struts & generating progress messages

[Kitching Simon <Si...@orange.ch>]

Hi,

I currently have a model-1 web application
ie in which the .jsp pages are responsible 
for invoking the business logic.

I am planning to move the webapp to struts,
for all the obvious and traditional reasons.
However, there is one nice feature of the 
current code that I can't see how to perform
inside an MVC framework like struts:

On submitting a particular form, there is a
sequence of 4 operations to perform, each of
which takes somewhere between 5 and 30
seconds to perform. Currently, what is done is
to generate an almost complete HTML page,
and force it to be flushed to the browser; as
each long-duration step starts and completes, 
a fragment of page containing CSS-positioned 
html/ javascript is flushed to the browser. The
effect is very nice - a list of the steps appears
on the screen, an hourglass appears next to 
each step as it starts, and a tick or error message
appears next to each one as it completes.

Does anyone have any idea how to generate the
same sort of effect (essentially alternating between
business logic and presentation output) in struts?

Thanks in advance,

Simon

Re: struts & generating progress messages

["Craig R. McClanahan" <Cr...@eng.sun.com>]

Kitching Simon wrote:

> Hi,
>
> I currently have a model-1 web application
> ie in which the .jsp pages are responsible
> for invoking the business logic.
>
> I am planning to move the webapp to struts,
> for all the obvious and traditional reasons.
> However, there is one nice feature of the
> current code that I can't see how to perform
> inside an MVC framework like struts:
>
> On submitting a particular form, there is a
> sequence of 4 operations to perform, each of
> which takes somewhere between 5 and 30
> seconds to perform. Currently, what is done is
> to generate an almost complete HTML page,
> and force it to be flushed to the browser; as
> each long-duration step starts and completes,
> a fragment of page containing CSS-positioned
> html/ javascript is flushed to the browser. The
> effect is very nice - a list of the steps appears
> on the screen, an hourglass appears next to
> each step as it starts, and a tick or error message
> appears next to each one as it completes.
>
> Does anyone have any idea how to generate the
> same sort of effect (essentially alternating between
> business logic and presentation output) in struts?
>

As you've gathered, a model 2 approach is not amenable to using flushing to show
progress.  A strategy that might work better is something like this, in the
initial Action that starts all of the work:

* Fire off a background thread to do the long-duration work.
* Give this thread access to the user's session, so it can
  update a status variable when it is done
* Return a "work in progress .. click here to check for completion"
  page, which returns to the same action
* When the action is entered, it will check to see that a
  background thread is currently running, so it will display
  another "work in progress" page, or the final results, depending
  on what's been done so far.
* When the background thread completes its work, it updates
  the final status and exits.

Variations on the theme:
* The status variable need not be binary -- for a multiple
  stage background operation, it could be a state identifier,
  or a percentage completed, or something like that, so you
  can be more descriptive on the "in progress" page.
* You might want to generate a meta refresh tag on the
  "in progress" page, so that the user doesn't have to remember
  to hit the link.

>
> Thanks in advance,
>
> Simon

Craig

Re: struts & generating progress messages

[Jim Newsham <ne...@hotu.com>]

Kitching Simon wrote:

> Hi,
>
> I currently have a model-1 web application
> ie in which the .jsp pages are responsible
> for invoking the business logic.
>
> I am planning to move the webapp to struts,
> for all the obvious and traditional reasons.
> However, there is one nice feature of the
> current code that I can't see how to perform
> inside an MVC framework like struts:
>
> On submitting a particular form, there is a
> sequence of 4 operations to perform, each of
> which takes somewhere between 5 and 30
> seconds to perform. Currently, what is done is
> to generate an almost complete HTML page,
> and force it to be flushed to the browser; as
> each long-duration step starts and completes,
> a fragment of page containing CSS-positioned
> html/ javascript is flushed to the browser. The
> effect is very nice - a list of the steps appears
> on the screen, an hourglass appears next to
> each step as it starts, and a tick or error message
> appears next to each one as it completes.
>
> Does anyone have any idea how to generate the
> same sort of effect (essentially alternating between
> business logic and presentation output) in struts?
>
> Thanks in advance,
>
> Simon

How about creating a separate thread to perform the tasks, and
forwarding immediately to the jsp page in your main thread.  Create an
object which can wait for the tasks to complete, and attach that object
to the request before forwarding.

As an example, create a TaskMonitor (sample code below; this code is
untested, and I usually don't program threads so don't trust it to work
without some scrutiny).

Task thread.  Create a thread to perform your tasks and give it a
reference to this monitor.  Each time a task is complete, call
monitor.taskComplete(taskid).

Main thread.  Attach monitor to request.  Forward to jsp.  The jsp can
call monitor.waitForTask(taskid) for each task in sequence.

public class TaskMonitor {

  boolean[] taskComplete;

  public TaskMonitor(int taskCount) {
    taskComplete = new boolean[taskCount];
  }

  public synchronized waitForTask(int taskid) {
    if (taskid < 0 || taskid >= taskComplete.length
|| taskComplete[taskid]) return;
    while(true) {
      try {
        wait();
      }
      catch(InterruptedException ie) { }
      if (taskComplete[taskid]) return;
    }
  }

  public synchronized taskComplete(int taskid) {
    if (taskid > 0 && taskid < taskComplete.length) {
      taskComplete[taskid] = true;
      notifyAll();
    }
  }

}