mysql - GWT+MySQL Web 应用程序 : compiling errors

Question

我的代码已更新。

In recall, I'm making a GWT+MySQL web application. I tried to modified a code from internet. Wen I run my project as a Web application (Debug As mode), I'm having the following compilation errors:

[ERROR] [deuxiemeapplicationgwt] - Unable to load module entry point class com.essai.client.DeuxiemeApplicationGwt (see associated exception for details)

[ERROR] [deuxiemeapplicationgwt] - Failed to load module 'deuxiemeapplicationgwt' from user agent 'Mozilla/5.0 (Windows; U; Windows NT 6.1; fr; rv:1.9.2.23) Gecko/20110920 Firefox/3.6.23' at 127.0.0.1:60265

    onModuleLoad() threw an exception

    Exception while loading module com.essai.client.DeuxiemeApplicationGwt. See Development Mode for details.
    java.lang.reflect.InvocationTargetException at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown Source) at com.google.gwt.dev.shell.ModuleSpace.onLoad(ModuleSpace.java:396) at com.google.gwt.dev.shell.OophmSessionHandler.loadModule(OophmSessionHandler.java:200) at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:525) at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:363) at java.lang.Thread.run(Unknown Source) Caused by: java.lang.NullPointerException at com.google.gwt.user.client.ui.ComplexPanel.add(ComplexPanel.java:88) at com.google.gwt.user.client.ui.AbsolutePanel.add(AbsolutePanel.java:97) at com.google.gwt.user.client.ui.Panel.add(Panel.java:71) at com.essai.client.DeuxiemeApplicationGwt.onModuleLoad(DeuxiemeApplicationGwt.java:28) ... 9 more

文件代码也已更新并在此处:

============================================= ===

package com.essai.client;

import com.google.gwt.user.client.rpc.RemoteService;

import com.essai.client.User;
import java.io.Serializable;

public interface ConnexionBD extends RemoteService
{
    public User authenticateUser(String user, String motDePasse);
}

==================================================

package com.essai.client;

import com.essai.client.User;

import com.google.gwt.user.client.rpc.AsyncCallback;

public interface ConnexionBDAsync {

    public void authenticateUser(String User, String motDePasse,
            AsyncCallback<User> callback);
}

============================================= ===

//EntryPoint file

package com.essai.client;

import com.essai.client.User;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.rpc.ServiceDefTarget;
import com.google.gwt.user.client.ui.*;

public class DeuxiemeApplicationGwt implements EntryPoint, ClickListener, IsWidget {
private ConnexionBDAsync rpc;
private TextBox usernameBox;
private TextBox passwordBox;
private Button OK;
public DeuxiemeApplicationGwt() {
rpc = (ConnexionBDAsync) GWT.create(ConnexionBD.class);
ServiceDefTarget target = (ServiceDefTarget) rpc;
// The path 'MySQLConnection' is determined in ./public/LoginScreen.gwt.xml
// This path directs Tomcat to listen for this context on the server side,
// thus intercepting the rpc requests.
String moduleRelativeURL = GWT.getModuleBaseURL() + "MySQLConnection";
target.setServiceEntryPoint(moduleRelativeURL);
initGUI();
}
public void onModuleLoad() {
RootPanel.get().add(this);
}

private void initGUI() {
Grid g = new Grid(3, 2);
usernameBox = new TextBox();
passwordBox = new TextBox();
OK = new Button("OK");
g.setWidget(0, 0, new Label("Username: "));
g.setWidget(0, 1, usernameBox);
g.setWidget(1, 0, new Label("Password: "));
g.setWidget(1, 1, passwordBox);
g.setWidget(2, 1, OK);
}
public void onClick(Widget sender) {

if (sender.equals(OK)) {
AsyncCallback<User> callback = new AuthenticationHandler<User>();
rpc.authenticateUser(usernameBox.getText(),passwordBox.getText(),
callback);
}
}
private class AuthenticationHandler<T> implements AsyncCallback<User> {
public void onFailure(Throwable ex) {
RootPanel.get().add(new HTML("RPC call failed. :-("));
}
public void onSuccess(User result) {
//do stuff on success with GUI, like load the next GUI element
}
}
@Override
public Widget asWidget() {
    // TODO Auto-generated method stub
    return null;
}
}

