I saw the same issue in many different locations and even after a good portion of googling, I could not resolve it. What I am trying to do (the bigger picture) is to go through
The ant task is not included in the JDK, even though there is a wsimport.exe which does exactly the same.
If you really want the ant task, you can download jaxws-ri and use the 23(!) jars in the lib folder.
Or you can use this workaround by calling wsimport.exe: