Help! Some confusing problems about gwt rpc with generics!

I make a test about gwt rpc these days, and meet some confusing
problems.
At first, I created a project named testGwtRpc by the gwt plug-in.
Certainly,this is a
helloworld project,and it works fine.
Second,I made some changes for the project,especially adding the
generics on the rpc service
interface. Just like this below:
public interface GenericsRpcService<T,K> extends RemoteService{

T callServer(List<K> id);

T callServer(K id);
}

@RemoteServiceRelativePath("greet")
public interface RpcService extends GenericsRpcService<String,
String>{

}

Then ,some strange happened.In the client page, it failed when I
called the function T callServer(K id) ,but worked fine when calling T
callServer(List<K> id). The correspongding exception is as follows:
Starting Jetty on port 8888
[WARN] testServlet: An IncompatibleRemoteServiceException was
thrown while processing this call.
com.google.gwt.user.client.rpc.IncompatibleRemoteServiceException:
java.lang.ClassNotFoundException: aaaaaa
at com.google.gwt.user.server.rpc.RPC.decodeRequest(RPC.java:315)
at
com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:
206)
at
com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:
248)
at
com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:
62)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:
487)
at
org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:
362)
at
org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:
216)
at
org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:
181)
at
org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:
729)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:
405)
at
org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:
152)
at
org.mortbay.jetty.handler.RequestLogHandler.handle(RequestLogHandler.java:
49)
at
org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:
152)
at org.mortbay.jetty.Server.handle(Server.java:324)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:
505)
at org.mortbay.jetty.HttpConnection
$RequestHandler.content(HttpConnection.java:843)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:647)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
at
org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:
395)
at org.mortbay.thread.QueuedThreadPool
$PoolThread.run(QueuedThreadPool.java:488)
Caused by: com.google.gwt.user.client.rpc.SerializationException:
java.lang.ClassNotFoundException: aaaaaa
at
com.google.gwt.user.server.rpc.impl.ServerSerializationStreamReader.deserialize(ServerSerializationStreamReader.java:
572)
at
com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamReader.readObject(AbstractSerializationStreamReader.java:
119)
at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamReader
$ValueReader$8.readValue(ServerSerializationStreamReader.java:137)
at
com.google.gwt.user.server.rpc.impl.ServerSerializationStreamReader.deserializeValue(ServerSerializationStreamReader.java:
381)
at com.google.gwt.user.server.rpc.RPC.decodeRequest(RPC.java:303)
... 22 more
Caused by: java.lang.ClassNotFoundException: aaaaaa
at java.lang.ClassLoader.findClass(ClassLoader.java:359)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at
org.mortbay.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:
352)
at
org.mortbay.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:
337)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at
com.google.gwt.user.server.rpc.impl.ServerSerializationStreamReader.deserialize(ServerSerializationStreamReader.java:
542)
... 26 more

Actually, I think it is because parameters would be compiled as
Object type when we used generics in the gwt rpc service interface. It
is undoubted that the Object type can't be serialized in the gwt rpc.
So the exception was threw. But after I tried the function T
callServer(List<K> id), I knew the problem was not so simple.
So, my questions are:
How does the gwt compiler work for the java generics in gwt rpc
service interface?
What differences are there between those two functions above?

Attached below is the testGwtRpc project.

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