目錄前言1、Filter接口2、FilterChain接口執行流程1、創建filterChain2、執行dofilter前言

Filter是什么？Filter是servlet規范中定義的java web組件, 在所有支持java web的容器中都可以使用 它是位于前端請求到servlet之間的一系列過濾器，也可以稱之為中間件，它主要是對請求到達servlet之前做一些額外的動作：

1、權限控制2、監控3、日志管理4、等等

這里涉及到兩個接口：Filter和FilterChain

Filter和FilterChain密不可分, Filter可以實現依次調用正是因為有了FilterChain。

1、Filter接口public interface Filter { // 容器創建的時候調用, 即啟動tomcat的時候調用 public void init(FilterConfig filterConfig) throws ServletException; // 由FilterChain調用, 并且傳入FilterChain本身, 最后回調FilterChain的doFilter()方法 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException; // 容器銷毀的時候調用, 即關閉tomcat的時候調用 public void destroy(); }2、FilterChain接口public interface FilterChain { // 由Filter.doFilter()中的chain.doFilter調用 public void doFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException;}執行流程

在前面的文章中，我們知道，tomcat啟動會執行StandardWrapperValve.java類的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) { } ......}

上面的代碼做了如下一些動作：

1、每次請求過來都會創建一個過濾器鏈(filterChain)，并把待執行的servlet對象存放到過濾器鏈中。對于每個url，對應的filter個數都是不固定的，filterchain需要保存每個請求所對應的一個filter數組，以及調用到的filter的position，以便繼續向下調用filter。2、創建了filterChain之后，就開始執行doFilter進行請求的鏈式處理。1、創建filterChain

下面我們具體來看看filterChain是怎么創建的

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;}

上面的代碼做了一下幾件事：

1、把要執行的servlet存放到過濾器鏈中。2、如果沒有配置過濾器則return一個空的過濾器鏈（只包含上面設置的servlet）。3、如果配置url-pattern過濾器，則把匹配的過濾器加入到過濾器鏈中4、如果配置servlet-name過濾器，則把匹配的過濾器加入到過濾器鏈中

注意: filterChain.addFilter()順序與web.xml中定義的Filter順序一致，所以過濾器的執行順序是按定義的上下順序決定的。

2、執行dofilter

創建了chain之后，就開始執行鏈式請求了，具體的邏輯如下：

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); } }}

上面的代碼邏輯如下：

1、通過position索引判斷是否執行完了所有的filter2、如果沒有，取出當前待執行的索引filter，調用其doFilter方法，在上面的接口說明中，我們看到，所有的filter類都繼承了filter接口，都實現了dofilter方法；我們也注意到，該方法接收一個filterChain對象。在這段代碼中，filter.doFilter(request, response, this);可以看到，將自身引用傳遞進去了，那么各個filter在dofilter的方法中，可以根據自身業務需要，來判斷是否需要繼續進行下面的filter鏈式執行，如果需要，就執行filterChain.doFilter方法，此時就又回到了此代碼中。如果反復3、如果執行完了所有的filter，則開始執行servlet業務模塊servlet.service(request, response);

以上就是詳解Tomcat中Filter是怎樣執行的的詳細內容，更多關于Tomcat Filter執行的資料請關注好吧啦網其它相關文章！