GWT SerializationException__Ljava_lang_String_2V while doing RPC

Below are my service, serviceImpl and async callback interfaces. Appreciate if someone can help me understand why the submitTeam(...) isn't being called (I howeve see that isValidEmail(...) is being invoked when used). At least provide me the approach for debugging this as currently I am unable to use eclipse debugger (eclipse is not stopping at breakpoints) and the sysout/syserr statements are not being logged to the console either :-(.

@RemoteServiceRelativePath("registrationService")
public interface RegistrationService extends RemoteService
{
    Boolean isValidEmail(String email);

    String submitTeam(String teamName, List<Player> players);
}

public interface RegistrationServiceAsync
{
    void isValidEmail(String email, AsyncCallback<Boolean> callback);

    void submitTeam(String teamName, List<Player> players, AsyncCallback<String> callback);
}

public class SubmitTeamCallback implements AsyncCallback<String> {
    private final Label status;

    public SubmitTeamCallback(Label s) {
        status = s;
    }

    public void onFailure(Throwable caught) {
        status.setText(caught.getMessage());
    }

    public void onSuccess(String result) {
        System.out.println("" + getClass().getName() + " : " + result);
        status.setText(result);
    }
}


public class RegistrationCallback implements AsyncCallback<Boolean>
{
    private final Label         status;
    private final PlayerWidget  playerWidget;
    private Boolean             isValidSharedFlag;

    public RegistrationCallback(Label s, PlayerWidget pw, Boolean isValid)
    {
        status = s;
        playerWidget = pw;
        setIsValidSharedFlag(isValid);
    }

    public void onFailure(Throwable caught)
    {
        status.setText(caught.getMessage());
    }

    public void onSuccess(Boolean result)
    {
        if (result.equals(Boolean.FALSE))
        {
            playerWidget.invalidEmail();
            String oldText = status.getText();

            status.setText(oldText + "Please specify a valid email address for the captain.");
            setIsValidSharedFlag(Boolean.FALSE);
        } else
        {
            playerWidget.validEmail();
            String newText = status.getText().replace("Please specify a valid email address for the captain.", "");
            status.setText(newText);
            setIsValidSharedFlag(Boolean.TRUE);
        }
    }

    public void setIsValidSharedFlag(Boolean isValidSharedFlag)
    {
        this.isValidSharedFlag = isValidSharedFlag;
    }

    public Boolean getIsValidSharedFlag()
    {
        return isValidSharedFlag;
    }
}

public class RegistrationServiceImpl extends RemoteServiceServlet implements
        RegistrationService {
    Logger logger = Logger.getLogger("");
    private final Emailer emailer = new Emailer();
    private final EmailValidator validator = new EmailValidator();

    public Boolean isValidEmail(String email) {
        return validator.validate(email);
    }

    public String submitTeam(String teamName, List<Player> players) {
        logger.log(Level.SEVERE, "This is a test log");

        System.out.println("This is a test log");
        boolean emailSent = false;
        for (Player p : players) {
            System.out.println("Emailing to captain");
            if (p instanceof Captain) {
                if (!validator.validate(p.getEmail())) {
                    return "Please specify a valid email";
                }
                System.out.println("Emailing to captain");
                emailSent = emailer.email(p);
            }
        }
        if (emailSent) {
            System.out.println("Email sent successfully");
            return teamName
                    + " has been successfully registered. Please pay the registration fee to confirm registration. Thank you.";
        } else {
            return "Unable to send email. Please email the team details [Team name, minimum of 6 players, captain's email address and phone number] to funpluscharity@gmail.com";
        }
    }
}

Below method is going to invoke the RPC calls

private void registerTeam() {
            System.out.println("Received request to dubmit team");
            StringBuilder statusText = new StringBuilder();
            try {
                Boolean isValid = true;

                RegistrationServiceAsync service = (RegistrationServiceAsync) GWT
                        .create(RegistrationService.class);
                ServiceDefTarget serviceDef = (ServiceDefTarget) service;
                System.err.println(".........." + GWT.getModuleBaseURL());
                serviceDef
                        .setServiceEntryPoint("/services/registrationService");

                if (teamName.getText() == null
                        || teamName.getText().length() == 0) {
                    isValid = false;
                    statusText.append("Please specify team name. ");
                }
                Player captain = getCaptain();
                if (!atleast6PlayersAreSpecified()) {
                    isValid = false;
                    statusText.append("Please specify atleast 6 players. ");
                }
                if (captain == null) {
                    isValid = false;
                    statusText.append("Please designate a captain. ");
                }
                System.out.println("Is request valid? " + isValid);
                if (isValid.equals(Boolean.TRUE)) {
                    System.out.println("RPC - submitTeam start ");
                    System.out.println("" + getPlayers());
                    SubmitTeamCallback callback = new SubmitTeamCallback(status);
                    service.submitTeam(teamName.getText(), getPlayers(),
                            callback);
                    System.out.println("RPC - submitTeam end");

                } else {
                    status.setText(statusText.toString());
                }

            } catch (Exception e) {
                e.printStackTrace();
                System.err.println(e.getMessage());
            }
        }

Made some progress after adding Window.alert(...) based on @Pistol suggestion and it is going to onFailure(Throwable t) method of the SubmitTeamCallback class.

unknown.com_google_gwt_user_client_rpc_SerializationExce开发者_StackOverflow社区ption_SerializationException__Ljava_lang_String_2V(Unknown Source)

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An alternative method to debug client side code is using Window.alert(...). For example placing it in your callback onFailure() method to see caught messages for a start. Or you can use FireBug or something similar to see if the rpc-call is actually being made?

Edit :

Are you sure your Player class implements IsSerializable and have an empty constructor? Check out Serializable User-Defined Classes.

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Figured this out. The POJO's should have a default constructor and the one's I am using didn't have one. GWT compiler didn't complain either. Not sure why.