Question

I am developing a restlet server inside a Glassfish server that will receive petitions from an Android client.

Based on the Hello World application and on the Object serialization tutorial

Here’s some code:

---

Server Side

MyUser.java

package com.server.common;

import java.io.Serializable;

public class MyUser implements Serializable {

private static final long serialVersionUID = 1L;
private String username;

public MyUser() {}

public MyUser(String username) {
    super();
    this.username = username;
}

public String getUsername() {
    return username;
}

public void setUsername(String username) {
    this.username = username;
}
}

UserResource.java

package com.server.common;

import org.restlet.resource.Get;

public interface UserResource {
@Get
public MyUser getUser();
}

UserServerResource.java

package com.server;

import org.restlet.resource.Get;
import org.restlet.resource.ServerResource;
import com.server.common.MyUser;
import com.server.common.UserResource;

public class UserServerResource extends ServerResource implements UserResource {

@Get
public MyUser getUser() {
    MyUser u = new MyUser("Nickname from server");
    return u;
}
}

---

Client Side

• The same UserResource.java and MyUser.java inside the package com.client.common

• MainActivity.java extract

  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  ClientResource cr = new ClientResource(
          "http://MYSERVERIP:8080/FamilyWeb/username");
  resource = cr.wrap(UserResource.class);
  
  try {
      MyUser u = resource.getUser();
      Log.w("success",u.toString());
  } catch (Exception e) {
      e.printStackTrace();
  }
  

---

LogCat

09-29 22:31:52.650: W/System.err(1409): java.lang.NullPointerException
09-29 22:31:52.650: W/System.err(1409):     at com.dimunoz.family.tablet.MainActivity.onCreate(MainActivity.java:33)
09-29 22:31:52.650: W/System.err(1409):     at android.app.Activity.performCreate(Activity.java:4465)
09-29 22:31:52.650: W/System.err(1409):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
09-29 22:31:52.660: W/System.err(1409):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
09-29 22:31:52.660: W/System.err(1409):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
09-29 22:31:52.660: W/System.err(1409):     at android.app.ActivityThread.access$600(ActivityThread.java:123)
09-29 22:31:52.660: W/System.err(1409):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
09-29 22:31:52.660: W/System.err(1409):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-29 22:31:52.660: W/System.err(1409):     at android.os.Looper.loop(Looper.java:137)
09-29 22:31:52.660: W/System.err(1409):     at android.app.ActivityThread.main(ActivityThread.java:4424)
09-29 22:31:52.660: W/System.err(1409):     at java.lang.reflect.Method.invokeNative(Native Method)
09-29 22:31:52.660: W/System.err(1409):     at java.lang.reflect.Method.invoke(Method.java:511)
09-29 22:31:52.660: W/System.err(1409):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
09-29 22:31:52.660: W/System.err(1409):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
09-29 22:31:52.660: W/System.err(1409):     at dalvik.system.NativeStart.main(Native Method)

If I change the getUser() method to return a String object and make the respective changes, my Android client gets the response, but if I run the code shown above, I get a NullPointerException.

What am I missing?

Answer 1

The problem was that the name of the common files packages was different in the server and the client, so the solution was to rename them to the same name.

In the Restlet Mailing List one of the moderators, Thierry Boileau, answered me why they should have the same name:

the common classes between the server and client parts (such as the classes
for the bean objects that are serialized/deserialized) must be located in
the same packages. The reason is that the serialization/deserialization
process leverages conventions, in particular convention regarding the full
path of the bean classes are referenced inside the JSON representation.
All other classes such as ClientResource, annotated interfaces could be
located in distinct packages between client/server projects.