Question

您好我是Android的新手，我正在开发一个应用程序，我通过连接到mysql数据库来检查登录凭据。

这是我遇到的错误，它出现在模拟器中。

显示java.lang.NullPointerException

这是我的代码。

LoginActivity.java

import android.app.Activity;
import java.util.ArrayList;
import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import android.os.Bundle;
import android.os.StrictMode;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class LoginActivity extends Activity {
    EditText un,pw;
    TextView error;
    Button ok;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    un=(EditText)findViewById(R.id.email_address);
    pw=(EditText)findViewById(R.id.password);
    ok=(Button)findViewById(R.id.login_button);
    error=(TextView)findViewById(R.id.tv_error);
    if (android.os.Build.VERSION.SDK_INT > 9) {
        StrictMode.ThreadPolicy policy = 
            new StrictMode.ThreadPolicy.Builder().permitAll().build();
        StrictMode.setThreadPolicy(policy);
    }
    ok.setOnClickListener(new View.OnClickListener() {

    @Override

    public void onClick(View v) {
    // TODO Auto-generated method stub
    ArrayList<NameValuePair> postParameters = new ArrayList<NameValuePair>();
    postParameters.add(new BasicNameValuePair("username", un.getText().toString()));
    postParameters.add(new BasicNameValuePair("password", pw.getText().toString()));

    //String valid = "1";
    String response = null;
    try {
    response = CustomHttpClient.executeHttpPost("http://engiguide.com/check.php", postParameters);  //Enetr Your remote PHP,ASP, Servlet file link
    String res=response.toString();
    // res = res.trim();
    res= res.replaceAll("\\s+","");
    //error.setText(res);
    if(res.equals("1"))
    error.setText("Correct Username or Password");
    else
    error.setText("Sorry!! Incorrect Username or Password");

    } catch (Exception e) {

    un.setText(e.toString());
    }}
    });
}
}

CustomHttpClient.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URI;
import java.util.ArrayList;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.conn.params.ConnManagerParams;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.params.HttpConnectionParams;
import org.apache.http.params.HttpParams;

public class CustomHttpClient {
/** The time it takes for our client to timeout */
public static final int HTTP_TIMEOUT = 30 * 1000; // milliseconds
/** Single instance of our HttpClient */
private static HttpClient mHttpClient;
/**
* Get our single instance of our HttpClient object.
*
* @return an HttpClient object with connection parameters set
*/
private static HttpClient getHttpClient() {
if (mHttpClient == null) {
mHttpClient = new DefaultHttpClient();
final HttpParams params = mHttpClient.getParams();
HttpConnectionParams.setConnectionTimeout(params, HTTP_TIMEOUT);
HttpConnectionParams.setSoTimeout(params, HTTP_TIMEOUT);
ConnManagerParams.setTimeout(params, HTTP_TIMEOUT);
}
return mHttpClient;
}
/**
* Performs an HTTP Post request to the specified url with the
* specified parameters.
*
* @param url The web address to post the request to
* @param postParameters The parameters to send via the request
* @return The result of the request
* @throws Exception
*/
public static String executeHttpPost(String url, ArrayList postParameters) throws Exception {
BufferedReader in = null;
try {
HttpClient client = getHttpClient();
HttpPost request = new HttpPost(url);
UrlEncodedFormEntity formEntity = new UrlEncodedFormEntity(postParameters);
request.setEntity(formEntity);
HttpResponse response = client.execute(request);
in = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
StringBuffer sb = new StringBuffer("");
String line = "";
String NL = System.getProperty("line.separator");
while ((line = in.readLine()) != null) {
sb.append(line + NL);
}
in.close();
String result = sb.toString();
return result;
} finally {
if (in != null) {
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}}}}
public static String executeHttpGet(String url) throws Exception {
BufferedReader in = null;
try {
HttpClient client = getHttpClient();
HttpGet request = new HttpGet();
request.setURI(new URI(url));
HttpResponse response = client.execute(request);
in = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
StringBuffer sb = new StringBuffer("");
String line = "";
String NL = System.getProperty("line.separator");
while ((line = in.readLine()) != null) {
sb.append(line + NL);
}
in.close();
String result = sb.toString();
return result;
} finally {
if (in != null) {
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}}}}}

这是用于连接mysql数据库的php代码。

<?php
$un=$_POST['username'];
$pw=$_POST['password'];
//connect to the db
$user = ‘username’;
$pswd = ‘password’;
$db = ‘loginactivity’;
$conn = mysql_connect(‘localhost’, $user, $pswd);
mysql_select_db($db, $conn);
//run the query to search for the username and password the match
$query = “SELECT * FROM userpass WHERE username = ‘$user’ AND password = ‘$pswd’”;
$result = mysql_query($query) or die(“Unable to verify user because : ” . mysql_error());
//this is where the actual verification happens
if(mysql_num_rows($result) > 0)
    echo 1;  // for correct login response
else
    echo 0; // for incorrect login response
?>

Answer 1

Please try this:

      String response = null;
        try {
        response = CustomHttpClient.executeHttpPost("http://engiguide.com/check.php", postParameters);  //Enetr Your remote PHP,ASP, Servlet file link
        String res=response.toString();
        // res = res.trim();
        res= res.replaceAll("\\s+","");
        //error.setText(res);
        if(res.equals("1"))
        error.setText("Correct Username or Password");
        else
        error.setText("Sorry!! Incorrect Username or Password");

            } catch (Exception e) {
    //the exception is caused in above code or from the server communication print the logs to //get the exact location
        e.printStackTrace();

        un.setText(e.toString());
        }}
        });

这将为您提供完整的堆栈跟踪，并帮助您精确定位空指针。检查logcat是否有异常。

连接到数据库时出现空指针异常

1 个答案: