Okay. Got it. The argument key and value must be several items in the args array.
Instead of:
args “${devLibs}/yuicompressor-2.4.2.jar”, “test.js”, “-o test-min.js”
it must be
args “${devLibs}/yuicompressor-2.4.2.jar”, “test.js”, “-o”, “test-min.js”
Reference: JavaExec args are not interpreted by the java command