Question

我的Android应用程序中的代码存在问题。我以前没有处理Async，我不确定如何调试问题。出于安全原因，URL已被替换。

class checkLogin extends AsyncTask<String, Void, Boolean> {
    protected Boolean doInBackground(String... params) {
        String urlStr = "http://example.com/api/?username=" + params[0] + "&password=" + params[1] + "&action=checkLogin";
        try{
            URL url = new URL(urlStr);
            HttpURLConnection connection= (HttpURLConnection)url.openConnection();
            connection.setRequestMethod("POST");
            connection.setDoOutput(true);
            connection.setUseCaches(false);
            connection.connect();
            BufferedReader reader= new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String output = "";
            String str;
            while ((str= reader.readLine()) != null) {
                output += "\r\n" + str;
            }
            createMsg(output,"Debug");
            return true;
        }catch(Exception e){
            createMsg("Encountered an error. Report this problem to the development department with steps to reproduce","Fatal Error");
            return false;
        }
    }
}

提前致谢！

编辑： createMsg创建一个android警告框。

错误：

06-07 18:19:35.066: E/AndroidRuntime(1206): FATAL EXCEPTION: AsyncTask #1
06-07 18:19:35.066: E/AndroidRuntime(1206): java.lang.RuntimeException: An error occured while executing doInBackground()
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.os.AsyncTask$3.done(AsyncTask.java:299)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.util.concurrent.FutureTask.setException(FutureTask.java:219)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.util.concurrent.FutureTask.run(FutureTask.java:239)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.lang.Thread.run(Thread.java:856)
06-07 18:19:35.066: E/AndroidRuntime(1206): Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.os.Handler.<init>(Handler.java:197)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.os.Handler.<init>(Handler.java:111)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.app.Dialog.<init>(Dialog.java:107)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.app.AlertDialog.<init>(AlertDialog.java:114)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.app.AlertDialog$Builder.create(AlertDialog.java:931)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at com.unrealonline.timesheet.MainActivity.createMsg(MainActivity.java:55)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at com.unrealonline.timesheet.MainActivity$checkLogin.doInBackground(MainActivity.java:79)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at com.unrealonline.timesheet.MainActivity$checkLogin.doInBackground(MainActivity.java:1)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at android.os.AsyncTask$2.call(AsyncTask.java:287)
06-07 18:19:35.066: E/AndroidRuntime(1206):     at java.util.concurrent.FutureTask.run(FutureTask.java:234)
06-07 18:19:35.066: E/AndroidRuntime(1206):     ... 4 more

Answer 1

您可以使用runOnUiThread或Hanlder来显示来自非UI线程的Toast或Alertbox（在您的情况下来自doInBackground）。这样做：

try{
 //.....your code here...
while ((str= reader.readLine()) != null) {
       output += "\r\n" + str;
     }
Your_Current_Activity.this.runOnUiThread(new Runnable() {
    public void run() {
        createMsg(output,"Debug"); // show message here
    }
});

  return true;
}catch(Exception e){
   //use runOnUiThread for showing Alert here....
  return false;

}

Answer 2

doInBackground中的AsyncTask方法不应在显示（UI）上执行任何活动，因为它特别不在UI线程上。虽然您可以使用@ρяσѕρєяK的答案中公布的方法来实现这一点，但AsyncTask的重点是提供一种从结构化方式从后台线程访问UI的简单机制。

当您从createMsg内拨打doInBackground时，您就打破了这种模式。原则上，AsyncTask中的所有UI操作都应该从以下开始：

onPreExecute，将在doInBackground
onPostExecute，将在doInBackground完成后运行
onProgressUpdate，只要publishProgress

doInBackground

Android dev page包含一个很好的用法示例。

在您的情况下，看起来您应该从doInBackground返回onPostExecute获取的成功或失败代码;如果收到失败代码，它应该调用createMsg。

执行doInBackground（）android时发生错误

2 个答案: