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<TITLE>Writing Advanced Applications, Chapter 7: Servlet Debugging</TITLE>
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<FONT SIZE="-1"><A HREF="/developer/onlineTraining/index.html">Training Index</A></FONT>
<H2>Writing Advanced Applications<BR>
<FONT SIZE="3">Chapter 7 Continued: Servlet Debugging</FONT></H2>
<FONT SIZE="-1">[<A HREF="running.html"><<BACK</A>] [<A HREF="index.html#contents">CONTENTS</A>] [<A HREF="awt.html">NEXT>></A>]</FONT></DIV>
<P>You can debug servlets with the same <CODE>jdb</CODE> commands
you use to debug an applet or an application.
The Java<FONT SIZE="-2"><SUP>TM</SUP></FONT>
Servlet Development Kit (JSDK) provides a standalone
program called <CODE>servletrunner</CODE> that lets you run a servlet
without a web browser. On most systems, this program simply runs
the <CODE>java sun.servlet.http.HttpServer</CODE> command.
You can, therefore, start a <CODE>jdb</CODE> session with the
<CODE>HttpServer</CODE> class.
<P>
A key point to remember when debugging servlets is that Java Web server and
<CODE>servletrunner</CODE> achieve servlet loading and unloading by
not including the <CODE>servlets</CODE> directory on the <CODE>CLASSPATH</CODE>.
This means the servlets are loaded using a
custom classloader and not the default system classloader.
<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#run">Running servletrunner in Debug Mode</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#web">Running Java Web Server<FONT SIZE="-2"><SUP>TM</SUP></FONT> in Debug Mode</A></FONT>
</UL>
<HR>
<A NAME="run"></A>
<H3>Running servletrunner in Debug Mode</H3>
In this example, the <CODE>servlets</CODE> examples directory is included
on the <CODE>CLASSPATH</CODE>.
You can configure the CLASSPATH for debug mode as follows:
</FONT>
<PRE>
Unix
$ export CLASSPATH=./lib/jsdk.jar:./examples:$CLASSPATH
Windows
$ set CLASSPATH=lib\jsdk.jar;examples;%classpath%
</PRE>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
To start the servletrunner program you can either run the
supplied startup script called <CODE>servletrunner</CODE> or just supply
the servletrunner classes as a parameter to <CODE>jdb</CODE>. This example uses the parameter to servletrunner.
</FONT>
<PRE>
$ jdb sun.servlet.http.HttpServer
Initializing jdb...
0xee2fa2f8:class(sun.servlet.http.HttpServer)
> stop in SnoopServlet.doGet
Breakpoint set in SnoopServlet.doGet
> run
run sun.servlet.http.HttpServer
running ...
main[1] servletrunner starting with settings:
port = 8080
backlog = 50
max handlers = 100
timeout = 5000
servlet dir = ./examples
document dir = ./examples
servlet propfile = ./examples/servlet.properties
</PRE>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<P>
To run <CODE>SnoopServlet</CODE> in debug mode, enter the following URL
in a browser where <CODE>yourmachine</CODE> is the machine where you started
servlet runner and <CODE>8080</CODE> is the port number displayed
in the settings output.
</FONT>
<PRE>
http://yourmachine:8080/servlet/SnoopServlet
</PRE>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<P>
In this example <CODE>jdb</CODE> stops at the first line of
the servlet's <CODE>doGet</CODE> method. The browser will wait for a
response from your servlet until a timeout is reached.
<p>
</FONT>
<PRE>
main[1] SnoopServlet: init
Breakpoint hit: SnoopServlet.doGet (SnoopServlet:45)
Thread-105[1]
</PRE>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
We can use the <CODE>list</CODE> command to work out where <CODE>jdb</CODE>
has stopped in the source.
<P>
</FONT>
<PRE>
Thread-105[1] list
41 throws ServletException, IOException
42 {
43 PrintWriter out;
44
45 => res.setContentType("text/html");
46 out = res.getWriter ();
47
48 out.println("<html>");
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