When the input request body is of the type "application/x-www-form-urlencoded", a.k.a. an HTML Form, you can inject individual form parameters from the request body into method parameter values.

<form method="POST" action="/resources/service">
First name: 
<input type="text" name="firstname">
<br>
Last name: 
<input type="text" name="lastname">
</form>

If you post through that form, this is what the service might look like:

@Path("/")
public class NameRegistry {

   @Path("/resources/service")
   @POST
   public void addName(@FormParam("firstname") String first, @FormParam("lastname") String last) {...}

You cannot combine @FormParam with the default "application/x-www-form-urlencoded" that unmarshalls to a MultivaluedMap<String, String>. i.e. This is illegal:

@Path("/")
public class NameRegistry {

   @Path("/resources/service")
   @POST
   @Consumes("application/x-www-form-urlencoded")
   public void addName(@FormParam("firstname") String first, MultivaluedMap<String, String> form) {...}