============================================= ===

//ConnexionMySQL.java

package com.essai.server;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Vector;

import com.google.gwt.user.server.rpc.RemoteServiceServlet;
import com.essai.client.User;
import com.essai.client.ConnexionBD;

public class ConnexionMySQL extends RemoteServiceServlet implements ConnexionBD
{
    private Connection conn = null;
    private String status;
    private String url = "jdbc:mysql://localhost/gestionpatients";
    private String utilisateurBD = "root";
    private String motDePasseBD = "";

    public ConnexionMySQL()
    {
    try
    {
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        conn = DriverManager.getConnection(url, utilisateurBD, motDePasseBD);
    }
    catch (Exception e)
    {

    }
}

public User authenticateUser(String user, String motDePasse)
{
    User utilisateur = null;
    try
    {
        PreparedStatement ps = conn.prepareStatement(
                "select readonly * from docteurs where Noms = \"\" + user + \"\" AND " + "ID = \"\" + pass + \"\""
                );
        ResultSet result = ps.executeQuery();
        while (result.next())
        {
            utilisateur = new User(result.getString(1), result.getString(2));
        }
        result.close();
        ps.close();
        }
        catch (SQLException sqle)
        {
            //Comportement en cas d'erreur
        }
        return utilisateur;
    }
}

============================================= ===

//User.java: user configuration

package com.essai.client;

import com.google.gwt.user.client.rpc.IsSerializable;
//import java.io.Serializable;

public class User implements IsSerializable
{
    private String nomUtilisateur;
    private String motDePasse;
    @SuppressWarnings("unused")
    private User() {}

    public User(String user, String motDePasse)
    {
        this.nomUtilisateur = user;
        this.motDePasse = motDePasse;
    }
}

============================================= ===

//XML file

<?xml version="1.0" encoding="UTF-8"?>
<module rename-to='deuxiemeapplicationgwt'>
  <!-- Inherit the core Web Toolkit stuff.                        -->
  <inherits name='com.google.gwt.user.User'/>

  <!-- Inherit the default GWT style sheet.  You can change       -->
  <!-- the theme of your GWT application by uncommenting          -->
  <!-- any one of the following lines.                            -->
  <inherits name='com.google.gwt.user.theme.clean.Clean'/>
  <!-- <inherits name='com.google.gwt.user.theme.standard.Standard'/> -->
  <!-- <inherits name='com.google.gwt.user.theme.chrome.Chrome'/> -->
  <!-- <inherits name='com.google.gwt.user.theme.dark.Dark'/>     -->

  <!-- Other module inherits                                      -->

  <!-- Specify the app entry point class.                         -->
  <entry-point class="com.essai.client.DeuxiemeApplicationGwt"/>

  <inherits name="com.google.gwt.user.theme.standard.Standard"/>
  <inherits name="com.google.gwt.user.theme.chrome.Chrome"/>
  <inherits name="com.google.gwt.user.theme.dark.Dark"/>

  <!-- Specify the paths for translatable code                    -->
  <source path='client'/>
  <source path='shared'/>
</module>

============================================= ===

项目结构可以在这里看到:http://tonguim.free.fr/divers/imgGWTtomcat.jpg

感谢您的帮助。

Answer 1

问题是您在 onModuleLoad() 中编写 RootPanel.get().add(this) 而 asWidget() 返回 空。这与编写 RootPanel.get().add(null) 相同。这就是在 onModuleLoad 中抛出 NullPointerException 的原因。或者更准确地说，ComplexPanel.add(Widget, Element) 中的 child.removeFromParent(); 行导致异常。

长话短说，让 asWidget() 返回 null 以外的内容。例如，使您在 initGui() 中创建的网格成为类成员并返回它。在当前形式下，initGui 无论如何都不会创建 GUI。

编辑 没有冒犯，但看看你的代码，你似乎并没有真正理解 GWT。除了将 null 添加到 RootPanel 之外，您的 Grid 没有附加到任何地方。此外，您实现了一个 ClickListener，但您从未将此监听器附加到实际监听事件。如果这是完整的代码(不仅仅是为了可读性而剥离)，您应该考虑阅读一些 gwt docs首先。