Resolution:

Class loading is not done due to incorrect or erroneous installation of the Hawk package.

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Resolution: Faced the below error while opening or running the Domainutility tool and if you are in GUI mode then able to see the popup error mentioning the "Unable to find the start class".

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Failed to find the start class, "com/tibco/administrator/du/DomainUtility" - bad classpath?
Unable to find the start class
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This happens because the domainutility tries to find the start class in the path provided and not able to locate it and fails with error. In such a scenario, try the following:

1). Run the domainutility with option "--debug" and check the class path in output. 
On Windows machine, please open a DOS prompt as Administrator then run below command:

cd %TIBCO_HOME%\tra\5.x\bin
wrap.exe --propFile domainutility.tra --debug

For Linux/Unix machine, please run domainutility with command like:

cd $TIBCO_HOME/tra/5.x/bin
./domainutility --debug

2). Try to locate "domainutility.jar" in the /tra/5.x/lib/ directory. If you do not find the file then try to copy the same file from a working environment to the non-working environment. Test the result after doing this.

3). If the above does not work then check the Hawk installation. In most cases the Hawk installation causes this problem and does not allow the domainulitity to start, load and execute. Try to reinstall the Hawk package and test the result.