SpringBoot中是如何建立WebServer的？

本文這裡環境是springboot 2.2.4.RELEASE。建立WebServer是在refresh方法的onRefresh方法中實作的。其也是refresh方法體系的一個重要步驟。

ServletWebServerApplicationContext的onRefresh方法。如下所示其首先調用父類的onRefresh方法初始化ThemeSource，然後調用createWebServer建立WebServer。

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

//GenericWebApplicationContext
@Override
protected void onRefresh() {
  this.themeSource = UiApplicationContextUtils.initThemeSource(this);
}

【1】createWebServer

ServletWebServerApplicationContext的createWebServer方法如下。

private void createWebServer() {
  WebServer webServer = this.webServer;
  // 擷取的是GenericWebApplicationContext的servletContext 
  ServletContext servletContext = getServletContext();
  
  if (webServer == null && servletContext == null) {
    // 本文環境擷取的是tomcatServletWebServerFactory
    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();
}

關于initPropertySources();方法可以參考博文：Spring中refresh分析之onRefresh方法詳解。

① 擷取WebServerFactory

如下所示，從容器中擷取ServletWebServerFactory類型的bean，唯一一個，否則抛出異常。本文環境擷取的是tomcatServletWebServerFactory。

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

② getSelfInitializer

ServletWebServerApplicationContext

的

getSelfInitializer

方法，傳回的是

ServletContextInitializer

。

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
  return this::selfInitialize;
}

看到

this::selfInitialize

是不是比較迷糊？典型的java8的lambda寫法。我們看一下ServletContextInitializer 可能就明白了。

如下所示，其是一個函數式接口，隻有一個onStartup方法。函數式接口(有且僅有一個抽象方法的接口)可以使用lambda式的寫法。

@FunctionalInterface
public interface ServletContextInitializer {
   // 初始化過程中，使用給定的servlets、filters、listeners
   //context-params and attributes necessary配置ServletContext
  void onStartup(ServletContext servletContext) throws ServletException;

}

我們這裡擷取到的本質是一個lambda，參數則是目前this，如下圖所示：

this::selfInitialize

中的selfInitialize則指的是ServletWebServerApplicationContext的selfInitialize方法。

this指的是AnnotationConfigServletWebServerApplicationContext，其繼承于ServletWebServerApplicationContext

private void selfInitialize(ServletContext servletContext) throws ServletException {
  prepareWebApplicationContext(servletContext);
  registerApplicationScope(servletContext);
  WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
  for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
    beans.onStartup(servletContext);
  }
}

其實換成匿名類的寫法則是：

new ServletContextInitializer() {
      @Override
      public void onStartup(ServletContext servletContext) throws ServletException {
    selfInitialize(servletContext);
      }
  };

【2】getWebServer

本文這裡是TomcatServletWebServerFactory的getWebServer方法。

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
  if (this.disableMBeanRegistry) {
  // registry = new NoDescriptorRegistry();
    Registry.disableRegistry();
  }
  //執行個體化Tomcat
  Tomcat tomcat = new Tomcat();

  //擷取臨時路徑 C:\Users\12746\AppData\Local\Temp\tomcat.9051357942624975261.8188
  File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
  //設定基礎路徑
  tomcat.setBaseDir(baseDir.getAbsolutePath());

  //執行個體化Connector 并進行配置
  Connector connector = new Connector(this.protocol);
  connector.setThrowOnFailure(true);

  //這裡會執行個體化server  service
  tomcat.getService().addConnector(connector);
  customizeConnector(connector);

  //對Connector做配置比如Protocol、URIEncoding
  tomcat.setConnector(connector);

  //這裡會執行個體化Engine、Host
  tomcat.getHost().setAutoDeploy(false);
  configureEngine(tomcat.getEngine());
  for (Connector additionalConnector : this.additionalTomcatConnectors) {
    tomcat.getService().addConnector(additionalConnector);
  }
  prepareContext(tomcat.getHost(), initializers);
  return getTomcatWebServer(tomcat);
}

getService

getService首先會觸發getServer然後擷取service。getServer如下所示會執行個體化Server并對其進行配置。

public Service getService() {
    return getServer().findServices()[0];
}
public Server getServer() {
       if (server != null) {
           return server;
       }
       System.setProperty("catalina.useNaming", "false");
      // 執行個體化 server
       server = new StandardServer();

