Can I make my servlet wait/delay for... maybe 500 - 1000 ms

[Ingo Gambin <IG...@brilliant.de>]

Hi,

this time I encountered another problem.
(my setup: Tomcat 5.5, itext 5.0.0)

i wrote a servlet that, using itext, extracts 1 page of a source pdf
from directory A file and writes this 1 page as temporary pdf-file into
another directory B. 

This works great but in the same move (after i called the extraction
method) i want to output said pdf-file to the browser 
e. g.
<embed src="/tmp/14153462345213.pdf...">

Basically with already existing pdf-files that works fine, but checking
the whole procedures inserting timing-outputs I realized that, although
the extraction method is done and the next methods are called, itext (or
the system) is still writing the file to directory B and therefore when
the browser tries to embed it (adobe plugin) its not fully written and
the adobe plugin can not read it because of that.

now I wonder how to solve that problem. So to my ideas:
        a) only continue after extraction when the file is fully written
                => Actually I have no idea about how to check that
        b) have the servlet wait/sleep for maybe up to a second
                => from what I found so far, sleeping a servlet is NOT
                good and on the other hand what if one pdf-page i want
                to extract is so big/has so many graphics in it that the
                process lasts longer than a second
                
would anybody have an idea or a hint to help me out ? 

Thanks in advance!

best regards

Ingo

RE: Can I make my servlet wait/delay for... maybe 500 - 1000 ms

["Caldarale, Charles R" <Ch...@unisys.com>]

> From: André Warnier [mailto:aw@ice-sa.com]
> Subject: Re: Can I make my servlet wait/delay for... maybe 500 - 1000
> ms
> 
> 1) I have no idea how one can call an external OS-level command from Java

Runtime.exec() is the traditional way; the ProcessBuilder class available in 1.5+ provides more control over the child process.

 - Chuck


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Re: Can I make my servlet wait/delay for... maybe 500 - 1000 ms

[André Warnier <aw...@ice-sa.com>]

Peter Crowther wrote:
> 2009/12/17 Ingo Gambin <IG...@brilliant.de>
> 
>> Basically with already existing pdf-files that works fine, but checking
>> the whole procedures inserting timing-outputs I realized that, although
>> the extraction method is done and the next methods are called, itext (or
>> the system) is still writing the file to directory B and therefore when
>> the browser tries to embed it (adobe plugin) its not fully written and
>> the adobe plugin can not read it because of that.
>>
> 
> I'm surprised, as I didn't think itext used background threads in order to
> write a document.  Certainly I've never had a problem with this, though I
> may just have been lucky!  Is the directory on the same machine, or is it
> remote?  If remote, network bandwidth/latency may be causing the issue you
> describe.
> 
> 
>> now I wonder how to solve that problem. So to my ideas:
>>        a) only continue after extraction when the file is fully written
>>                => Actually I have no idea about how to check that
>>
> 
> I'd ask Bruno and co on the itext list how you would know that itext has
> finished.
> 
> 
>>        b) have the servlet wait/sleep for maybe up to a second
>>                => from what I found so far, sleeping a servlet is NOT
>>                good and on the other hand what if one pdf-page i want
>>                to extract is so big/has so many graphics in it that the
>>                process lasts longer than a second
>>
>> "Not good" is a generalisation; let's look at the specifics.  First off,
> you're not sleeping "a servlet"; you're sleeping a thread.  Sleeping a
> thread means that the request being handled by that thread takes longer to
> complete.  Therefore, the thread is returned to the pool later.  Therefore,
> more threads are needed to achieve the same throughput.  This is only a
> problem if your server can't handle the extra load.
> 
> I wouldn't just throw sleep calls around the code like they were confetti,
> but I'll confess to having one place in my own code where a HTTP request has
> to wait for an external executable to complete its task and write some rows
> into a relational database.  Unfortunately the only approach here is to
> poll: repeatedly try to retrieve the "I'm done" row, sleeping a while
> between each test.  It works well enough, until we get a better
> architectural solution to the problem.
> 
1) I have no idea how one can call an external OS-level command from Java
2) this being said, if it happened to be possible to call this command 
and swallow it's output, then I would do it as follows :
- open an input stream on the output of the external command
- have the external command run and produce its output to STDOUT (which 
my stream swallows)
- read the input stream until I get an EOF (at which point the external 
command is presumably done writing)
- output what I read, as the response of my servlet, back to the client
- when I'm done, cleanup whatever needs to be

In perl, I would do this approximately as :
open(PDFPAGE,'here comes the command | ');
while { read PDFPAGE until EOF
   print CLIENT what I read;
}
close PDFPAGE;


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Re: Can I make my servlet wait/delay for... maybe 500 - 1000 ms

[Peter Crowther <pe...@melandra.com>]

2009/12/17 Ingo Gambin <IG...@brilliant.de>

> Basically with already existing pdf-files that works fine, but checking
> the whole procedures inserting timing-outputs I realized that, although
> the extraction method is done and the next methods are called, itext (or
> the system) is still writing the file to directory B and therefore when
> the browser tries to embed it (adobe plugin) its not fully written and
> the adobe plugin can not read it because of that.
>

I'm surprised, as I didn't think itext used background threads in order to
write a document.  Certainly I've never had a problem with this, though I
may just have been lucky!  Is the directory on the same machine, or is it
remote?  If remote, network bandwidth/latency may be causing the issue you
describe.


> now I wonder how to solve that problem. So to my ideas:
>        a) only continue after extraction when the file is fully written
>                => Actually I have no idea about how to check that
>

I'd ask Bruno and co on the itext list how you would know that itext has
finished.


>        b) have the servlet wait/sleep for maybe up to a second
>                => from what I found so far, sleeping a servlet is NOT
>                good and on the other hand what if one pdf-page i want
>                to extract is so big/has so many graphics in it that the
>                process lasts longer than a second
>
> "Not good" is a generalisation; let's look at the specifics.  First off,
you're not sleeping "a servlet"; you're sleeping a thread.  Sleeping a
thread means that the request being handled by that thread takes longer to
complete.  Therefore, the thread is returned to the pool later.  Therefore,
more threads are needed to achieve the same throughput.  This is only a
problem if your server can't handle the extra load.

I wouldn't just throw sleep calls around the code like they were confetti,
but I'll confess to having one place in my own code where a HTTP request has
to wait for an external executable to complete its task and write some rows
into a relational database.  Unfortunately the only approach here is to
poll: repeatedly try to retrieve the "I'm done" row, sleeping a while
between each test.  It works well enough, until we get a better
architectural solution to the problem.

- Peter