Android 尝试在Asynctask中设置水平进度条会引发NullPointerException

我正在尝试使用

AsyncTask

将CSV文件导出到SD卡，并使用

ProgressBar

告诉用户该任务是如何运行的。然而，当我试图用

setProgress

设置进度条时，我得到一个空指针异常。我的Logcat将我传递的整数显示为8，因此它不是null。如果我注释掉设置进度条的尝试，它将正确运行。当我尝试设置进度条时，运行时看到的是null吗

完整的异步任务实现：

public class Exporter extends AsyncTask<File, Integer, Boolean> {
    private Context ctx;
    private final static String TAG = "Exporter";

    private ProgressDialog progress;

    private GeoDatabase db;
    private RecordListActivity parent;

    public Exporter(Context ctx) {
        super();
        Log.d(TAG, "Building " + this + " from " + ctx);
        this.ctx = ctx;
        parent = (RecordListActivity) ctx;
        db = (GeoDatabase) ctx.getApplicationContext();
    }

    @Override
    protected void onPreExecute() {
        // Initialize the progress dialog
        Log.i(TAG, "Pre-executing exporter");
        final String title = "Exporting";
        final String msg = "Initializing export...";
        progress = ProgressDialog.show(ctx, title, msg, true);
        progress.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);

        super.onPreExecute();
    }

    /**
     * Begins operation of the exporter and determines if it was a
     * success.
     * 
     * @param args the destination file
     * 
     * @return whether the transfer was successful
     */
    @Override
    protected Boolean doInBackground(final File... args) {
        Log.d(TAG, "src-> " + "N/A" + ", dest-> " + args[0]);

        // Get the ArrayList of records inside the current folder
        ArrayList<Record> records = db.getRecordsIn(parent.getCurrentFolder());

        FileWriter writer;
        try {
            // Build a FileWriter targeting the destination
            writer = new FileWriter(args[0]);

            // Iterate through the records to write their CSV string out
            final int total = records.size();
            int current = 1;
            for (Record record : records) {
                Log.v(TAG, "Accessing " + record);
                writer.write(record.getCSVString() + "\n");
                Log.v(TAG, record.getCSVString());

                // Update the progress bar
                onProgressUpdate(total, current);
                current++;
            }

            // Close the writer
            writer.close();
            Log.d(TAG, "Writer closed");
        } catch (IOException e) {
            // Something has gone horribly wrong
            e.printStackTrace();
            Log.e(TAG, "Writing to file failed!");
            return false;
        }

        return true;
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
            // values[1] is the current record being written
        // values[0] is the total number of records being written
        final int total = values[0];
        final int current = values[1];

        // Updates the text for the current record
        final String msg = "Writing record " + current + " of " + total;
        progress.setMessage(msg);
        Log.d(TAG, msg);

        // Updates the progress bar
        final int position = 100 * current / total;
        Log.d(TAG, "Setting position to " + position);
        progress.setProgress(position);
    }

    // can use UI thread here
    protected void onPostExecute(final Boolean success) {
        // TODO: Alert the user to success or failure - the dialog drops
        // to quickly now
        if (success) {
            progress.setMessage("Export successful");
            Log.i(TAG, "Export successful");
        } else {
            progress.setMessage("Export failed");
            Log.e(TAG, "Export failed");
        }
        progress.dismiss();
    }
}

---

找到了问题和解决方法。在Android API中，使用

ProgressDialog.show（params）

创建

ProgressDialog

的工厂方法没有正确设置更新处理程序，这似乎是一个错误。因此，修订后的工作守则如下：

@Override
protected void onPreExecute() {
    // Initialize the progress dialog
    Log.i(TAG, "Pre-executing exporter");
    final String title = "Exporting";
    final String msg = "Initializing export...";
    progress = new ProgressDialog(ctx);
    progress.setTitle(title);
    progress.setMessage(msg);       
    progress.setIndeterminate(false);
    progress.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
    super.onPreExecute();
}

---

找到了问题和解决方法。在Android API中，使用

ProgressDialog.show（params）

创建

ProgressDialog

的工厂方法没有正确设置更新处理程序，这似乎是一个错误。因此，修订后的工作守则如下：

@Override
protected void onPreExecute() {
    // Initialize the progress dialog
    Log.i(TAG, "Pre-executing exporter");
    final String title = "Exporting";
    final String msg = "Initializing export...";
    progress = new ProgressDialog(ctx);
    progress.setTitle(title);
    progress.setMessage(msg);       
    progress.setIndeterminate(false);
    progress.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
    super.onPreExecute();
}

---

您能否发布完整日志以及完整的

AsyncTask

，包括

doInBackround

（）？@LuxuryMode Done。没有太多额外的内容，但希望能有所帮助。您能发布完整的日志以及完整的

AsyncTask

，包括

doInBackround

（）？@LuxuryMode Done。没有太多额外的，但希望它有帮助。