I'm now currently extracting out data from my database using rpc but 
whenever I click the button it does generates a error and my onFailure 
command is being triggered always.
Below is my Code for the program.

public class PleaseWork implements EntryPoint {private TextBox textbox2 = new 
TextBox();private Label Hi = new Label("New 
label");@SuppressWarnings("deprecation")public void onModuleLoad() {
    RootPanel rootPanel = RootPanel.get();
    Button btnNewButton = new Button("New button");
    btnNewButton.addClickListener(new ClickListener() {
        public void onClick(Widget event) {
            HelpConnectionAsync Abra =(HelpConnectionAsync) 
GWT.create(HelpConnection.class);
            ServiceDefTarget target = (ServiceDefTarget) Abra;
            String moduleRelativeURL = GWT.getModuleBaseURL() + 
"MySQLConnection";
            target.setServiceEntryPoint(moduleRelativeURL);
            AsyncCallback callback = new AsyncCallback(){
            public void onSuccess (Object result){
                textbox2.setText((String)result);
                Hi.setText("You Pass!");            }
            public void onFailure(Throwable caught) {
                caught.printStackTrace();
                Hi.setText("You fail!");            }
            };
            Abra.sensors(callback);
        }
    });
    rootPanel.add(btnNewButton, 23, 30);
    rootPanel.add(textbox2,23, 70);     
    rootPanel.add(Hi, 23, 130);}

And this is my Serverside code 
public class MySQLConnection extends RemoteServiceServlet implements 
HelpConnection {
        private Connection conn = null;
        private ResultSet rs = null;
    private PreparedStatement pstmt = null;
        
        public static Connection getConnection() throws Exception {
            String driver = "com.mysql.jdbc.Driver";
            String url = "jdbc:mysql://localhost:3306/temperature";
            String username = "root";
            String password = "123456";
            Class.forName(driver);
            Connection conn = DriverManager.getConnection(url, username, 
password);
            return conn;
          }

        @Override
        public String[] sensors() throws Exception {
                String [] user = null;
                try {
                      conn = getConnection();

                      rs = pstmt.executeQuery("SELECT ID, times FROM sensor 
ORDER BY id DESC LIMIT 1;");
                      // extract data from the ResultSet
                      while (rs.next()) {
                        user = new String[] {rs.getString(1),rs.getString(2)};
                      }
                    } catch(SQLException e) {
                      e.printStackTrace();
                    } finally {
                      try {
                        rs.close();
                        pstmt.close();
                        conn.close();
                      } catch (SQLException e) {
                        e.printStackTrace();
                      }
                    }
                    return user;
        }

}

While running the application it generates this error message on the console 
side which is telling me why onFailure Command is always being
 triggered but i do not understand about it.
java.lang.NullPointerException
        at 
com.google.gwt.user.client.rpc.core.java.lang.NullPointerException_FieldSerializer.instantiate(NullPointerException_FieldSerializer.java:16)
        at 
com.google.gwt.user.client.rpc.core.java.lang.NullPointerException_FieldSerializer.create(NullPointerException_FieldSerializer.java:25)
        at 
com.google.gwt.user.client.rpc.impl.SerializerBase.instantiate(SerializerBase.java:115)
        at 
com.google.gwt.user.client.rpc.impl.ClientSerializationStreamReader.deserialize(ClientSerializationStreamReader.java:396)
        at 
com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamReader.readObject(AbstractSerializationStreamReader.java:119)
        at 
com.google.gwt.user.client.rpc.impl.RequestCallbackAdapter.onResponseReceived(RequestCallbackAdapter.java:216)
        at 
com.google.gwt.http.client.Request.fireOnResponseReceived(Request.java:258)
        at 
com.google.gwt.http.client.RequestBuilder$1.onReadyStateChange(RequestBuilder.java:412)
        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.MethodAdaptor.invoke(MethodAdaptor.java:103)
        at 
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
        at 
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
        at 
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:338)
        at 
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:219)
        at 
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
        at 
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:571)
        at 
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:279)
        at 
com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
        at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
        at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:242)
        at sun.reflect.GeneratedMethodAccessor38.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
        at 
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
        at 
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
        at 
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:293)
        at 
com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:547)
        at 
com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:364)
        at java.lang.Thread.run(Unknown Source)



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