How to pass new header to sendRedirect

问题

I feel like this should be easy. I have an app where all I am trying to do is have a form page (index.jsp) that calls a servlet (CheckInfo.java) which sets a new header (myHeader) and redirects the user to another page (redirect.jsp). All of these files are on the same server. The index.jsp is sending the request just fine and CheckInfo is processing and redirecting, but myHeader is not showing up on redirect.jsp. I've read several posts talking about response.sendRedirect sends a 302 which doesn't pass headers and that I should use RequestDispatcher, but nothing seems to work. Is there no way to send headers from a servlet to a jsp? Here is the servlet code:

response.setHeader("myHeader", "hey there");
response.sendRedirect("redirect.jsp");

I have also tried this:

response.setHeader("myHeader", "hey there");
RequestDispatcher view = request.getRequestDispatcher("redirect.jsp");
view.forward(request, response);

And I have this in redirect.jsp:

System.out.println(request.getHeader("myHeader"));

This does not print anything.

If the answer to my question is no... then I would settle for a way to set the header once I got back to the jsp. My reverse proxy is looking for a specific header to determine whether or not to perform an action. Obviously I tried response.addHeader() on redirect.jsp, but the page has already loaded at that point so that just made me feel dumb.

回答1:

response.setHeader("myHeader", "hey there");
response.sendRedirect("redirect.jsp");

You are adding it as response header and it is 302 response. Browser on seeing a 302 response will just look for Location header and fire a new request to this location. Custom headers in the response are untouched whereas you are expecting these custom response headers to be included in the request (to new redirect location) which is not being sent.

Solution:-
1. you can use request dispatcher and forward the request instead of external redirect. And you need to use request attributes here.

2. you can call submit form using an ajax request may be jquery like and handle the response manually(for 302 response) but would not suggest you to use this approach as it is not a cleaner and intuitive approach. Just mentioning so that you know there are other ways to achieve this.

回答2:

The problem is that the redirect() method of the response initiates a new request altogether, thereby loosing the attributes that were set before redirecting. Luckily there is a fluent way of solving the problem still. See below

response.setHeader("myHeader", "hey there"); request.getRequestDispatcher("redirect.jsp").forward(request, response);

Then in your destination you can do response.getHeaders("myHeader")

I have tested the code.

回答3:

I hope it's clear that in case of asking the client to redirect to another URL - the browser shall not honor the cookies.

However, the 2nd method - where server forwards the request is feasible. The main mistake appears to be in mutating the response while we are supposed to change the request.

Then again, one cannot directly mutate a HttpServletRequest object. Here is one way to do so:

HttpServletRequestWrapper requestWrapper = new HttpServletRequestWrapper(request){
    public String getHeader(String name) {
        String value = super.getHeader(name);
        if(Strings.isNullOrEmpty(value)) {
            ...
            value = myNewHeader;
        }
        return value;
    }

    public Enumeration<String> getHeaders(String name) {
        List<String> values = Collections.list(super.getHeaders(name));
        if(values.size()==0) {
            ...
            values.add(myNewHeader);
        }
        return Collections.enumeration(values);
    }

    public Enumeration<String> getHeaderNames() {
        List<String> names = Collections.list(super.getHeaderNames());
        names.add(myNewHeaderName);
        ...
        return Collections.enumeration(names);
    }
}

Followed by:

RequestDispatcher view = request.getRequestDispatcher("redirect.jsp");
// OR (If you can get servletContext)
RequestDispatcher view = servletContext.getRequestDispatcher("redirect.jsp");

view.forward(requestWrapper, response);

Reference:

• https://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpServletRequestWrapper.html
  • For the headers case - getHeader(), getHeaders() and getHeaderNames() fn in the reqWrapper obj need Overriding.
  • Similarly you can override cookies and params.
• See also: Modify request parameter with servlet filter

NOTE: It might not be possible to forward a req to an endpoint which expects a different MIME type.

回答4:

A client side redirect creates a new HTTP request/response pair. This link may help you more on debugging perspective -

Sending Custom headers

来源：https://stackoverflow.com/questions/34972006/how-to-pass-new-header-to-sendredirect