For my use case, none of the previous answers worked because I was writing a server-side unit test which was failing due the following error message as described in the Unable to Mock Glassfish Jersey Client Response Object question:
java.lang.IllegalStateException: Method not supported on an outbound message.
at org.glassfish.jersey.message.internal.OutboundJaxrsResponse.readEntity(OutboundJaxrsResponse.java:145)
at ...
This exception occurred on the following line of code:
String actJsonBody = actResponse.readEntity(String.class);
The fix was to turn the problem line of code into:
String actJsonBody = (String) actResponse.getEntity();