[英]Debugging AsyncTask - strange behaviour, debug jumping into the code
我正在調試一個僅下載文件的Asynctask:這里的代碼:
public class AsyncDownloadFilesTask extends AsyncTask<String, Integer, Boolean> {
public AsyncResponse<Boolean> delegate=null;
protected Boolean doInBackground(String... params) {
android.os.Debug.waitForDebugger();
try {
URL url = new URL(params[0]);
int count;
String fileName = new String(params[1]);
URLConnection connessione = url.openConnection();
connessione.connect();
int lenghtOfFile = connessione.getContentLength();
InputStream input = new BufferedInputStream(url.openStream());
OutputStream output = new FileOutputStream(fileName);
long total = 0;
byte data[] = new byte[1024];
while ((count = input.read(data)) != -1) {
total += count;
publishProgress((int)((total*100)/lenghtOfFile));
output.write(data, 0, count);
}
output.flush();
output.close();
input.close();
return Boolean.valueOf(true);
} catch (Exception e) {
return null;
}
}
protected void onPostExecute(Boolean result) {
delegate.processFinish(result);
}
}
我得到一個奇怪的行為:當執行到達時返回Boolean.valueOf(true); 跳過返回null; 進入catch塊,但Exception e為null,然后調試器轉到AsyncTask的第1行,即打包com.example.compa.asynctasks; 然后繼續執行(執行onPostExecute方法),當然,返回的結果為null
怎么了? 為什么以這種方式調試跳轉?
任務正確下載文件。
這是實例化並調用異步任務的活動的代碼
package com.example.compa.activities;
import android.app.Activity;
import ...
public class CoverActivity extends Activity implements AsyncResponse<Boolean>{
ImageView coverImg;
Drawable d;
CompassesFileManager cfm;
int coverId;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_cover);
coverId = getIntent().getExtras().getInt("coverId");
cfm = new CompassesFileManager(this);
ImageView coverImg = (ImageView)findViewById(R.id.cover_image);
d = cfm.getCover(coverId);
if (d!=null){
coverImg.setImageDrawable(d);
} else {
AsyncDownloadFilesTask task = new AsyncDownloadFilesTask();
task.delegate = this;
task.execute(cfm.getCoverURL(coverId), cfm.getCoverFileName(coverId));
}
}
@Override
public void processFinish(Boolean output) {
if (output){
Drawable d = cfm.getCover(coverId);
coverImg.setImageDrawable(d);
} else {
finish();
}
}
}
錯誤的堆棧跟蹤:
02-21 19:37:29.520: E/AndroidRuntime(407): FATAL EXCEPTION: main
02-21 19:37:29.520: E/AndroidRuntime(407): java.lang.NullPointerException
02-21 19:37:29.520: E/AndroidRuntime(407): at com.example.compa.asynctasks.AsyncDownloadFilesTask.onPostExecute(AsyncDownloadFilesTask.java:65)
02-21 19:37:29.520: E/AndroidRuntime(407): at com.example.compa.asynctasks.AsyncDownloadFilesTask.onPostExecute(AsyncDownloadFilesTask.java:1)
02-21 19:37:29.520: E/AndroidRuntime(407): at android.os.AsyncTask.finish(AsyncTask.java:631)
02-21 19:37:29.520: E/AndroidRuntime(407): at android.os.AsyncTask.access$600(AsyncTask.java:177)
02-21 19:37:29.520: E/AndroidRuntime(407): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
02-21 19:37:29.520: E/AndroidRuntime(407): at android.os.Handler.dispatchMessage(Handler.java:99)
02-21 19:37:29.520: E/AndroidRuntime(407): at android.os.Looper.loop(Looper.java:176)
02-21 19:37:29.520: E/AndroidRuntime(407): at android.app.ActivityThread.main(ActivityThread.java:5419)
02-21 19:37:29.520: E/AndroidRuntime(407): at java.lang.reflect.Method.invokeNative(Native Method)
02-21 19:37:29.520: E/AndroidRuntime(407): at java.lang.reflect.Method.invoke(Method.java:525)
02-21 19:37:29.520: E/AndroidRuntime(407): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
02-21 19:37:29.520: E/AndroidRuntime(407): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
02-21 19:37:29.520: E/AndroidRuntime(407): at dalvik.system.NativeStart.main(Native Method)
行:02-21 19:37:29.520:E / AndroidRuntime(407):位於com.example.compa.asynctasks.AsyncDownloadFilesTask.onPostExecute(AsyncDownloadFilesTask.java:65)是AsyncDownloadFilesTask類的最后一個,並且是一個括弧,}
謝謝
我沒有足夠的要評論的地方,但是看來onPostExecute中的委托為null
delegate.processFinish(result); // delegate is null
如果不是這種情況,則上面的代碼存根並沒有定義它。
我自己解決了。
1,我將調用移到onStart()方法而不是onCreate()中的異步任務上
2,我在改行中犯了一個錯誤
ImageView coverImg = (ImageView)findViewById(R.id.cover_image);
在
coverImg = (ImageView)findViewById(R.id.cover_image);
避免一個愚蠢的空指針(我已經聲明了CoverImg)!
無論如何,我仍然不了解調試的行為,但是我解決了我的問題。
謝謝大家
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