java 中filter的執行順序是怎樣的

問題：#

filterChain 是什麼時候執行的呢？
filterChain 中的 filter 來源何處？
standardContext 什麼時候開始收集的過濾器集合

1. filterChain 是什麼時候執行的呢？#

在 org.apache.catalina.core.StandardWrapperValve 類中有如下一個方法

public final void invoke(Request request, Response response)
        throws IOException, ServletException {

				//省略掉大段無關代碼
        Servlet servlet = null;
        Context context = (Context) wrapper.getParent();
				//省略掉大段無關代碼
        try {
            if (!unavailable) {
              	//1.create servlet
                servlet = wrapper.allocate();
            }
        } catch(){
          ...
        }
        // 2. Create the filter chain for this request
        ApplicationFilterChain filterChain =
                ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

        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 {
                          	//execute doFilter method of the FilterChain object.
                            filterChain.doFilter(request.getRequest(),
                                    response.getResponse());
                        }
                    } finally {
                      
                    }
                } else {
                    if (request.isAsyncDispatching()) {
                        request.getAsyncContextInternal().doInternalDispatch();
                    } else {
                        filterChain.doFilter
                            (request.getRequest(), response.getResponse());
                    }
                }

            }
        } catch(){
        } finally {
            // Release the filter chain (if any) for this request
            if (filterChain != null) {
                filterChain.release();
            }

            // Deallocate the allocated servlet instance
            try {
                if (servlet != null) {
                    wrapper.deallocate(servlet);
                }
            }
        }
    }

我們一會兒回過頭來看它是如何創建過濾器鏈對象的代碼，我們先來看它是如何執行過濾器鏈的，過濾器鏈對象執行的流程實現：

public final class ApplicationFilterChain implements FilterChain {
   //cur executing filter index
   private int pos = 0;
  //matched filter count 
   private int n = 0;
  //servlet 
   private Servlet servlet = null;
  //matched filter arrays
   private ApplicationFilterConfig[] filters = new ApplicationFilterConfig[0];

  @Override
  public void doFilter(ServletRequest request, ServletResponse response)
        throws IOException, ServletException {

    		//security been turned on
        if( Globals.IS_SECURITY_ENABLED ) {
          //... security bean filter
        } else {
          	//execute filter logic 
            internalDoFilter(request,response);
        }
    }

    private void internalDoFilter(ServletRequest request,
                                  ServletResponse response)
        throws IOException, ServletException {
      
      	// loop and  Call the next filter if there is one
        if (pos < n) {
          	//pos index ++
            ApplicationFilterConfig filterConfig = filters[pos++];
            try {
                Filter filter = filterConfig.getFilter();
                if( Globals.IS_SECURITY_ENABLED ) {
                } else {
                  	//execute the doFilter() of the cur filter in the filter chain
                    filter.doFilter(request, response, this);
                }
            } catch(){
              ...
            }
            return;
        }

        // We fell off the end of the chain -- call the servlet instance
        try {
            if ((request instanceof HttpServletRequest) &&
                    (response instanceof HttpServletResponse) &&
                    Globals.IS_SECURITY_ENABLED ) {
            } else {
              	//if there are no filters to be executed, the servlet's service method will be executed, which 									contains the business logic that we have written.
                servlet.service(request, response);
            }
        }catch(){
          ...
        }finally{
          ...
        }
    }
}

從 filterChain 類的源碼可以看出底層是包含了所匹配上的 filter 陣列，也就是添加進去匹配上過濾器對象是有序的，添加的時候就決定了。

那麼它是什麼時候添加的呢？

2. filterChain 中的 filter 來源何處？#

其實在 org.apache.catalina.core.StandardWrapperValve 類的 invoke 方法中

ApplicationFilterChain filterChain =
               ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

    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();
      	//Get an array of all the filters registered in the ServletContext object
        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 (int i = 0; i < filterMaps.length; i++) {
            if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
                continue;
            }
            if (!matchFiltersURL(filterMaps[i], requestPath))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                context.findFilterConfig(filterMaps[i].getFilterName());
            if (filterConfig == null) {
                // FIXME - log configuration problem
                continue;
            }
            filterChain.addFilter(filterConfig);
        }

        // Add filters that match on servlet name second
        for (int i = 0; i < filterMaps.length; i++) {
            if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
                continue;
            }
            if (!matchFiltersServlet(filterMaps[i], servletName))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                context.findFilterConfig(filterMaps[i].getFilterName());
            if (filterConfig == null) {
                // FIXME - log configuration problem
                continue;
            }
            filterChain.addFilter(filterConfig);
        }

        // Return the completed filter chain
        return filterChain;
    }

可以看出在創建 filterChain 對象時，從 ServletContext 獲取所有註冊的 filter 的陣列取出需要的添加到這次請求創建的 filterChain 對象，而且 servletContext 對象的註冊的所有的過濾器本身就是一個陣列（本身就是有序的），所以遍歷匹配的時候，也就是有序的。

3. ServletContext 什麼時候開始收集的陣列#

filterMaps 在 StandardContext 類中

public class StandardContext extends ContainerBase
        implements Context, NotificationEmitter {

  private final ContextFilterMaps filterMaps = new ContextFilterMaps();
  
  //
  @Override
    public void addFilterMap(FilterMap filterMap) {
        validateFilterMap(filterMap);
        // Add this filter mapping to our registered set
        filterMaps.add(filterMap);
        fireContainerEvent("addFilterMap", filterMap);
    }

}

啟動 tomcat，我們可以根據堆棧查看調用順序

StandardContext.startInternal()-->fireLifecycleEvent()-->ContextConfig.lifecycleEvent()-->ContextConfig.lifecycleEventcon-->ContextConfig.con.figureStart()-->ContextConfig.webConfig()

查看方法

protected void webConfig() {
      if (ok) {
          configureContext(webXml);
      }
}

private void configureContext(WebXml webxml) {
      for (FilterMap filterMap : webxml.getFilterMappings()) {
          context.addFilterMap(filterMap);
      }
}

這裡就將 web.xml 收集到所有 filter 的 set 集合，然後將收集到的 filter 集合轉換成陣列，設置給 StandardContext 對象