深入讲解springboot中servlet的启动过程与原理

StopWatch stopWatch = new StopWatch();
 stopWatch.start();
 ConfigurableApplicationContext cOntext= null;
 FailureAnalyzers analyzers = null;
 configureHeadlessProperty();
 SpringApplicationRunListeners listeners = getRunListeners(args);
 listeners.starting();
 try {
  ApplicationArguments applicatiOnArguments= new DefaultApplicationArguments(
   args);
  ConfigurableEnvironment envirOnment= prepareEnvironment(listeners,
   applicationArguments);
  Banner printedBanner = printBanner(environment);
   //创建一个ApplicationContext容器
  cOntext= createApplicationContext();
  analyzers = new FailureAnalyzers(context);
  prepareContext(context, environment, listeners, applicationArguments,
   printedBanner);
   //刷新IOC容器
  refreshContext(context);
  afterRefresh(context, applicationArguments);
  listeners.finished(context, null);
  stopWatch.stop();
  if (this.logStartupInfo) {
  new StartupInfoLogger(this.mainApplicationClass)
   .logStarted(getApplicationLog(), stopWatch);
  }
  return context;
 }
 catch (Throwable ex) {
  handleRunFailure(context, listeners, analyzers, ex);
  throw new IllegalStateException(ex);
 }

public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
  + "annotation.AnnotationConfigApplicationContext";

 /**
 * The class name of application context that will be used by default for web
 * environments.
 */
 public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework."
  + "boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext";


protected ConfigurableApplicationContext createApplicationContext() {
 Class<&＃63;> cOntextClass= this.applicationContextClass;
 if (cOntextClass== null) {
  try {
　　　　　　　　　　//根据应用环境,创建不同的IOC容器
  cOntextClass= Class.forName(this.webEnvironment
   &＃63; DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
  }
  catch (ClassNotFoundException ex) {
  throw new IllegalStateException(
   "Unable create a default ApplicationContext, "
    + "please specify an ApplicationContextClass",
   ex);
  }
 }
 return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
 }

private void refreshContext(ConfigurableApplicationContext context) {
 refresh(context);
 if (this.registerShutdownHook) {
  try {
  context.registerShutdownHook();
  }
  catch (AccessControlException ex) {
  // Not allowed in some environments.
  }
 }
 }

protected void refresh(ApplicationContext applicationContext) {
 Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
 ((AbstractApplicationContext) applicationContext).refresh();
 }

public void refresh() throws BeansException, IllegalStateException {
 Object var1 = this.startupShutdownMonitor;
 synchronized(this.startupShutdownMonitor) {
  this.prepareRefresh();
  ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
  this.prepareBeanFactory(beanFactory);

  try {
  this.postProcessBeanFactory(beanFactory);
  this.invokeBeanFactoryPostProcessors(beanFactory);
  this.registerBeanPostProcessors(beanFactory);
  this.initMessageSource();
  this.initApplicationEventMulticaster();
  this.onRefresh();
  this.registerListeners();
  this.finishBeanFactoryInitialization(beanFactory);
  this.finishRefresh();
  } catch (BeansException var9) {
  if (this.logger.isWarnEnabled()) {
   this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
  }

  this.destroyBeans();
  this.cancelRefresh(var9);
  throw var9;
  } finally {
  this.resetCommonCaches();
  }

 }
 }

@Override
 protected void onRefresh() {
 super.onRefresh();
 try {
  createEmbeddedServletContainer();
 }
 catch (Throwable ex) {
  throw new ApplicationContextException("Unable to start embedded container",
   ex);
 }
 }

private void createEmbeddedServletContainer() {
 EmbeddedServletContainer localCOntainer= this.embeddedServletContainer;
 ServletContext localServletCOntext= getServletContext();
 if (localCOntainer== null && localServletCOntext== null) {
　　　　　　　　　　　　　　　　//获取嵌入式Servlet容器工厂
  EmbeddedServletContainerFactory cOntainerFactory= getEmbeddedServletContainerFactory();
　　　　　　　　　　//根据容器工厂获取对应嵌入式Servlet容器
  this.embeddedServletCOntainer= containerFactory
   .getEmbeddedServletContainer(getSelfInitializer());
 }
 else if (localServletContext != null) {
  try {
  getSelfInitializer().onStartup(localServletContext);
  }
  catch (ServletException ex) {
  throw new ApplicationContextException("Cannot initialize servlet context",
   ex);
  }
 }
 initPropertySources();
 }

