[web] Provide before and after handlers
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Key: LABS-240
URL: https://issues.apache.org/jira/browse/LABS-240
Project: Labs
Issue Type: Bug
Components: Magma
Affects Versions: Current
Reporter: Simone Gianni
Assignee: Simone Gianni
Fix For: Current
In a number of situation a WebHandler may want to perform some actions before
any call to a do or handle method. While this is achievable with aspects,
aspect method patterns knowledge is not require for the role of web programmer,
so a simpler method should be provided.
A good way could be to use an already existing "standard", for example using
@Before and @After annotations similar to those provided by Junit 4 or even
predefined names like setup and tearDown like it was in Junit 3 and how it is
in the startup package.
A good use case is, for example, when a certain value could be passed via
different environment methods. For example, a user preference which could be
present as a cookie or as a session parameter, giving precedence to one of
them, for example to cookies. In that case, every doMethod who wants to use
that user preference will end up checking if one of the two has been passed,
and using the most appropiate one. This could be done in a setup method, as if
it was an aspect.
Also, there are actions that need to be done before or after every request, or
every request in a certain zone of the site.
A good use case for this is the automatic user login. A login handler could use
cookies to mark a user, and then automatically log in when he comes back to the
site. Anyway, this should be done before any processing of the site is done,
cause having or not having a user could change the behaviour of a number of
site elements (like, display or not a login box, or a messages box and so on).
For this situation, adding setup methods to the RootWebHandler will mean "call
me every request". Nesting the setup method in another handler will limit the
scope.
Using setup/teardown methods could bring a number of problems :
- Performances : for the automatic login system, we need many data passed to
the handler, for example sessionUser, cookieUser and the like. We don't need to
pass the cookies if the user is already logged in for example, or even if a
session is already present (we need to check only the first request that
arrives). In more complicated situations ths could be a huge performance impact.
- Conflicts : anyone wanting to do something before every request will create a
setup method in the RootWebHandler, and if they need environment informations
(like sessionUser) they will try to add the setter. This can easily lead to
conflicts.
Also there are some limitations, for example the user may expect to be able to
do something in the setup method, like redirecting to a login page, or
displaying an error page telling something as a result of parameters evaluation.
While we don't want to provide something with all the features of an aspect
(using which all this stuff is already possible), it could help to have the
setup method return an HtmlProducer. If it return null, nothing happens, if it
returns something, that something is returned instead of the doMethod that was
originally being called.
This solution can also help mitigate the performance problem. Infact an
"AutomaticLoginHandler" could handle all the automatic login stuff in it's
doCheckForLogin method, environmental getters and setters would be placed only
in that handler. That do would return null by default, since there is no need
to do anything if the user cannot be automatically logged in. In the
rootWebHandler only a setup method calling that do method if no session user is
present is needed, avoiding the binding of all cookies if not necessary.
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