You could have your Java client add a header when talking to your webservice, possibly a specific user agent. Or have it send a accept header which includes a version:
GET /jobs HTTP/1.1
Host: api.example.com
Accept: application/vnd.example.api+json;version=2
Your webservice can check the presence of the Accept header and the version string in it and decide what to do based on that.
You could return a value that is compatible with the old client and a different value that is compatible with the new client. or you can simply return a 404 or a 501 status code with an error message to the old client.
The generic process is explained here in detail:
Applying this to PHP and Java is relatively simple:
As an alternative you can add a version value to your request using a post variable, querystring parameter or as part of the request body (in JSON). I personally like it less than the use of accept headers, since the last is more flexible in the long run. You can even add multiple values to an accept header to indicate which versions you support. Your server can reply with the version it responded with. Giving you a better long-term solution.