我在使用正确的文字在我的Android应用中收到提醒时遇到问题。我使用相同的代码向服务器发送Get请求并且工作正常，但在这种情况下，我使用Post方法，而我发送的数据存储在数据库中，Logcat显示正确/期望的字符串返回，当它到达onPostExecute部分而不是显示警报时，应用程序崩溃。
我错过了什么，是否有其他方法可以处理Post方法的响应？
这是我的代码：
package com.example.aplikacija;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
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.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.protocol.BasicHttpContext;
import org.apache.http.protocol.HttpContext;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.AsyncTask;
import android.text.Html;
import android.util.Log;
public class REST_VpisOcene extends AsyncTask<Void, Void, String> {
String restURL = "*REST IP removed*";
private Activity activity;
private String predmet;
private String ocena;
private String IDucenca;
private String IDprofesorja;
public REST_VpisOcene(String predmet, String ocena, String IDucenca, String IDprofesorja,
Activity activity) {
this.predmet = predmet;
this.ocena = ocena;
this.IDucenca = IDucenca;
this.IDprofesorja = IDprofesorja;
this.activity = activity;
}
protected String getASCIIContentFromEntity(HttpEntity entity)
throws IllegalStateException, IOException {
InputStream in = entity.getContent();
StringBuffer out = new StringBuffer();
int n = 1;
while (n > 0) {
byte[] b = new byte[4096];
n = in.read(b);
if (n > 0)
out.append(new String(b, 0, n));
}
return out.toString();
}
@Override
protected String doInBackground(Void... params) {
String sporocilo = "";
HttpClient httpclient = new DefaultHttpClient();
HttpContext localContext = new BasicHttpContext();
HttpPost httppost = new HttpPost(restURL);
httppost.addHeader("Accept", "text/html");
try {
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs
.add(new BasicNameValuePair("predmet", predmet));
nameValuePairs.add(new BasicNameValuePair("ocena", ocena));
nameValuePairs.add(new BasicNameValuePair("IDucenca", IDucenca));
nameValuePairs.add(new BasicNameValuePair("IDprofesorja", IDprofesorja));
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
HttpResponse response = httpclient.execute(httppost, localContext);
HttpEntity entity = response.getEntity();
sporocilo = getASCIIContentFromEntity(entity);
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
Log.i("odgovor vpisa", sporocilo);
return sporocilo;
}
protected void onPostExecute(String results) {
if (results.equals("0")) {
AlertDialog.Builder builder = new AlertDialog.Builder(activity);
builder.setMessage("Pri vpisu je prišlo do napake!")
.setCancelable(false)
.setNegativeButton("OK",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int id) {
dialog.cancel();
}
});
AlertDialog alert = builder.create();
alert.show();
} else if (results.equals("1")) {
AlertDialog.Builder builder = new AlertDialog.Builder(activity);
builder.setMessage("Vpisano!")
.setCancelable(false)
.setNegativeButton("OK",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog,
int id) {
dialog.cancel();
}
});
AlertDialog alert = builder.create();
alert.show();
}
}
}
我的Logcat：
02-08 14:56:11.856: I/odgovor vpisa(6424): 1
02-08 14:56:11.859: D/AndroidRuntime(6424): Shutting down VM
02-08 14:56:11.860: E/AndroidRuntime(6424): FATAL EXCEPTION: main
02-08 14:56:11.860: E/AndroidRuntime(6424): Process: com.example.aplikacija, PID: 6424
02-08 14:56:11.860: E/AndroidRuntime(6424): java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources$Theme android.content.Context.getTheme()' on a null object reference
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:154)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.app.AlertDialog$Builder.<init>(AlertDialog.java:379)
02-08 14:56:11.860: E/AndroidRuntime(6424): at com.example.aplikacija.REST_VpisOcene.onPostExecute(REST_VpisOcene.java:121)
02-08 14:56:11.860: E/AndroidRuntime(6424): at com.example.aplikacija.REST_VpisOcene.onPostExecute(REST_VpisOcene.java:1)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.os.AsyncTask.finish(AsyncTask.java:632)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.os.AsyncTask.access$600(AsyncTask.java:177)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.os.Handler.dispatchMessage(Handler.java:102)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.os.Looper.loop(Looper.java:135)
02-08 14:56:11.860: E/AndroidRuntime(6424): at android.app.ActivityThread.main(ActivityThread.java:5221)
02-08 14:56:11.860: E/AndroidRuntime(6424): at java.lang.reflect.Method.invoke(Native Method)
02-08 14:56:11.860: E/AndroidRuntime(6424): at java.lang.reflect.Method.invoke(Method.java:372)
02-08 14:56:11.860: E/AndroidRuntime(6424): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
02-08 14:56:11.860: E/AndroidRuntime(6424): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
它在第121行失败，即
AlertDialog.Builder builder = new AlertDialog.Builder(activity);
在第二个if子句中，意味着它通过
results.equals("1")
检查，然后失败。
谢谢。
答案 0 :(得分：10)
问题在于，无法保证在调用onPostExecute()时活动存在。
onPostExecute()是在后台线程处理完成后调用的方法，与Activity的生命周期无关。
解决方案是检查当时是否存在活动，然后执行预期的逻辑。
您可以看到this更详细的答案（不同的情况，但都与您在此处遇到的核心问题相关）。还有一篇博客文章详细定义了这一主题，可用here。
答案 1 :(得分：2)
一年后，如果其他人有这个问题，也许这个解决方案可以提供帮助。
当活动要显示对话框或需要活动上下文的任何操作时，检查活动是否可见之前检查是否重要。
要完成此任务，您需要一个从Application：
扩展的类public class App extends Application {
private static Context mContext;
@Override
public void onCreate() {
super.onCreate();
mContext = this;
}
public static Context getContext(){
return mContext;
}
public static boolean isActivityVisible() {
return activityVisible;
}
public static void activityResumed() {
activityVisible = true;
}
public static void activityPaused() {
activityVisible = false;
}
private static boolean activityVisible;
}
在您的MainActivity类中，您需要在完成其中一些操作时注册应用的当前状态：
@Override
protected void onResume() {
super.onResume();
App.activityResumed();
}
@Override
protected void onPause() {
super.onPause();
App.activityPaused();
}
最后，在执行某些操作（如显示对话框）之前，只需检查应用是否可见：
if (App.isActivityVisible()) {
// We can show the dialog
} else {
// Better not show the dialog to avoid a crash
}
祝你有愉快的一天！ ：）
答案 2 :(得分：0)
将private Activity context;更改为private Context context
和
public REST_VpisOcene(String predmet, String ocena, String IDucenca, String IDprofesorja,
Context context) {
this.predmet = predmet;
this.ocena = ocena;
this.IDucenca = IDucenca;
this.IDprofesorja = IDprofesorja;
this.context= context;
}
或使用getApplicationContext（）代替上下文，另见this Question