ch11.htm

JAVA Developing Professional JavaApplets

know when a model has changed. For this situation, the Observable
class and Observer interface in the java.util package offer an
effective solution. The Observable class represents the model.
To create an Observable model, you need to build a subclass of
Observable. Observer objects can then attach to your model; when
the model changes, you notify the observers.
<P>
In the applet portion of the chapter project, a class called ElectionTable
maintains a local cache of election results on a state-by-state
and grand-total basis; it is a direct subclass of Observable.
When its data is modified, it notifies the Observer objects of
the changes:
<BLOCKQUOTE>
<TT>setChanged();<BR>
notifyObservers();</TT>
</BLOCKQUOTE>
<P>
The first method, <TT>setChanged()</TT>,
tells the Observable class that the model has been modified. When
a notify method is next called (immediately afterward, in this
case), the Observable class checks to see whether the data has
changed and, if so, broadcasts a notification to the observers.
Table 11.1 lists a summary of the Observable class's methods.
<BR>
<P>
<CENTER><B>Table 11.1. The Observable class's methods.</B></CENTER>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD WIDTH=163><I>Method</I></TD><TD WIDTH=427><I>Description</I>
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>addObserver</TT>
</TD><TD WIDTH=427>Add an Observer object to the list of observers.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>deleteObserver</TT>
</TD><TD WIDTH=427>Remove an Observer from the observer list.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>deleteObservers</TT>
</TD><TD WIDTH=427>Clear the observer list.</TD></TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>notifyObservers</TT>
</TD><TD WIDTH=427>Notify all the Observers that the data has changed. One version of this method has an optional parameter to send data about what was modified.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>setChanged</TT>
</TD><TD WIDTH=427>Signals internally that a change has occurred.
</TD></TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>hasChanged</TT>
</TD><TD WIDTH=427>Returns whether the data has changed.</TD>
</TR>
<TR VALIGN=TOP><TD WIDTH=163><TT>countObservers</TT>
</TD><TD WIDTH=427>Returns a count of all the Observers.</TD>
</TR>
</TABLE></CENTER>
<P>
<P>
The Observer interface has only one method, <TT>update()</TT>.
It takes as its parameters the Observable object and an object
that can be used to convey more information about what has changed.
The Observer object hooks to the Observable object through the
<TT>addObserver()</TT> method. In
this chapter's project, the two observers simply repaint when
they get an <TT>update()</TT> message.
<H2><A NAME="ChapterProject"><FONT SIZE=5 COLOR=#FF0000>Chapter
Project</FONT></A></H2>
<P>
This chapter project demonstrates an applet and corresponding
Java server used to show you election night returns as they come
in. When you select the Election home page, the applet connects
to the server and is added to a list of registrants to be notified
whenever election data changes. The server uses its local database
to check for any incoming election returns. When they occur, the
server broadcasts the results to the client applets. The applets,
in turn, update themselves to show the latest totals.
<P>
To add a little fun to the project, the system was designed for
the likely 1996 presidential election between the incumbent and
his challenger. (Note that it wouldn't be hard to change the project
if a third-party candidate throws a monkey wrench into this mix,
since the project is mostly driven by the database. However, a
few assumptions about there being two candidates were made.) Figure
11.1 shows the Election applet before the returns start rolling
in, Figure 11.2 illustrates what the applet might look like later
in the evening, and Figure 11.3 shows the applet as the election
reaches its possible conclusion. This last figure demonstrates
the applet with its graphical view turned on.
<P>
<A HREF="f11-1.gif" ><B>Figure 11.1 : </B><I>The Election applet before the election returns start rolling in.</I></A>
<P>
<A HREF="f11-2.gif" ><B>Figure 11.2 : </B><I>The Election applet as the evening progresses. It's a close one.</I></A>
<P>
<A HREF="f11-3.gif" ><B>Figure 11.3 : </B><I>The applet as the election reaches in climax-who is going to win?</I></A>
<P>
You are given a database consisting of filled-in states and corresponding
electoral votes. However, the totals are left blank. If you want,
you can rig the election in favor of the candidate of your choice!
<H3><A NAME="GeneralArchitectureoftheProject">General Architecture
of the Project</A></H3>
<P>
The general architecture of this project is based on the project
architecture from <A HREF="ch10.htm" >Chapter 10</A>, &quot;Native
Methods and Java.&quot; For the sake of clarity, a visual overview
of the architecture is repeated in Figure 11.4. Refer to <A HREF="ch10.htm" >Chapter 10</A>
for a description of the general architecture and detailed server
and database implementation, but a couple of additional points
about the structure of the project are covered here. Furthermore,
a full description of the client applet will follow in the section
&quot;Applet Client.&quot;
<P>
<A HREF="f11-4.gif" ><B>Figure 11.4 : </B><I>The project architecture.</I></A>
<P>
The hierarchy of the server environment is as follows: The server
classes should be placed in the parent directory (which could
be in a variety of places); the database (ELECT.DBF) can also
be in this directory, but if your ODBC data source is set up properly,
the database can be located elsewhere.
<P>
Underneath the parent is a directory called <I>htdocs</I>. The
only file it must have is index.html, which is the HTML for the
applet at hand. Located underneath htdocs is a <I>classes</I>
