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Charles Proxy is a great tool, and I heartily recommend it to web developers. It allows you to see all the details of a web request (kind of like the live headers Mozilla plugin), but system wide. It is very useful when you're trying to figure out what parameters/etc are actually getting passed around. It even works with ssl requests.

To tell Groovy/Java to use the proxy, choose one:

Properties props= new Properties(System.getProperties());
props.put("http.proxySet", "true");
props.put("http.proxyHost", "localhost");
props.put("http.proxyPort", "8888");
Properties newprops = new Properties(props);
System.setProperties(newprops);

Or add these parameters to your invocation of java:
-Dhttp.proxyHost=proxyhostURL
-Dhttp.proxyPort=proxyPortNumber