Tomcat启动流程一

Spring Boot版本是2.1.0.RELEASE

Spring Boot项目中只需要引入依赖spring-boot-starter-web，项目就能自动运行在Tomcat容器中。

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

Spring Boot中web容器自动配置类是ServletWebServerFactoryAutoConfiguration，spring.factories文件在spring-boot-autoconfigure-2.1.0.RELEASE.jar中

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


@Import了EmbeddedTomcat类

@Configuration
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedTomcat {
 
   @Bean
   public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
      return new TomcatServletWebServerFactory();
   }
 
}

根据注解
ConditionalOnClass，加载条件是classpath下有Servlet.class, Tomcat.class, UpgradeProtocol.class
ConditionalOnMissingBean，加载条件是当前没有类型为ServletWebServerFactory的其他bean加载

org.apache.catalina.startup.Tomcat
org.apache.coyote.UpgradeProtocol

这两个类在jar包tomcat-embed-core-9.0.12.jar中，我们没有在Pom文件中显示引入tomcat的jar，是在

spring-boot-starter-web中依赖了spring-boot-starter-tomcat

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-tomcat</artifactId>
  <version>2.1.0.RELEASE</version>
  <scope>compile</scope>
</dependency>

TomcatServletWebServerFactory中有一个很重要的方法getWebServer

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
   Tomcat tomcat = new Tomcat();
   File baseDir = (this.baseDirectory != null) ? this.baseDirectory
         : createTempDir("tomcat");
   tomcat.setBaseDir(baseDir.getAbsolutePath());
   Connector connector = new Connector(this.protocol);
   tomcat.getService().addConnector(connector);
   customizeConnector(connector);
   tomcat.setConnector(connector);
   tomcat.getHost().setAutoDeploy(false);
   configureEngine(tomcat.getEngine());
   for (Connector additionalConnector : this.additionalTomcatConnectors) {
      tomcat.getService().addConnector(additionalConnector);
   }
   prepareContext(tomcat.getHost(), initializers);
   return getTomcatWebServer(tomcat); // 跟进去
}


protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
   return new TomcatWebServer(tomcat, getPort() >= 0);
}
 
 
public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
   Assert.notNull(tomcat, "Tomcat Server must not be null");
   this.tomcat = tomcat;
   this.autoStart = autoStart;
   initialize();
}
 
private void initialize() throws WebServerException {
   logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));
   synchronized (this.monitor) {
      try {
         addInstanceIdToEngineName();
 
         Context context = findContext();
         context.addLifecycleListener((event) -> {
            if (context.equals(event.getSource())
                  && Lifecycle.START_EVENT.equals(event.getType())) {
               // Remove service connectors so that protocol binding doesn't
               // happen when the service is started.
               removeServiceConnectors();
            }
         });
 
         // Start the server to trigger initialization listeners
        // 启动tomcat
         this.tomcat.start();
 
         // We can re-throw failure exception directly in the main thread
         rethrowDeferredStartupExceptions();
 
         try {
            ContextBindings.bindClassLoader(context, context.getNamingToken(),
                  getClass().getClassLoader());
         }
         catch (NamingException ex) {
            // Naming is not enabled. Continue
         }
 
         // Unlike Jetty, all Tomcat threads are daemon threads. We create a
         // blocking non-daemon to stop immediate shutdown
         startDaemonAwaitThread();
      }
      catch (Exception ex) {
         stopSilently();
         throw new WebServerException("Unable to start embedded Tomcat", ex);
      }
   }
}

TomcatServletWebServerFactory#getWebServer()的调用时机，需要从启动类SpringApplication开始看

public static ConfigurableApplicationContext run(Class<?> primarySource,
      String... args) {
   return run(new Class<?>[] { primarySource }, args);
}
 
 
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
      String[] args) {
   return new SpringApplication(primarySources).run(args);
}
 
 
public ConfigurableApplicationContext run(String... args) {
   StopWatch stopWatch = new StopWatch();
   stopWatch.start();
   ConfigurableApplicationContext context = null;
   Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
   configureHeadlessProperty();
   SpringApplicationRunListeners listeners = getRunListeners(args);
   listeners.starting();
   try {
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(
            args);
      ConfigurableEnvironment environment = prepareEnvironment(listeners,
            applicationArguments);
      configureIgnoreBeanInfo(environment);
      Banner printedBanner = printBanner(environment);
     // 创建应用上下文，类型为AnnotationConfigServletWebServerApplicationContext
      context = createApplicationContext();
      exceptionReporters = getSpringFactoriesInstances(
            SpringBootExceptionReporter.class,
            new Class[] { ConfigurableApplicationContext.class }, context);
      prepareContext(context, environment, listeners, applicationArguments,
            printedBanner);
    // 刷新应用上下文
      refreshContext(context);
      afterRefresh(context, applicationArguments);
      stopWatch.stop();
      if (this.logStartupInfo) {
         new StartupInfoLogger(this.mainApplicationClass)
               .logStarted(getApplicationLog(), stopWatch);
      }
      listeners.started(context);
      callRunners(context, applicationArguments);
   }
   catch (Throwable ex) {
      handleRunFailure(context, ex, exceptionReporters, listeners);
      throw new IllegalStateException(ex);
   }
 
   try {
      listeners.running(context);
   }
   catch (Throwable ex) {
      handleRunFailure(context, ex, exceptionReporters, null);
      throw new IllegalStateException(ex);
   }
   return context;
}
 
 
private void refreshContext(ConfigurableApplicationContext context) {
   refresh(context);
   if (this.registerShutdownHook) {
      try {
         context.registerShutdownHook();
      }
      catch (AccessControlException ex) {
         // Not allowed in some environments.
      }
   }
}

org.springframework.context.support.AbstractApplicationContext#refresh

@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      prepareRefresh();
 
      // Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
 
      // 省略...
 
         // Initialize message source for this context.
         initMessageSource();
 
         // Initialize event multicaster for this context.
         initApplicationEventMulticaster();
 
         // Initialize other special beans in specific context subclasses.
        // AnnotationConfigServletWebServerApplicationContext继承自ServletWebServerApplicationContext
        // 这里会调用ServletWebServerApplicationContext的onRefresh()
         onRefresh();
 
         // 省略...
          
   }
}

org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext#onRefresh

@Override
protected void onRefresh() {
   super.onRefresh();
   try {
      createWebServer();
   }
   catch (Throwable ex) {
      throw new ApplicationContextException("Unable to start web server", ex);
   }
}
 
 
private void createWebServer() {
   WebServer webServer = this.webServer;
   ServletContext servletContext = getServletContext();
   if (webServer == null && servletContext == null) {
    // 这里获取的就是最开始提到的TomcatServletWebServerFactory
      ServletWebServerFactory factory = getWebServerFactory();
    // 调用TomcatServletWebServerFactory.getWebServer()
      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();
}```