STS launches Tomcat with the parameters specified in the launch configuration. It does not evaluate the regular startup files for setting JVM or command line parameters.
As Christian pointed out in the other blog post: From the Servers view, double click on the Server to open the Server Editor. In the Server Editor click "Open Launch Configuration" in the left column. This brings up the configuration dialog. Click on the Arguments tab and enter your JVM arguments.
Have you tried adding your parameter that way?
Steffen
Steffen Pingel
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Committer, SpringSource Tool Suite
Senior Developer, Tasktop