBeanDefinitions(registry);
}
// 第7步
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {  
    do {
        parser.parse(candidates);
    }
}
// 第8步
org.springframework.context.annotation.ConfigurationClassParser  
protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {  
    do {
        sourceClass = doProcessConfigurationClass(configClass, sourceClass);
    }
}
// 第9步
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)  
        throws IOException {
    // Process any @PropertySource annotations
    for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
            sourceClass.getMetadata(), PropertySources.class,
            org.springframework.context.annotation.PropertySource.class)) {
        if (this.environment instanceof ConfigurableEnvironment) {
            processPropertySource(propertySource);
        }
        else {
            logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
                    "]. Reason: Environment must implement ConfigurableEnvironment");
        }
    }
    // Process any @ComponentScan annotations
    Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
            sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
    if (!componentScans.isEmpty() &&
            !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
        for (AnnotationAttributes componentScan : componentScans) {
            // The config class is annotated with @ComponentScan -> perform the scan immediately
            Set<BeanDefinitionHolder> scannedBeanDefinitions =
                    this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
            // Check the set of scanned definitions for any further config classes and parse recursively if needed
            for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
                BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
                if (bdCand == null) {
                    bdCand = holder.getBeanDefinition();
                }
                if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
                    parse(bdCand.getBeanClassName(), holder.getBeanName());
                }
            }
        }
    }
    // Process any @Import annotations
    processImports(configClass, sourceClass, getImports(sourceClass), true);
    // Process any @ImportResource annotations
    AnnotationAttributes importResource =
            AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
    if (importResource != null) {
        String[] resources = importResource.getStringArray("locations");
        Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
        for (String resource : resources) {
            String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
            configClass.addImportedResource(resolvedResource, readerClass);
        }
    }
}
// 第10步
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,  
        Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
    String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
    Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
    processImports(configClass, currentSourceClass, importSourceClasses, false);
}

// 第11步
org.springframework.boot.autoconfigure.AutoConfigurationImportSelector  
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {  
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
            autoConfigurationMetadata, annotationMetadata);
}
// 第12步
private List<String> filter(List<String> configurations,  
        AutoConfigurationMetadata autoConfigurationMetadata) {
    for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
        invokeAwareMethods(filter);
        boolean[] match = filter.match(candidates, autoConfigurationMetadata);
        for (int i = 0; i < match.length; i++) {
            if (!match[i]) {
                skip[i] = true;
                candidates[i] = null;
                skipped = true;
            }
        }
    }
    return new ArrayList<>(result);
}
// 第13步
protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {  
    return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
        this.beanClassLoader);
}
// 第14步
org.springframework.core.io.support.SpringFactoriesLoader  
public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {  
    List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
    return result;
}
// 第15步
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {  
    try {
        Enumeration<URL> urls = (classLoader != null ?
                classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        return result;
    }
}
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

也就是说，其实 spring.factories 是 spring-core 的功能。

看一下 spring-boot-autoconfigure 项目里面的 spring.factories 。

# 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.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.reactor.core.ReactorCoreAutoConfiguration,/  
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,/  
org.springframework.boot.autoconfigure.security.servlet.SecurityRequestMatcherProviderAutoConfiguration,/  
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.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.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

看到这里，是不是有一点解惑了，为什么我们什么都没有干，它就已经具备了那么多的功能。因为在项目启动的时候，已经就给我们内置了这么多的服务。

3、容器在哪儿启动的？

3.1 为什么是Tomcat默认启动？

我们再回到开始的时候，为什么启动的时候看到了 Tomcat 的日志呢？

Connected to the target VM, address: '127.0.0.1:56945', transport: 'socket'

  .                 _            __ _ _
 /// / ___'_ __ _ _(_)_ __  __ _ / / / /
( ( )/___ | '_ | '_| | '_ // _` | / / / /
 ///  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |    | .__|_| |_|_| |_/__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                        

2019-06-13 09:15:11.818  INFO 16978 --- [           main] c.e.s.SpringBootDemoApplication          : Starting SpringBootDemoApplication on bogon with PID 16978 (/Users/chenyuan/Dropbox/Workspaces/IdeaProjects/spring-boot-demo/target/classes started by chenyuan in /Users/chenyuan/Dropbox/Workspaces/IdeaProjects/spring-src-leaning)  
2019-06-13 09:15:11.823  INFO 16978 --- [           main] c.e.s.SpringBootDemoApplication          : No active profile set, falling back to default profiles: default

