Generally you can debug a program by double-clicking the left margin of the editor until you get a round blob which represents a breakpoint (blue on eclipse, red on NetBeans). Your execution stops when you get to the 1st breakpoint, then you can use the step-into, step-over and step-return instructions to go to the next line, to the next line without seeing details of the method call, and to the end of the method. At all stages you can get a debug perspective which shows details of all the fields and local variables. You can probably use f keys (I think f5 f6 f7) instead of step-into step-over and step-return.
Don't know whether there are special methods for Struts and Tomcat. Sorry
There's no need for special debugging assistance in Struts, but in order to debug any Tomcat application, the Eclipse debugger has to connect to Tomcat.
If you're using the sysdeo Tomcat plug-in or something similar, this is done when you use Eclipse to start Tomcat. For stand-alone Tomcat applications, start Tomcat with remote debugging turned on (this doesn't have to be on a remote machine). Then create a remote debugging profile in Eclipse (use the Debug dialog) and connect to the running copy of Tomcat in the same way you'd connect to any external Java application.
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