详解Tomcat中Filter的执行流程

public interface Filter {
   // 容器创建的时候调用, 即启动tomcat的时候调用
   public void init(FilterConfig filterConfig) throws ServletException;
   // 由FilterChain调用, 并且传入FilterChain本身, 最后回调FilterChain的doFilter()方法
   public void doFilter(ServletRequest request, ServletResponse response,
       FilterChain chain) throws IOException, ServletException;
   // 容器销毁的时候调用, 即关闭tomcat的时候调用
   public void destroy();
 }

public interface FilterChain {
   // 由Filter.doFilter()中的chain.doFilter调用
   public void doFilter(ServletRequest request, ServletResponse response)
     throws IOException, ServletException;
}

public final void invoke(Request request, Response response){
  ......
  MessageBytes requestPathMB = request.getRequestPathMB();
  DispatcherType dispatcherType = DispatcherType.REQUEST;
  if (request.getDispatcherType()==DispatcherType.ASYNC) dispatcherType = DispatcherType.ASYNC;
  request.setAttribute(Globals.DISPATCHER_TYPE_ATTR,dispatcherType);
  request.setAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR,
      requestPathMB);
  // Create the filter chain for this request
  ApplicationFilterChain filterChain =
      ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);
  // Call the filter chain for this request
  // NOTE: This also calls the servlet's service() method
  Container container = this.container;
  try {
    if ((servlet != null) && (filterChain != null)) {
      // Swallow output if needed
      if (context.getSwallowOutput()) {
        try {
          SystemLogHandler.startCapture();
          if (request.isAsyncDispatching()) {
            request.getAsyncContextInternal().doInternalDispatch();
          } else {
            filterChain.doFilter(request.getRequest(),
                response.getResponse());
          }
        } finally {
          String log = SystemLogHandler.stopCapture();
          if (log != null && log.length() > 0) {
            context.getLogger().info(log);
          }
        }
      } else {
        if (request.isAsyncDispatching()) {
          request.getAsyncContextInternal().doInternalDispatch();
        } else {
          filterChain.doFilter
            (request.getRequest(), response.getResponse());
        }
      }
    }
  } catch (ClientAbortException | CloseNowException e) {
  }
  ......
}

public static ApplicationFilterChain createFilterChain(ServletRequest request,
            Wrapper wrapper, Servlet servlet) {
  // If there is no servlet to execute, return null
  if (servlet == null)
    return null;
  // Create and initialize a filter chain object
  ApplicationFilterChain filterChain = null;
  if (request instanceof Request) {
    Request req = (Request) request;
    if (Globals.IS_SECURITY_ENABLED) {
      // Security: Do not recycle
      filterChain = new ApplicationFilterChain();
    } else {
      filterChain = (ApplicationFilterChain) req.getFilterChain();
      if (filterChain == null) {
        filterChain = new ApplicationFilterChain();
        req.setFilterChain(filterChain);
      }
    }
  } else {
    // Request dispatcher in use
    filterChain = new ApplicationFilterChain();
  }
  filterChain.setServlet(servlet);
  filterChain.setServletSupportsAsync(wrapper.isAsyncSupported());
  // Acquire the filter mappings for this Context
  StandardContext context = (StandardContext) wrapper.getParent();
  FilterMap filterMaps[] = context.findFilterMaps();
  // If there are no filter mappings, we are done
  if ((filterMaps == null) || (filterMaps.length == 0))
    return filterChain;
  // Acquire the information we will need to match filter mappings
  DispatcherType dispatcher =
      (DispatcherType) request.getAttribute(Globals.DISPATCHER_TYPE_ATTR);
  String requestPath = null;
  Object attribute = request.getAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR);
  if (attribute != null){
    requestPath = attribute.toString();
  }
  String servletName = wrapper.getName();
  // Add the relevant path-mapped filters to this filter chain
  for (FilterMap filterMap : filterMaps) {
    if (!matchDispatcher(filterMap, dispatcher)) {
      continue;
    }
    if (!matchFiltersURL(filterMap, requestPath))
      continue;
    ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
        context.findFilterConfig(filterMap.getFilterName());
    if (filterConfig == null) {
      // FIXME - log configuration problem
      continue;
    }
    filterChain.addFilter(filterConfig);
  }
  // Add filters that match on servlet name second
  for (FilterMap filterMap : filterMaps) {
    if (!matchDispatcher(filterMap, dispatcher)) {
      continue;
    }
    if (!matchFiltersServlet(filterMap, servletName))
      continue;
    ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
        context.findFilterConfig(filterMap.getFilterName());
    if (filterConfig == null) {
      // FIXME - log configuration problem
      continue;
    }
    filterChain.addFilter(filterConfig);
  }
  // Return the completed filter chain
  return filterChain;
}

private void internalDoFilter(ServletRequest request,
                                  ServletResponse response)
        throws IOException, ServletException {
  // Call the next filter if there is one
  if (pos < n) {
    ApplicationFilterConfig filterConfig = filters[pos++];
    try {
      Filter filter = filterConfig.getFilter();
      if (request.isAsyncSupported() && "false".equalsIgnoreCase(
          filterConfig.getFilterDef().getAsyncSupported())) {
        request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);
      }
      if( Globals.IS_SECURITY_ENABLED ) {
        final ServletRequest req = request;
        final ServletResponse res = response;
        Principal principal =
          ((HttpServletRequest) req).getUserPrincipal();
        Object[] args = new Object[]{req, res, this};
        SecurityUtil.doAsPrivilege ("doFilter", filter, classType, args, principal);
      } else {
        filter.doFilter(request, response, this);
      }
    } catch (IOException | ServletException | RuntimeException e) {
      throw e;
    } catch (Throwable e) {
      e = ExceptionUtils.unwrapInvocationTargetException(e);
      ExceptionUtils.handleThrowable(e);
      throw new ServletException(sm.getString("filterChain.filter"), e);
    }
    return;
  }
  // We fell off the end of the chain -- call the servlet instance
  try {
    if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
      lastServicedRequest.set(request);
      lastServicedResponse.set(response);
    }
    if (request.isAsyncSupported() && !servletSupportsAsync) {
      request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,
          Boolean.FALSE);
    }
    // Use potentially wrapped request from this point
    if ((request instanceof HttpServletRequest) &&
        (response instanceof HttpServletResponse) &&
        Globals.IS_SECURITY_ENABLED ) {
      final ServletRequest req = request;
      final ServletResponse res = response;
      Principal principal =
        ((HttpServletRequest) req).getUserPrincipal();
      Object[] args = new Object[]{req, res};
      SecurityUtil.doAsPrivilege("service",
                     servlet,
                     classTypeUsedInService,
                     args,
                     principal);
    } else {
      servlet.service(request, response);
    }
  } catch (IOException | ServletException | RuntimeException e) {
    throw e;
  } catch (Throwable e) {
    e = ExceptionUtils.unwrapInvocationTargetException(e);
    ExceptionUtils.handleThrowable(e);
    throw new ServletException(sm.getString("filterChain.servlet"), e);
  } finally {
    if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
      lastServicedRequest.set(null);
      lastServicedResponse.set(null);
    }
  }
}