subdirectory. This will contain classes and any additional files
the client applet will need.
<P>
To start the server, go to the parent directory and type:
<BLOCKQUOTE>
<TT>java BasicWebServer</TT>
</BLOCKQUOTE>
<P>
An additional <TT>debug 1</TT> parameter
will display debug information about the server.
<H3><A NAME="ChangestotheServer">Changes to the Server</A></H3>
<P>
As in the previous chapter, the ElectionServer class is used to
send data to multiple client applets. It does this at the behest
of the BasicWebServer class because it implements the LiveDataServer
interface. The main part of this Thread class is its <TT>run()</TT>
method, which constantly looks for new data and broadcasts its
results to the client applets. This code was slightly modified
from <A HREF="ch10.htm" >Chapter 10</A> for a fuller implementation
of the election night project. Only the <TT>run()</TT>
method was modified, as illustrated in Listing 11.1.
<HR>
<BLOCKQUOTE>
<B>Listing 11.1. Changes to the </B><TT><B><FONT SIZE=1 FACE="Courier">run()</FONT></B></TT><B>
method of the ElectionServer Class.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>/**<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Run method for this thread.<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Recheck the database every 30 seconds
<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * and send changed data<BR>
&nbsp;&nbsp;&nbsp;&nbsp; * Resend all data every couple minutes.
<BR>
&nbsp;&nbsp;&nbsp;&nbsp; */<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public void run()<BR>
&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp; int timestamp = 0;<BR>
&nbsp;&nbsp;&nbsp;&nbsp; int iterationsBeforeFull = 60;&nbsp;&nbsp;//
Fifteen minutes<BR>
&nbsp;&nbsp;&nbsp;&nbsp; int iteration = 0;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if ( results !=
null )<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;servThread.start();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while(true)
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sleep(30);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;SQLStmt
nn;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
After so many iterations, resend all data...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(iteration &gt;= iterationsBeforeFull) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
nn = election.sql(&quot;select * from election order by </TT><FONT FACE="ZapfDingbats">&Acirc;</FONT><TT>state,candidate&quot;);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
synchronized (results)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;results
= nn;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
servThread.sendToUsers(<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;formatResults(&quot;RESULTS&quot;,
results));<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
iteration = 0;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
// end full if<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else
{<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Otherwise just check timestamps...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String
partialSQL =<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&quot;select
* from election where updated &gt; '&quot; +<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;timestampToChar(timestamp)
+<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&quot;'
order by state,candidate&quot;;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;nn
= election.sql(partialSQL);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Make sure a valid number of rows is returned...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
((nn.numRows() &gt; 0) &amp;&amp; ((nn.numRows()%2) == 0) ) {
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Send data and update timestamp...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;servThread.sendToUsers(
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;formatResults(&quot;RESULTS&quot;,
nn));<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;++timestamp;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;++iteration;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch
(DBException de)<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println(&quot;Error:
&quot; + de);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}</TT>
</BLOCKQUOTE>
<HR>
<P><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD><B>Note</B></TD></TR>
<TR VALIGN=TOP><TD>
<BLOCKQUOTE>
Note that the database used here does not have a &quot;triggering&quot; capability as many relational databases do. With triggers, the database would update the server, much like the model-view paradigm discussed earlier. This is much better than having the server query the database all the time.</BLOCKQUOTE>

</TD></TR>
</TABLE></CENTER>
<P>
<P>
There were mainly two changes to the method. The first was the
introduction of partial updates. After broadcasting all the election
data (in the <TT>NewRegistrant()</TT>
method, which is not listed here), the server loops and waits
for partial updates. The server uses the timestamp mechanism discussed
in the next section, &quot;Database,&quot; to see whether data
has changed. Every 30 seconds, the server queries the database
to see whether any data has been added with a timestamp older
than the current timestamp. If not, the server sleeps again and
then retries. If new data is found, the changes are broadcast
to the client applets. The timestamp is then incremented and the
process repeats itself.
<P>
The other change in the server's <TT>run()</TT>
method is indicated by the <I>iteration</I> variable. Every 15
minutes, the server rebroadcasts <I>all</I> of the data to the
clients. This might seem unnecessary, but remember that the datagram
protocol used to transmit data is unreliable, and packets could