Tomcat中Filter是怎样执行的

tunsuy

发布于 2022-10-27 09:58:28

5750

发布于 2022-10-27 09:58:28

前言

Filter是什么？Filter是servlet规范中定义的java web组件, 在所有支持java web的容器中都可以使用它是位于前端请求到servlet之间的一系列过滤器，也可以称之为中间件，它主要是对请求到达servlet之前做一些额外的动作：

1、权限控制
2、监控
3、日志管理
4、等等

这里涉及到两个接口：Filter和FilterChain

Filter和FilterChain密不可分, Filter可以实现依次调用正是因为有了FilterChain。

1、Filter接口

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();
 }

2、FilterChain接口

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

执行流程

在前面的文章中，我们知道，tomcat启动会执行StandardWrapperValve.java类的invoke方法：

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) {
  
  }
  ......
}

上面的代码做了如下一些动作：

1、每次请求过来都会创建一个过滤器链(filterChain)，并把待执行的servlet对象存放到过滤器链中。对于每个url，对应的filter个数都是不固定的，filterchain需要保存每个请求所对应的一个filter数组，以及调用到的filter的position，以便继续向下调用filter。
2、创建了filterChain之后，就开始执行doFilter进行请求的链式处理。

1、创建filterChain

下面我们具体来看看filterChain是怎么创建的

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;
}

上面的代码做了一下几件事：

1、把要执行的servlet存放到过滤器链中。
2、如果没有配置过滤器则return一个空的过滤器链（只包含上面设置的servlet）。
3、如果配置url-pattern过滤器，则把匹配的过滤器加入到过滤器链中
4、如果配置servlet-name过滤器，则把匹配的过滤器加入到过滤器链中

注意: filterChain.addFilter()顺序与web.xml中定义的Filter顺序一致，所以过滤器的执行顺序是按定义的上下顺序决定的。

2、执行dofilter

创建了chain之后，就开始执行链式请求了，具体的逻辑如下：

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);
    }
  }
}

上面的代码逻辑如下：

1、通过position索引判断是否执行完了所有的filter
2、如果没有，取出当前待执行的索引filter，调用其doFilter方法，在上面的接口说明中，我们看到，所有的filter类都继承了filter接口，都实现了dofilter方法；我们也注意到，该方法接收一个filterChain对象。在这段代码中，filter.doFilter(request, response, this);可以看到，将自身引用传递进去了，那么各个filter在dofilter的方法中，可以根据自身业务需要，来判断是否需要继续进行下面的filter链式执行，如果需要，就执行filterChain.doFilter方法，此时就又回到了此代码中。如果反复
3、如果执行完了所有的filter，则开始执行servlet业务模块servlet.service(request, response);

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原始发表：2021-02-28，如有侵权请联系 cloudcommunity@tencent.com 删除

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