      // 對basedir做處理
       initBaseDir();

       // Set configuration source
       ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(new File(basedir), null));
      // 為server設定port和service
       server.setPort( -1 );
       //執行個體化service
       Service service = new StandardService();
       service.setName("Tomcat");
       server.addService(service);
       return server;
   }

prepareContext

這裡會執行個體化TomcatEmbeddedContext并對其進行配置。

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
  File documentRoot = getValidDocumentRoot();
  TomcatEmbeddedContext context = new TomcatEmbeddedContext();
  if (documentRoot != null) {
    context.setResources(new LoaderHidingResourceRoot(context));
  }
  context.setName(getContextPath());
  context.setDisplayName(getDisplayName());
  context.setPath(getContextPath());
  File docBase = (documentRoot != null) ? documentRoot : createTempDir("tomcat-docbase");
  context.setDocBase(docBase.getAbsolutePath());
  context.addLifecycleListener(new FixContextListener());
  context.setParentClassLoader((this.resourceLoader != null) ? this.resourceLoader.getClassLoader()
      : ClassUtils.getDefaultClassLoader());
  resetDefaultLocaleMapping(context);
  addLocaleMappings(context);
  context.setUseRelativeRedirects(false);
  try {
    context.setCreateUploadTargets(true);
  }
  catch (NoSuchMethodError ex) {
    // Tomcat is < 8.5.39. Continue.
  }
  configureTldSkipPatterns(context);
  WebappLoader loader = new WebappLoader(context.getParentClassLoader());
  loader.setLoaderClass(TomcatEmbeddedWebappClassLoader.class.getName());
  loader.setDelegate(true);
  context.setLoader(loader);
  if (isRegisterDefaultServlet()) {
    addDefaultServlet(context);
  }
  if (shouldRegisterJspServlet()) {
    addJspServlet(context);
    addJasperInitializer(context);
  }
  context.addLifecycleListener(new StaticResourceConfigurer(context));
  ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
  host.addChild(context);
  configureContext(context, initializersToUse);
  postProcessContext(context);
}

getTomcatWebServer

這個方法很簡單，隻是直接執行個體化了TomcatWebServer傳回。其構造方法觸發了initialize，這會引起後續一系列動作，包括tomcat.start。

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();
}

【3】selfInitialize

擷取到TomcatWebServer後，就觸發了selfInitialize方法。這裡servletContext其實是擷取了ApplicationContext的一個門面/外觀–ApplicationContextCade。

// ServletWebServerApplicationContext
private void selfInitialize(ServletContext servletContext) throws ServletException {
  prepareWebApplicationContext(servletContext);
  registerApplicationScope(servletContext);
  WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
  for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
    beans.onStartup(servletContext);
  }
}

① prepareWebApplicationContext

ServletWebServerApplicationContext

的

prepareWebApplicationContext

方法如下所示，簡單來講就是為servletContext設定根容器屬性并為目前應用上下文ApplicationContext設定servletContext引用。

protected void prepareWebApplicationContext(ServletContext servletContext) {
//嘗試從servletContext中擷取rootContext 
  Object rootContext = servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
  if (rootContext != null) {
    if (rootContext == this) {
      throw new IllegalStateException(
          "Cannot initialize context because there is already a root application context present - "
              + "check whether you have multiple ServletContextInitializers!");
    }
    return;
  }
  Log logger = LogFactory.getLog(ContextLoader.class);
  // 這個日志是不是很熟悉？！
  servletContext.log("Initializing Spring embedded WebApplicationContext");
  try {
    
//向servletContext設定屬性 ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this);
    if (logger.isDebugEnabled()) {
      logger.debug("Published root WebApplicationContext as ServletContext attribute with name ["
          + WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
    }
    // 為ApplicationContext設定servletContext引用
    setServletContext(servletContext);
    if (logger.isInfoEnabled()) {
      long elapsedTime = System.currentTimeMillis() - getStartupDate();
      logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
    }
  }
  catch (RuntimeException | Error ex) {
    logger.error("Context initialization failed", ex);
    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
    throw ex;
  }
}

