Fall 2005, David Matuszek
The model is the part that actually plays the Animals Game; it is not concerned in any way with communicating with the user. All communcation with the "outside world" is through method calls.This means that it can be used equally well in an applet, an application, or a web application; with a GUI, a text interface, or an HTML interface.
A model often consists of more than one class. In this case, you probably have
some "master class" called something like
that uses my
BinaryTree class. You might have other classes as
When the servlet starts, it should create a new object of type
(or whatever you called it). This should only be done once. Remember that servlets
init() method for one-time initializations.
To avoid dealing with special cases at startup, the Animals Game should be initialized with a nonempty binary tree. The one I usually use is:
Does it live in the water?
You can, of course, use any initial binary tree you like.
The binary tree grows only at the leaves. The animal name in the leaf node is replaced by a question; two children are added to the node, one containing the animal name that was in it, and the other containing the name of the new animal.
Does it live in the water?
Yes: Does it eat people?
Notice that there is no need to replace a leaf node; you only need to replace its contents. In the above example, the value "frog" is replaced by the value "Does it eat people?"
Since (we will pretend) many thousands of people may be using our binary tree every second, and parts of it will be changing, we must synchronize access to the parts that can change. We can synchronize on individual nodes; we can synchronize on certain methods. Doing this correctly is a difficult problem; I don't expect anyone to really get it right, but I hope you will think about it.
The animals binary tree should persist (and grow) throughout the lifetime of
the servlet. The most obvious (and probably the best) place to keep this binary
tree is within the Animals Game class. It is worth noting, however, that the
ServletContext object also has the desired lifetime, and can be
used to store arbitrary objects..
In this program you have an
HttpServlet that acts as the controller.
Its main job is to get a request from the user and give it to the Animals Game
model; and to take the response from the Animals Game model and forward it to
the view (JSP). It doesn't actually have to know anything about what kind of
data is passing through it.
The controller also maintains session information. In this assignment, session information is simply knowledge about where this particular player is in the game. Many games can be played in a session, but only one at a time. Again, though, the controller can keep this information without actually knowing what it means.
Where do you keep session information? You have basically two choices:
HttpSessionobject. This object can hold any number of attributes and their values, where the values can be objects of any kind.
The JSP view takes a response and formats it into a Web page, which
it then returns to the client (user). It does not return to the
The flow of control goes: User -->
HttpServlet --> Model --> HttpServlet --> JSP --> User.
The user sees four or five different kinds of pages:
You could write a single JSP file that handles all these cases (it has to look at the information from the model to decide which kind of page to produce). The disadvantage is that the result can be quite lengthy.
You could have the controller look at the information from the model and decide to which JSP to forward the request. The disadvantage is that the controller has to know something about the program in order to make this decision.
You could write a JSP file that examines the information from the model, and chooses to which other JSP it will forward the request. The disadvantage is that you could end up with duplicated code in the various JSPs.
In any case, notice that when the controller forwards a request to a JSP, the
JSP only gets two objects: the
HttpRequest object and the
object. Any information that the controller wants to send to the JSP must
be put in the
HttpRequest object. The JSP can, of course, import
the Animals Game class, but this gives it access only to static
information in that class.