// 这里日志显示，有一个embedded的tomcat启动了8080端口
2019-06-13 09:15:13.597  INFO 16978 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)  
2019-06-13 09:15:13.644  INFO 16978 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]  
2019-06-13 09:15:13.645  INFO 16978 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.14]  
2019-06-13 09:15:13.653  INFO 16978 --- [           main] o.a.catalina.core.AprLifecycleListener   : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/chenyuan/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]  
2019-06-13 09:15:13.752  INFO 16978 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext  
2019-06-13 09:15:13.752  INFO 16978 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1862 ms  
2019-06-13 09:15:14.018  INFO 16978 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'  
2019-06-13 09:15:14.226  INFO 16978 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''  
2019-06-13 09:15:14.231  INFO 16978 --- [           main] c.e.s.SpringBootDemoApplication          : Started SpringBootDemoApplication in 3.007 seconds (JVM running for 3.924)

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=/

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

<!-- spring-boot-demo/pom.xml -->  
<dependency>  
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
</dependency>

<!-- spring-boot-starter-web/pom.xml -->  
<dependency>  
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-tomcat</artifactId>
</dependency>

<!-- spring-boot-starters/spring-boot-starter-tomcat/pom.xml -->  
<dependency>  
  <groupId>org.apache.tomcat.embed</groupId>
  <artifactId>tomcat-embed-core</artifactId>
  <exclusions>
    <exclusion>
      <groupId>org.apache.tomcat</groupId>
      <artifactId>tomcat-annotations-api</artifactId>
    </exclusion>
  </exclusions>
</dependency>

3.2 如果要指定其他的容器呢？

<dependencies>  
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <exclusions>
            <exclusion>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-tomcat</artifactId>
            </exclusion>
        </exclusions>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-jetty</artifactId>
    </dependency>
</dependencies>

把项目中Tomca的starter注释掉，引入Jetty容器即可。

这么更换后，为什么就又能引入了呢？

org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryConfiguration  
@Configuration
@ConditionalOnClass({ Servlet.class, Server.class, Loader.class,WebAppContext.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedJetty {  
  @Bean
  public JettyServletWebServerFactory JettyServletWebServerFactory() {
    return new JettyServletWebServerFactory();
  }
}

The @ConditionalOnClass and @ConditionalOnMissingClass annotations let @Configuration classes be included based on the presence or absence of specific classes.

这里的 Servlet.class, Server.class, Loader.class,WebAppContext.class 就是Jetty里面的包。

这里还设计了一个工厂模式，获取一个WebServer。

@FunctionalInterface
public interface ServletWebServerFactory {

    /**
     * Gets a new fully configured but paused {@link WebServer} instance. Clients should
     * not be able to connect to the returned server until {@link WebServer#start()} is
     * called (which happens when the {@link ApplicationContext} has been fully
     * refreshed).
     * @param initializers {@link ServletContextInitializer}s that should be applied as
     * the server starts
     * @return a fully configured and started {@link WebServer}
     * @see WebServer#stop()
     */
    WebServer getWebServer(ServletContextInitializer... initializers);

}

3.3 那是在什么时候初始化容器的呢？

// 第1步
org.springframework.context.support.AbstractApplicationContext  
public void refresh() throws BeansException, IllegalStateException {  
  synchronized (this.startupShutdownMonitor) {
    try {
      // Initialize other special beans in specific context subclasses.
      onRefresh();
    }
}
// 第2步
org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext  
protected void onRefresh() {  
    super.onRefresh();
    try {
        createWebServer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start web server", ex);
    }
}
// 第3步
private void createWebServer() {  
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    if (webServer == null && servletContext == null) {
        ServletWebServerFactory factory = getWebServerFactory();
        this.webServer = factory.getWebServer(getSelfInitializer());
    }
    else if (servletContext != null) {
        try {
            getSelfInitializer().onStartup(servletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context",
                    ex);
        }
    }
    initPropertySources();
}

在看到 factory.getWebServer ，是不是就全部都串联起来了？