A Guest Book Servlet
You have probably heard quite a bit of talk about Java applets. We
discussed in Chapter 8, however, how doing
database access in the client is a really bad idea. We have packaged
with the examples in this book an example that contains a real
application that uses the JDBC knowledge we have discussed in this
chapter to create a server-side Java class known as a servlet. While
servlets are not in themselves part of the three-tier solution we
discussed in Chapter 8, this example should
provide a useful example of how JDBC can be used. The servlet in
question is a web page that lets people visiting your site enter
comments about it. Others can then view these comments. For this
example, all you need to know about servlets is that the
doPost()
method handles HTTP
POST
events and doGet()
handles
HTTP GET
events.
There are two pieces to this servlet: the get and the post. In both
pieces, a call is made to
printComments()
to show the comments in the guest
book. In this method, we encounter something we have not yet seen in
the previous simple examples, a call to
wasNull()
after each column value is retrieved. As
its name implies, wasNull()
returns
true
if the last value fetched was SQL
NULL
. For calls returning a Java object, the value
will generally be NULL
when a SQL
NULL
is read from the database. In these
instances, wasNull()
may appear somewhat
redundant. For primitive datatypes, however, a valid value may be
returned on a fetch. The wasNull()
method gives you ...
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