slioch wrote:
Thanks much for the responses. I understand.
It makes sense to use HttpServletRequestWrapper, but there are no methods in
HttpServletRequestWrapper or HttpServletRequest that initiates the sending
of the response back to the client. So, by subclassing HttpServletRequest
there doesn't appear to be any way to suppress the sending on the response
back to the client (until I've completed processing).
Yes there is.
To create output for the client, the application calls something, right
? (I mean a method of HttpRequest).
So, redefine this same method in your subclass.
Then, whenever the application calls this method, thinking it is sending
output to the client, in fact it calls your method in your subclass.
Sneaky, he ?
Now your method in your subclas gets whatever the webapp thought it was
sending directly out.
Now it's up to your method, whatever it does with that data, before
itself calling the real Httprequest method to really send the data out.
For example, you could buffer all the data somewhere waiting for the
webapp to have finished its complete response.
The above all happens *during* your call to doFilter(), because it is
during that call that the webapp executes. The webapp just "calls back"
into the methods you have defined in your wrapper.
*Then* when your doFilter() returns (meaning the webapp is done), all
the data is in the buffer, and you start processing it and outputting
whatever you want.
(Maybe not recommended for megabytes of data, that, but I am sure you'll
think of something.)
Me speak in mine not really Java-like approximative language, but hope
you understand what me means.
What I would like to do is to prevent the response from being sent back to
the client until my filter has completed it's processing (and this is after
my filter delegates the doFilter() call to the other filters).
Mike
Pid-2 wrote:
André Warnier wrote:
slioch wrote:
[...]
I'll risk an explanation here..
I think maybe the issue is a misunderstanding of how a servlet filter
works. It took me a while too, but I think I've got it in the end.
Sorry if this is level 101, that's my own level.
What was confusing to me at first, is that this is different from, for
instance, Apache input and output filters. There there is a clear
distinction between an input filter, which sees all the data on the way
in to the application, and an output filter, which sees all the data
that the application produces, before it goes out to the browser. An
input filter and and output filter are two separate pieces of code, and
you can install them independently.
In the Java servlet view of things, a filter is a "wrapper", within
which the other filters and the webapp run.
It's like an onion : your filter is the outer layer, within which there
are possibly further layers (other filters), and at the center is the
webapp. When your filter calls doFilter(), it executes all its inner
layers in one go. The bummer is that, unless you take pains to change
that, each of these layers has a direct access to the output buffers,
which live outside the onion. So unless you prevent them from doing
that, they will start putting bytes there, and by the time your
doFilter() returns, it's too late to change that.
When you execute doFilter(), in fact you execute, at that point, all the
further filters that are in the chain, and the webapp at the deepest
level. If any of these starts sending output, then by the time the
doFilter() returns, that output is already "past" your filter, and there
is nothing you can do anymore to modify it.
If you want something else to happen, then you have to do something like
this :
- in your filter, subclass the HttpRequest, say as "myHttpRequest". In
this subclass, redefine the methods that the underlying filters and
webapp will (presumably) use to send output to the buffer. In these
redefined methods, you can then do whatever you want to transform what
the application sends out via these methods.
- then, instead of passing the original HttpRequest to the doFilter(),
pass your own myHttpRequest instance of it.
Close enough; the spec has some stuff that covers this type of problem,
so I'd recommend investigating:
javax.servlet.http.HttpServletRequestWrapper
javax.servlet.http.HttpServletResponseWrapper (hereafter HSRsW)
Using the latter should allow you to modify the appropriate header.
HSRsW wrappedHres = new HttpServletResponseWrapper(hres);
chain.doFilter(hreq, wrappedHres);
Where your HSRsW contains appropriate code to modify the header, perhaps
in the construction phase.
p
This way, whenever the application "thinks" it is just using the (say)
HttpRequest.setHeader() method, it is in fact using *your*
myHttpRequest.setHeader() method, in which you can catch and "pervert"
whatever you want, before passing it on to the "real"
HttpRequest.setHeader() method.
The point is, if you let any underlying (from the point of your filter)
other filter or webapp call i.e. the original setHeader(), then that's
it : that line of output is now already in the HTTP output buffer queue,
and by the time your doFilter() returns, it no longer can "claw it back".
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