//EmbeddedWebApplicationContext#getEmbeddedServletContainerFactory 
protected EmbeddedServletContainerFactory getEmbeddedServletContainerFactory() { 
 // Use bean names so that we don't consider the hierarchy 
 String[] beanNames = getBeanFactory() 
 .getBeanNamesForType(EmbeddedServletContainerFactory.class); 
 if (beanNames.length == 0) { 
 throw new ApplicationContextException( 
  "Unable to start EmbeddedWebApplicationContext due to missing " 
  + "EmbeddedServletContainerFactory bean."); 
 } 
 if (beanNames.length > 1) { 
 throw new ApplicationContextException( 
  "Unable to start EmbeddedWebApplicationContext due to multiple " 
  + "EmbeddedServletContainerFactory beans : " 
  + StringUtils.arrayToCommaDelimitedString(beanNames)); 
 } 
 return getBeanFactory().getBean(beanNames[0], 
     EmbeddedServletContainerFactory.class); 
}

this.embeddedServletCOntainer= containerFactory 
  .getEmbeddedServletContainer(getSelfInitializer()); 

// Instantiate all remaining (non-lazy-init) singletons.
  finishBeanFactoryInitialization(beanFactory);

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
 // Initialize conversion service for this context.
 if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
  beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
  beanFactory.setConversionService(
   beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
 }

 // Register a default embedded value resolver if no bean post-processor
 // (such as a PropertyPlaceholderConfigurer bean) registered any before:
 // at this point, primarily for resolution in annotation attribute values.
 if (!beanFactory.hasEmbeddedValueResolver()) {
  beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
  @Override
  public String resolveStringValue(String strVal) {
   return getEnvironment().resolvePlaceholders(strVal);
  }
  });
 }

 // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
 String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
 for (String weaverAwareName : weaverAwareNames) {
  getBean(weaverAwareName);
 }

 // Stop using the temporary ClassLoader for type matching.
 beanFactory.setTempClassLoader(null);

 // Allow for caching all bean definition metadata, not expecting further changes.
 beanFactory.freezeConfiguration();

 // Instantiate all remaining (non-lazy-init) singletons.
 beanFactory.preInstantiateSingletons();
 }

public void preInstantiateSingletons() throws BeansException {
  if (this.logger.isDebugEnabled()) {
   this.logger.debug("Pre-instantiating singletons in " + this);
  }

  List beanNames = new ArrayList(this.beanDefinitionNames);
  Iterator var2 = beanNames.iterator();

  while(true) {
   while(true) {
    String beanName;
    RootBeanDefinition bd;
    do {
     do {
      do {
       if (!var2.hasNext()) {
        var2 = beanNames.iterator();

        while(var2.hasNext()) {
         beanName = (String)var2.next();
         Object singletOnInstance= this.getSingleton(beanName);
         if (singletonInstance instanceof SmartInitializingSingleton) {
          final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton)singletonInstance;
          if (System.getSecurityManager() != null) {
           AccessController.doPrivileged(new PrivilegedAction() {
            public Object run() {
             smartSingleton.afterSingletonsInstantiated();
             return null;
            }
           }, this.getAccessControlContext());
          } else {
           smartSingleton.afterSingletonsInstantiated();
          }
         }
        }

        return;
       }

       beanName = (String)var2.next();
       bd = this.getMergedLocalBeanDefinition(beanName);
      } while(bd.isAbstract());
     } while(!bd.isSingleton());
    } while(bd.isLazyInit());

    if (this.isFactoryBean(beanName)) {
     final FactoryBean<&＃63;> factory = (FactoryBean)this.getBean("&" + beanName);
     boolean isEagerInit;
     if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
      isEagerInit = ((Boolean)AccessController.doPrivileged(new PrivilegedAction() {
       public Boolean run() {
        return ((SmartFactoryBean)factory).isEagerInit();
       }
      }, this.getAccessControlContext())).booleanValue();
     } else {
      isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean)factory).isEagerInit();
     }

     if (isEagerInit) {
      this.getBean(beanName);
     }
    } else {
　　　　　　　　　　　　//注册bean
     this.getBean(beanName);
    }
   }
  }
 }

public class ServletInitializer extends SpringBootServletInitializer { 
 
 @Override 
 protected SpringApplicationBuilder configure(SpringApplicationBuilder application) { 
  return application.sources(SpringBoot04WebJspApplication.class); 
 } 
}

