我的代码有什么问题（GWT）

Question

I'm creating a login application on Eclipse using Google Web Toolkit(GWT). The code checks for the username and password and if its correct, it shows the o/p as welcome. Still after compiling it is giving me errors.I'm sharing both code and the error message. Please help me out.

    package com.vin.client;
import java.sql.DriverManager;
import java.sql.ResultSet;
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.dev.generator.ast.Statement;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.Window;
import com.google.gwt.user.client.ui.*;

public class HelloWorld implements EntryPoint{
    public void onModuleLoad() {
        Button click=new Button("Click Here");
        Label name=new Label("Enter Name");
        Label passwrd=new Label("Enter Password");
        final TextBox t_name=new TextBox();
        final TextBox t_passwrd=new TextBox();
        click.addClickHandler(new ClickHandler() {
        public void onClick(ClickEvent ev) {
            try {
                String temp_user=t_name.getText();
                String temp_pass=t_passwrd.getText();
                java.sql.Connection con = null;
                Class.forName("org.hsqldb.jdbcDriver");
                con = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/", "SA", "");
                Statement st=(Statement) con.createStatement();
                ResultSet rs=((java.sql.Statement) st).executeQuery("select username,password from lgfrm");
                String user=rs.getString(1);
                String pass=rs.getString(2);
                if(temp_user.equals(user) && temp_pass.equals(pass)) {
                    Window.alert("Welcome");
                }
                else {
                    Window.alert("Please enter valid details");
                }
        }
        catch (Exception ae) {}
        }
    });
        RootPanel.get().add(name);
        RootPanel.get().add(t_name);
        RootPanel.get().add(passwrd);
        RootPanel.get().add(t_passwrd);
        RootPanel.get().add(click);
    }
    }

Error Message is----------

Compiling module com.vin.HelloWorld Exception in thread "UnitCacheLoader" java.lang.RuntimeException: Unable to read from byte cache at com.google.gwt.dev.util.DiskCache.transferFromStream(DiskCache.java:166) at com.google.gwt.dev.util.DiskCacheToken.readObject(DiskCacheToken.java:87) at sun.reflect.GeneratedMethodAccessor19.invoke(Unknown Source) ..............and many more like this.... Please help me out

Answer 1

Try something like following for Server side :

UserService.java

@RemoteServiceRelativePath("userService")
public interface UserService extends RemoteService {
    String loginUser(String username,String password);
}

UserServiceAsync.java

public interface UserServiceAsync {
    void loginUser(String username, String password, AsyncCallback<String> callback);
}

UserServiceImpl.java

public class UserServiceImpl extends RemoteServiceServlet  implements UserService {

        public String loginUser(String username, String password){
            //database interaction
            return "result"; //return success or failure depending upon logic
    }
}

Follow Communicate with a Server in GWT and the Anatomy of service

For Client Side :

public class HelloWorld implements EntryPoint{
    //(1) Create the client proxy.
    private UserServiceAsync userService = (UserServiceAsync) GWT.create(UserService.class);
    public void onModuleLoad() {
        Button click=new Button("Click Here");
        Label name=new Label("Enter Name");
        Label passwrd=new Label("Enter Password");
        final TextBox t_name=new TextBox();
        final TextBox t_passwrd=new TextBox();
        click.addClickHandler(new ClickHandler() {
        public void onClick(ClickEvent ev) {
                 String temp_user=t_name.getText();
                 String temp_pass=t_passwrd.getText();   
                 /// (2) Create an asynchronous callback and Make the call
                 userService.loginUser(temp_user, temp_pass, new AsyncCallback<String>() {
                     public void onFailure(Throwable caught) {
                             Window.alert("Please enter valid details");
                      }

                     public void onSuccess(String result) {
                         Window.alert("Welcome");
                     }
                 });//end of service call
       });//end of clickhandler
        RootPanel.get().add(name);
        RootPanel.get().add(t_name);
        RootPanel.get().add(passwrd);
        RootPanel.get().add(t_passwrd);
        RootPanel.get().add(click);
    }
 }

Answer 2

You can not put DB related code in Entry point class, you need to call GWT-RPC on click method.

Actually this EntryPoint class would be compiled by GWT processor and it will create javascript in output which going to run in browser. So there is no justification you can call db in javascript.

GWT-RPC is asynchronous call which code reside in server. Here you can write all business logic, db interactivity etc.

LINK