將我的登錄應用程序從android（ADT）連接到ms sql server 2008

Question

我在Java和android編程上還很新。 我正在嘗試將android應用程序連接到Windows筆記本電腦上安裝的SQL Server數據庫。 我正在Windows下使用ADT進行編程。我也添加了jtds-1.2.5.jar代碼如下（放置在MainActivity.java中）：

          package com.test;
     import android.app.Activity;
     import android.os.AsyncTask;
     import android.os.Bundle;
     import android.view.View;
     import android.widget.Button;
     import android.widget.EditText;
     import android.widget.ProgressBar;
     import android.widget.TextView;
     import android.widget.Toast;
     import java.sql.Connection;
     import java.sql.ResultSet;
     import java.sql.Statement;
       public class MainActivity extends Activity {
       ConnectionClass connectionClass;
       EditText e1, e2;
       Button btnlogin;
       TextView t1, t2;
       ProgressBar progressBar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    connectionClass = new ConnectionClass();
    e1 = (EditText) findViewById(R.id.editText1);
    e2 = (EditText) findViewById(R.id.editText2);
    btnlogin = (Button) findViewById(R.id.button1);

    btnlogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            DoLogin doLogin = new DoLogin();
            doLogin.execute("");

        }
    });

}

public class DoLogin extends AsyncTask<String, String, String> {
    String z = "";
    Boolean isSuccess = false;

    String userid = e1.getText().toString();
    String password = e2.getText().toString();

    @Override
    protected void onPreExecute() {
        progressBar.setVisibility(View.VISIBLE);
    }

    @Override
    protected void onPostExecute(String r) {
        progressBar.setVisibility(View.GONE);
        Toast.makeText(MainActivity.this, r, Toast.LENGTH_SHORT).show();
        if (isSuccess) {
            Toast.makeText(MainActivity.this, "Login Successfull",
                    Toast.LENGTH_LONG).show();
            // finish();
        }
    }

    @Override
    protected String doInBackground(String... params) {
        if (userid.trim().equals("") || password.trim().equals(""))
            z = "Please enter User Id and Password";
        else {
            try {
                Connection con = connectionClass.CONN();
                if (con == null) {
                    z = "Error in connection with SQL server";
                } else {
                    String query = "select * from LoginTable where username='"
                            + userid + "' and password='" + password + "'";
                    Statement stmt = con.createStatement();
                    ResultSet rs = stmt.executeQuery(query);

                    if (rs.next()) {

                        z = "Login successfull";
                        isSuccess = true;
                    } else {
                        z = "Invalid Credentials";
                        isSuccess = false;
                    }

                }
            } catch (Exception ex) {
                isSuccess = false;
                z = "Exceptions";
            }
        }
        return z;
    }
   }
 }

當我按下“連接到數據庫”按鈕時，我收到以下錯誤消息：有人可以幫我嗎？

     09-05 11:32:42.505: E/AndroidRuntime(1402): FATAL EXCEPTION: main
     09-05 
     11:32:42.505:E/AndroidRuntime(1402):java.lang.NullPointerException
     09-05 11:32:42.505: E/AndroidRuntime(1402):    at 
     com.test.MainActivity$DoLogin.onPreExecute(MainActivity.java:54)
     09-05 11:32:42.505: E/AndroidRuntime(1402):    at 
      android.os.AsyncTask.executeOnExecutor(AsyncTask.java:586)
     09-05 11:32:42.505: E/AndroidRuntime(1402):    at 
    android.os.AsyncTask.execute(AsyncTask.java:534)
   09-05 11:32:42.505: E/AndroidRuntime(1402):  at 
      com.test.MainActivity$1.onClick(MainActivity.java:38)
      09-05 11:32:42.505: E/AndroidRuntime(1402):   at 
    android.view.View.performClick(View.java:4240)
     09-05 11:32:42.505: E/AndroidRuntime(1402):    at 
       android.view.View$PerformClick.run(View.java:17721)
      09-05 11:32:42.505: E/AndroidRuntime(1402):   at 
        android.os.Handler.handleCallback(Handler.java:730)
       09-05 11:32:42.505: E/AndroidRuntime(1402):  at 
        android.os.Handler.dispatchMessage(Handler.java:92)
        09-05 11:32:42.505: E/AndroidRuntime(1402):     at 
        android.os.Looper.loop(Looper.java:137)
        09-05 11:32:42.505: E/AndroidRuntime(1402):     at 
            android.app.ActivityThread.main(ActivityThread.java:5103)
        09-05 11:32:42.505: E/AndroidRuntime(1402):     at 
              java.lang.reflect.Method.invokeNative(Native Method)
               09-05 11:32:42.505: E/AndroidRuntime(1402):  at 
              java.lang.reflect.Method.invoke(Method.java:525)
             09-05 11:32:42.505: 
            E/AndroidRuntime(1402):atcom.android.internal.os
           .ZygoteInit$MethodAndArgsCaller
                       .run(ZygoteInit.java:737)
                     09-05 11:32:42.505: E/AndroidRuntime(1402):    at 
             com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
                        09-05 11:32:42.505: E/AndroidRuntime(1402):     at 
                       dalvik.system.NativeStart.main(Native Method)
                      09-05 11:32:46.115: I/Process(1402): Sending signal. 
                         PID: 1402 SIG: 9

提前致謝

Answer 1

在DoLogin類中

 @Override
    protected void onPreExecute() {
        progressBar.setVisibility(View.VISIBLE);
    }

在這里，您會得到空指針異常，因為您沒有在范圍中定義progressBar 。