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

 /**
  * Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
  * implementations present on the application classpath.
  * 
Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
  * Servlet 3.0+ containers will automatically scan the classpath for implementations
  * of Spring's {@code WebApplicationInitializer} interface and provide the set of all
  * such types to the {@code webAppInitializerClasses} parameter of this method.
  * 
If no {@code WebApplicationInitializer} implementations are found on the classpath,
  * this method is effectively a no-op. An INFO-level log message will be issued notifying
  * the user that the {@code ServletContainerInitializer} has indeed been invoked but that
  * no {@code WebApplicationInitializer} implementations were found.
  * 
Assuming that one or more {@code WebApplicationInitializer} types are detected,
  * they will be instantiated (and sorted if the @{@link
  * org.springframework.core.annotation.Order @Order} annotation is present or
  * the {@link org.springframework.core.Ordered Ordered} interface has been
  * implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
  * method will be invoked on each instance, delegating the {@code ServletContext} such
  * that each instance may register and configure servlets such as Spring's
  * {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
  * or any other Servlet API componentry such as filters.
  * @param webAppInitializerClasses all implementations of
  * {@link WebApplicationInitializer} found on the application classpath
  * @param servletContext the servlet context to be initialized
  * @see WebApplicationInitializer#onStartup(ServletContext)
  * @see AnnotationAwareOrderComparator
  */
 @Override
 public void onStartup(Set> webAppInitializerClasses, ServletContext servletContext)
   throws ServletException {

  List initializers = new LinkedList();

  if (webAppInitializerClasses != null) {
   for (Class<&＃63;> waiClass : webAppInitializerClasses) {
    // Be defensive: Some servlet containers provide us with invalid classes,
    // no matter what @HandlesTypes says...
    if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
      WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
     try {
　　　　　　　　　　　　　　　　//为所有的WebApplicationInitializer类型创建实例,并加入集合中
      initializers.add((WebApplicationInitializer) waiClass.newInstance());
     }
     catch (Throwable ex) {
      throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
     }
    }
   }
  }

  if (initializers.isEmpty()) {
   servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
   return;
  }

  servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
  AnnotationAwareOrderComparator.sort(initializers);
　　　　　　//调用每一个WebApplicationInitializer实例的onstartup方法
  for (WebApplicationInitializer initializer : initializers) {
   initializer.onStartup(servletContext);
  }
 }
}

public abstract class SpringBootServletInitializer implements WebApplicationInitializer { 
 
  //other code... 
   
  @Override 
  public void onStartup(ServletContext servletContext) throws ServletException { 
    // Logger initialization is deferred in case a ordered 
    // LogServletContextInitializer is being used 
    this.logger = LogFactory.getLog(getClass()); 
    //创建IOC容器 
    WebApplicationContext rootAppCOntext= createRootApplicationContext( 
        servletContext); 
    if (rootAppContext != null) { 
      servletContext.addListener(new ContextLoaderListener(rootAppContext) { 
        @Override 
        public void contextInitialized(ServletContextEvent event) { 
          // no-op because the application context is already initialized 
        } 
      }); 
    } 
    else { 
      this.logger.debug("No ContextLoaderListener registered, as " 
          + "createRootApplicationContext() did not " 
          + "return an application context"); 
    } 
  } 
 
  protected WebApplicationContext createRootApplicationContext( 
      ServletContext servletContext) { 
    //创建Spring应用构建器，并进行相关属性设置 
    SpringApplicationBuilder builder = createSpringApplicationBuilder(); 
    StandardServletEnvironment envirOnment= new StandardServletEnvironment(); 
    environment.initPropertySources(servletContext, null); 
    builder.environment(environment); 
    builder.main(getClass()); 
    ApplicationContext parent = getExistingRootWebApplicationContext(servletContext); 
    if (parent != null) { 
      this.logger.info("Root context already created (using as parent)."); 
      servletContext.setAttribute( 
          WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null); 
      builder.initializers(new ParentContextApplicationContextInitializer(parent)); 
    } 
    builder.initializers( 
        new ServletContextApplicationContextInitializer(servletContext)); 
    builder.contextClass(AnnotationConfigEmbeddedWebApplicationContext.class); 
     
    //调用configure方法，创建war类型的web项目后，由于编写SpringBootServletInitializer的子类重写configure方法，所以此处调用的是我们定义的子类重写的configure方法 
    builder = configure(builder); 
     
    //通过构建器构建了一个Spring应用 
    SpringApplication application = builder.build(); 
    if (application.getSources().isEmpty() && AnnotationUtils 
        .findAnnotation(getClass(), Configuration.class) != null) { 
      application.getSources().add(getClass()); 
    } 
    Assert.state(!application.getSources().isEmpty(), 
        "No SpringApplication sources have been defined. Either override the " 
            + "configure method or add an @Configuration annotation"); 
    // Ensure error pages are registered 
    if (this.registerErrorPageFilter) { 
      application.getSources().add(ErrorPageFilterConfiguration.class); 
    } 
    //启动Spring应用 
    return run(application); 
  } 
   
  //Spring应用启动，创建并返回IOC容器 
  protected WebApplicationContext run(SpringApplication application) { 
    return (WebApplicationContext) application.run(); 
  }   
}