调试AsyncTask-奇怪的行为，调试跳入代码

Question

I'm debugging an Asynctask that simply downloads a file: here the code:

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);
}
}

I obtain a strange behaviour: when execution arrive to return Boolean.valueOf(true); it skips to return null; into the catch block, but Exception e is null, and then debugger goto line 1 of AsyncTask, that is simply package com.example.compa.asynctasks; Then execution goes on (executing onPostExecute method) and, of course, returned result is null

What happens? Why debug jump in this way?

Task download correctly the file.

Here code of the Activity that instantiates and calls Async Task

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();
    }

}
}

Stacktrace of error:

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)

line: 02-21 19:37:29.520: E/AndroidRuntime(407): at com.example.compa.asynctasks.AsyncDownloadFilesTask.onPostExecute(AsyncDownloadFilesTask.java:65) is the last one of AsyncDownloadFilesTask class, and is a closing bracket, }

Thank you

Answer 1

I don't have enough points to comment, but it looks like delegate is null in your onPostExecute

delegate.processFinish(result); // delegate is null

if that's not the case, you're code stub above doesn't define it though.

Answer 2

I solved on my own.

1st, I move call to the Async Task in the onStart() method, instead of onCreate()

2nd, I made a mistake, in change line

ImageView coverImg = (ImageView)findViewById(R.id.cover_image);

in

coverImg = (ImageView)findViewById(R.id.cover_image);

to avoid a stupid null pointer (I already declared coverImg)!

Anyway, I still don't understand debug's behaviour, but I solved my problem.

Thank you everybody