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HitCount tutorial: Using Debugger and OLT
6.7.hc: HitCount tutorial: Using Debugger and OLT
HitCount is an example servlet that installs by default with IBM Websphere Application
Server. This tutorial demonstrates how to use the IBM
Distributed Debugger and Object Level Trace (OLT) on the HitCount servlet,
referring to screen captures from the administrative console (WebSphere
Administrative Console) used with the Application Server Advanced Single
Server Edition.
Let's begin.
- Start the Administrative Console:

- Select the Default Server for your node in the topology view on the left
side of the console, and then select the JVM Settings tab in the right
side of the console:

- Click the Advanced JVM Settings pushbutton and then, in the Advanced
JVM Settings dialog box, select the Enable
IBM Distributed Debugger checkbox:

You will be prompted with a Confirm
dialog message, asking you if you would like to enable OLT. Click Yes
to enable OLT:

The entry in the Advanced JVM Settings dialog box Source Path field indicates to the Debugger where
servlet
source files, EJB source files, and JSP files can be found. Use semicolons to separate all
entries in the Source Path. For this sample, add the following to the source
path:
<WAS_ROOT>\installedApps\sampleApp.ear\examples.war;<WAS_ROOT>\installedApps\sampleApp.ear\examples.war\WEB-INF\classes
where WAS_ROOT is where WebSphere Application Server is installed
(eg. c:\WebSphere\AppServer).
Click OK when you are finished, and then click Apply
in the JVM Settings tab to save your changes.
- To see the OLT settings, select the Services tab:

Select the Object Level Trace Service and then click Edit Properties.

In the OLT server host field, specify the OLT host by entering the name of
the machine where you will be running the OLT tool. The default value is localhost.
The OLT server port should not be
changed from its default of 2102, unless there is a port conflict (see the Note
below). If you do not need to change any of the OLT properties in the Object
Level Trace dialog box, click Cancel. If you do change any of
the OLT properties, click OK and then Apply to save your changes.
- Start the Default Server by right-clicking on it and selecting Start
from the pop-up menu.
- Once you have started the application server, start OLT by issuing
the olt command at a command prompt. You will then see the following screen:

Ensure that the Execution mode is set to Trace and debug. If it is not, click the
drop-down list and select it. In the Debugger hostname field, specify the name of the machine
where the debugger tool is located. Assuming for this example that
the Debugger is installed locally, enter the host name of the machine on which
you are working. Keep the Debugger TCP/IP port set to 8001 (change it only if
port 8001 is already in use on your machine). Click Apply.
- Invoke the HitCount servlet by typing its URL in a Web browser:
http://<your_machine>/webapp/examples/HitCount
OLT will trace the initial calls to the HitCount servlet and JSP:

Your browser should now be displaying the following:

-
Let's enable step-by-step debugging now.

-
In the browser, choose the Enterprise Java Bean radio button and Transaction Type
None. Click the Increment button, and OLT will start tracing the events taking
place:
-
Highlight the service method and Click OK. This will cause the debugger to attach and suspend in the first executable line of the
HitCount service method:

-
You can now choose to step over methods, step into others, or run the
program. Click Run in the Debugger toolbar.
Because OLT is in Step-by-Step mode, when it encounters the next method that
can be debugged, it will ask you again (via the Method breakpoints dialog box)
if you would like to step into the method. Indicate that you would like to
skip breaking into the enterprise bean. An enterprise bean is displayed as two
distinct objects in the trace: the enterprise bean object itself, and an
object called the ProgrammingModel. The ProgrammingModel is an EJB Factory
instance that is beyond application logic and is not intended to be debugged.
-
To skip breaking into a method, click Cancel in the Method breakpoints
dialog:
The EJB trace lines in the trace are IncBean and ProgrammingModel, as
shown:

-
To break into the JSP, step into the _jspService method:
The debugger will enter the JSP code, displaying the following:

-
You can now choose to step over, step debug, run, and so on. If you
click Run, then HitCount should run to completion. Note that your browser may have
timed out by this time. For information about
increasing browser timeout, please see OLT Troubleshooting - Browsers
in the OLT online documentation.
Note:
If you modify the Port in the IBM Distributed Debugger and
Object Level Trace settings frame to a port other than 2102, you must start
OLT and modify the OLT Server TCP/IP port
accordingly. The OLT Server TCP/IP port is set in the Browser Preferences
dialog box (accessed by selecting File > Preferences from
the OLT menu bar and then selecting the OLT node). If there is a port conflict and you
are unable to start OLT, go to the %userprofile%\DbgProf directory and modify the
OLT_TRC_SRVAPP_PORT in the dertrenv.dat file accordingly.
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