我正在尝试演示AsyncTask和Asynctask中方法的执行顺序。我已将Toast放入每个方法中以查看顺序。以下是我使用的代码片段：

private class UploadImage extends AsyncTask<Void, Void, Void> {

    Bitmap image;
    String name;

    public UploadImage(Bitmap image, String name){
        this.image = image;
        this.name = name;

        Toast.makeText(getApplicationContext(), "Constructor - UpImg",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected Void doInBackground(Void... arg0) {

        Toast.makeText(getApplicationContext(), "DoinBgd - UpImg",Toast.LENGTH_SHORT).show();


/*      ByteArrayOutputStream baos = new ByteArrayOutputStream();
        image.compress(Bitmap.CompressFormat.PNG, 100, baos);
        String encodedImage = Base64.encodeToString(baos.toByteArray(), Base64.DEFAULT);
        Toast.makeText(getApplicationContext(), "DoInBgd - UpImg",Toast.LENGTH_LONG).show();

        ArrayList<NameValuePair> dataToSend = new ArrayList();
        dataToSend.add(new BasicNameValuePair("image", encodedImage));
        dataToSend.add(new BasicNameValuePair("name", name));

        HttpParams httpRequestParams = getHttpRequestParams();
        HttpClient client = new DefaultHttpClient(httpRequestParams);
        HttpPost post = new HttpPost(SERVERADDRESS + "/SavePicture.php");

        try{
            post.setEntity(new UrlEncodedFormEntity(dataToSend));
            client.execute(post);
        }catch(Exception e)
        {
            e.printStackTrace();
        }*/
        return null; 
    }

    @Override
    protected void onPostExecute(Void result) {
        // TODO Auto-generated method stub

        Toast.makeText(getApplicationContext(), "OnPost exec - UpImg",Toast.LENGTH_SHORT).show();
        super.onPostExecute(result);
        //Toast.makeText(getApplicationContext(), "Image Uploaded" , Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onPreExecute() {
        // TODO Auto-generated method stub
        Toast.makeText(getApplicationContext(), "PreExec - UpImg",Toast.LENGTH_SHORT).show();
        /*ByteArrayOutputStream baos = new ByteArrayOutputStream();
        image.compress(Bitmap.CompressFormat.PNG, 100, baos);
        String encodedImage = Base64.encodeToString(baos.toByteArray(), Base64.DEFAULT); */
        super.onPreExecute();
    }
}

其他方法的Toast正确显示。但是，当我将Toast放入doInBac…（）方法时，我收到错误。以下是LogCat（）输出：

05-17 10:38:51.888: E/AndroidRuntime(18508): FATAL EXCEPTION: AsyncTask #3
05-17 10:38:51.888: E/AndroidRuntime(18508): java.lang.RuntimeException: An error occured while executing doInBackground()
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.os.AsyncTask$3.done(AsyncTask.java:299)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at java.util.concurrent.FutureTask.setException(FutureTask.java:219)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at java.util.concurrent.FutureTask.run(FutureTask.java:239)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at java.lang.Thread.run(Thread.java:856)
05-17 10:38:51.888: E/AndroidRuntime(18508): Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.os.Handler.<init>(Handler.java:197)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.os.Handler.<init>(Handler.java:111)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.widget.Toast$TN.<init>(Toast.java:324)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.widget.Toast.<init>(Toast.java:91)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at android.widget.Toast.makeText(Toast.java:238)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at com.msrohit.imageselect.MainActivity$UploadImage.doInBackground(MainActivity.java:191)
05-17 10:38:51.888: E/AndroidRuntime(18508):    at com.msrohit.imageselect.MainActivity$UploadImage.doInBackground(MainActivity.java:1)

RunTimeException背后的原因是什么？