② registerApplicationScope

ServletWebServerApplicationContext

的

registerApplicationScope

方法如下所示，簡單來講就是(擴充)注冊scope-application。這裡會執行個體化一個ServletContextScope (包裝了servletContext)，然後注冊到BeanFactory中并為servletContext設定屬性。

private void registerApplicationScope(ServletContext servletContext) {
  ServletContextScope appScope = new ServletContextScope(servletContext);
  // application
  getBeanFactory().registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
  // Register as ServletContext attribute, for ContextCleanupListener to detect it.
  servletContext.setAttribute(ServletContextScope.class.getName(), appScope);
}

我們在Spring中refresh分析之postProcessBeanFactory方法詳解提到了request-RequestScope，session–SessionScope的注冊，本文這裡注冊了application-ServletContextScope注冊。

③ registerEnvironmentBeans

WebApplicationContextUtils的registerEnvironmentBeans方法。

public static void registerEnvironmentBeans(ConfigurableListableBeanFactory bf, @Nullable ServletContext sc) {
  registerEnvironmentBeans(bf, sc, null);
}

public static void registerEnvironmentBeans(ConfigurableListableBeanFactory bf,
    @Nullable ServletContext servletContext, @Nullable ServletConfig servletConfig) {

//将servletContext作為單例注冊容器
  if (servletContext != null && !bf.containsBean(WebApplicationContext.SERVLET_CONTEXT_BEAN_NAME)) {
    bf.registerSingleton(WebApplicationContext.SERVLET_CONTEXT_BEAN_NAME, servletContext);
  }

// 将servletConfig 作為單例注冊容器本文這裡沒有觸發
  if (servletConfig != null && !bf.containsBean(ConfigurableWebApplicationContext.SERVLET_CONFIG_BEAN_NAME)) {
    bf.registerSingleton(ConfigurableWebApplicationContext.SERVLET_CONFIG_BEAN_NAME, servletConfig);
  }
// String CONTEXT_PARAMETERS_BEAN_NAME = "contextParameters";
  if (!bf.containsBean(WebApplicationContext.CONTEXT_PARAMETERS_BEAN_NAME)) {
    Map<String, String> parameterMap = new HashMap<>();
    if (servletContext != null) {
    // 擷取servletContextd的初始化參數
      Enumeration<?> paramNameEnum = servletContext.getInitParameterNames();
      while (paramNameEnum.hasMoreElements()) {
        String paramName = (String) paramNameEnum.nextElement();
        parameterMap.put(paramName, servletContext.getInitParameter(paramName));
      }
    }
    // 本文這裡servletConfig 為null
    if (servletConfig != null) {
    // // 擷取servletConfig的初始化參數
      Enumeration<?> paramNameEnum = servletConfig.getInitParameterNames();
      while (paramNameEnum.hasMoreElements()) {
        String paramName = (String) paramNameEnum.nextElement();
        parameterMap.put(paramName, servletConfig.getInitParameter(paramName));
      }
    }
    // 将contextParameters作為單例注冊到容器
    bf.registerSingleton(WebApplicationContext.CONTEXT_PARAMETERS_BEAN_NAME,
        Collections.unmodifiableMap(parameterMap));
  }

// String CONTEXT_ATTRIBUTES_BEAN_NAME = "contextAttributes";
  if (!bf.containsBean(WebApplicationContext.CONTEXT_ATTRIBUTES_BEAN_NAME)) {
    Map<String, Object> attributeMap = new HashMap<>();
    if (servletContext != null) {
      Enumeration<?> attrNameEnum = servletContext.getAttributeNames();
      while (attrNameEnum.hasMoreElements()) {
        String attrName = (String) attrNameEnum.nextElement();
        attributeMap.put(attrName, servletContext.getAttribute(attrName));
      }
    }
    // 将contextAttributes作為單例注冊到容器
    bf.registerSingleton(WebApplicationContext.CONTEXT_ATTRIBUTES_BEAN_NAME,
        Collections.unmodifiableMap(attributeMap));
  }
}

④ 觸發ServletContextInitializer的onStartup

如下所示，這裡會擷取ServletContextInitializer的所有執行個體，周遊觸發其onStartup方法。

for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
  beans.onStartup(servletContext);
}

SpringBoot中是如何建立WebServer的？

【1】createWebServer

① 擷取WebServerFactory

② getSelfInitializer

【2】getWebServer

getService

prepareContext

getTomcatWebServer

【3】selfInitialize

① prepareWebApplicationContext

② registerApplicationScope

③ registerEnvironmentBeans

④ 觸發ServletContextInitializer的onStartup

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