doFilter called twice, intended behaviour?

问题:

I'm working through the Java EE servlet tutorial and tried the mood example. I noticed the doFilter is getting called twice, once the servlet call is in the chain and the second time it isnt.

I added some printlns in the TimeOfDayFilter.java and in the MoodServlet.java to show this.

TimeOfDayFilter.java:

    ...
    System.out.println("TimeOfDay before"); //added
    chain.doFilter(req, res);
    System.out.println("TimeOfDay after"); //added
    ...

MoodServlet.java:

    ...
    response.setContentType("text/html;charset=UTF-8");

    System.out.println("MoodServlet"); //added

    PrintWriter out = response.getWriter();
    ...

The result from the glassfish server (3.1) window when calling the servlet is the following:

    INFO: mood was successfully deployed in 406 milliseconds.
    INFO: TimeOfDay before
    INFO: MoodServlet
    INFO: TimeOfDay after
    INFO: TimeOfDay before
    INFO: TimeOfDay after

Is this intended behaviour? If so, what is the reason for the extra call?

回答1:

chain.doFilter(request,response);

This will pass the control to the servlet the filter is associated with. But after the corresponding servlet is executed, the control comes back at the end of the above line and all the lines thereafter in the current doFilter() is executed.

If you want to pass the control permanently to the servlet and not letting it return to the filter, just add a

return;

at the end of chain.doFilter(request,response) line in the current filter.

回答2:

The Filter.doFilter method is called once per request. You can execute some code before other filters in the chain are called and also afterwards (in the order specified in the filter chain, as per the web.xml filter-mapping order), something like the following example:

public MyFilter implements Filter {
    public void doFilter(ServletRequest request, ServletResponse response,
           FilterChain chain) 
           throws IOException, ServletException
    {
        codeToExecuteBeforeOtherFiltersInTheChain(request, response);

        chain.doFilter(request, response);

        codeToExecuteAfterOtherFiltersInTheChain(request, response);

    }
}

If your filter is configured to dispatch REQUEST and FORWARD requests, then the MyFilter.doFilter method will be called once for the original request and once if the request has been forwarded:

Configure filter mapping using web.xml file:

...
<filter-mapping>
      <filter-name>MyFilter</filter-name>
      <url-pattern>/*</url-pattern>
      <dispatcher>REQUEST</dispatcher>
      <dispatcher>FORWARD</dispatcher>
</filter-mapping>
...

Configure filter mapping using @WebFilter annotation:

@WebFilter(urlPatterns = "/*", dispatcherTypes = {
    DispatcherType.REQUEST, DispatcherType.FORWARD
}) public MyFilter implements Filter {
    ...
}

To be able to check if the request has been forwarded, you can use the request attribute described here: How to know when the request is forwarded in a RequestWrapper object

For details about filters see: https://docs.oracle.com/cd/B32110_01/web.1013/b28959/filters.htm

回答3:

I solved the same issue after removing @Component in CustomFilter class.

回答4:

The reason why the filter is called twice are the images used in the response creation, as for instance

out.println("<img src=\"resources/images/duke.snooze.gif\" alt=\"Duke sleeping\"/><br/>");

Please see the log output

2016-01-16T11:25:34.894+0100|Info: TimeOfDay doFilter method before sending to chain

2016-01-16T11:25:34.895+0100|Info: MoodServlet get method called

2016-01-16T11:25:34.895+0100|Info: TimeOfDay doFilter method after sending to chain

2016-01-16T11:25:34.942+0100|Info: TimeOfDay doFilter method before sending to chain

2016-01-16T11:25:34.942+0100|Info: TimeOfDay doFilter method after sending to chain

src in img tag is nothing else than the second request for server to take care of. Please notice the url pattern used in the @WebFilter

@WebFilter(filterName = "TimeOfDayFilter",
urlPatterns = {"/*"},
initParams = {
    @WebInitParam(name = "mood", value = "awake")})

It will intercept all the requests coming into mood application. As an exercise simply try to remove images from the response or change url pattern to intercept only requests ending up in MoodServlet

@WebFilter(filterName = "TimeOfDayFilter",
urlPatterns = {"/report"},
initParams = {
    @WebInitParam(name = "mood", value = "awake")})

Both will result in one call of doFilter as you originally expected

2016-01-16T11:28:53.485+0100|Info: TimeOfDay doFilter method before sending to chain

2016-01-16T11:28:53.486+0100|Info: MoodServlet get method called

2016-01-16T11:28:53.487+0100|Info: TimeOfDay doFilter method after sending to chain

回答5:

Yes it is, Filter is executed twice in the life cycle, first calls when request from client come to servlet and second time when response is served to the client after execution of servlet.

The order of execution is somehow looks like this.

回答6:

As Mohan has stated, @Component will make your filter be called twice if you have it already registered in you Application class, like this:

resources.add(new MyFilter());

If that's the case you have to choose between annotating it or registering it. But this is only valid for JAX-RS applications that use Spring. Not the topic of this question.

回答7:

I came across the same issue when the doFilter is called twice (or multiple times). The problem was that the filter handles every request including css, js, image and all other files while I expected one request for each page, so I solved the issue by adding the following code:

@WebFilter(filterName = "MyCustomFilter")
public class MyCustomFilter implements Filter {

  public void doFilter(ServletRequest request,ServletResponse response,
          FilterChain chain) throws IOException, ServletException {
    HttpServletRequest httpRequest = (HttpServletRequest) request;
    String accept = httpRequest.getHeader("accept");

    // Since the filter handles every request
    // we have to ensure that the request is asking for text/html
    if (accept == null || !accept.toLowerCase().startsWith("text/html")) {
      chain.doFilter(request, response);
      return;
    }

    // your code goes here

Hope this will help people like me who googled this question .

回答8:

As @BalusC said, in my case the browser was requesting the favicon.ico by default, so there was an error in the browser's console

Failed to load resource: the server responded with a status of 404 (Not Found)

So I followed this answer and the error dissapeared but then I got the filter to get called twice.

MY SOLUTION

I replaced

<link rel="shortcut icon" href="#" />

For this:

<link rel="shortcut icon" href="../assets/img/valid_icon.png" />