doFilter called twice, intended behaviour?

Question

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 secon

Answer 1

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:

...

      MyFilter
      /*
      REQUEST
      FORWARD

...

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