Thinking about filters, again.

[Hussein Baghdadi]

Hi.
Shamefully, there is something I don't understand about Servlets filters...
My basic idea of filters :
If the request satifies your requirements, allow it to hit the resource (you know, chain.doFilter), if not, return from the method.
But recently, I found this snippet from Hibernate CaveatEmptor:

try {
  chain.doFilter(request, response);
  HibernateUtil.commitTransaction();
} finally {
  HibernateUtil.closeSession();
}

How the hell they put another statement after doFilter( ) call ?

[Bear Bibeault]

doFilter() is a Java method like any other. When it completes, it returns and execution continues from the point of call.

Why should this surprise you?
[ April 04, 2007: Message edited by: Bear Bibeault ]

[Jeanne Boyarsky]

John,
There are three basic patterns for filters.

1) Execute filter before rest of request. Good for restricting access.
// do filter stuff
chain.doFilter(request, response);

2) Execute filter after rest of request. Good for post processing of response.
chain.doFilter(request, response);
// do filter stuff

3) Execute request in between filter. Good for more complex filters like the open source compression filter.
// do filter stuff
chain.doFilter(request, response);
// do more filter stuff

I'm assuming that Hibernate is an example of case 3 where they make sure every request is in a transaction. (What I'm assuming is that they begin the transaction earlier in the filter.)

[Hussein Baghdadi]

When it completes

completes here means, after (lets say) JSP had been rendered.
right ?

[Bear Bibeault]

Correct. Remember this is all done in Java code, so all the usual Java rules apply.

[Hussein Baghdadi]

Ok, lets say we have two filters :
Filter one :

//print 1
chaing.doFilter( );
//print 2

Filter two :

//print 3
chaing.doFilter( );
//print 4

So, the workflow is as the following :
- print 1
- filter1 chain.doFilter( );
- print 3
- filter2 chain.doFilter( );
- hit the JSP
- print 4
- print 2
Right ?
[ April 04, 2007: Message edited by: John Todd ]