嗨，我是 android 新手，我正在开发一个应用程序，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。
java.lang.NullPointerException
这是我的代码。
登录活动.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
?>