检查网址是否存在

Question

For my project I need to check a URL to see if it exists or not. So I created a HttpURLConnection and connected to the url and wait for the response code. Since I can't use that HttpURLConnection in my Main Thread, i had to move it to an AsyncTask and let it run on a separate Thread.

This is what my code for that AsyncTask looks like (only the important parts):

private class URLExists extends AsyncTask<String, Void, Boolean> {

    protected Boolean doInBackground(String... urls)
    {
        boolean isValid = false;

        try
        {
            URL myurl = new URL(urls[0]);
            HttpURLConnection.setFollowRedirects(false);

            HttpURLConnection connection = (HttpURLConnection) myurl.openConnection();
            connection.setUseCaches(true);
            connection.setRequestMethod("HEAD");
            connection.connect();
            int code = connection.getResponseCode();

            ...

            connection.disconnect();

            if(code != 404)
                isValid = true;

            else
                isValid = false;
        }

            ...
        return isValid;
    }

    protected void onPostExecute(Boolean isValid)
    {
        isTrue = isValid;
    }
}

So basically i then get a boolean value isTrue which tells me if the url is valid. When I run this i get the right results, everything works.

But now, when i execute this AsyncTask, it runs parallel to my main Thread, obviously.

I have this code:

new URLExists().execute("https://www.example.com");
if(isTrue)
{
    myButton.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                //Do Something
            }
        });
}

Since my AsyncTask runs parallel, my Main Thread executes the next operation without waiting for the result of the AsyncTask. Since isTrue is false by default, the if statement never runs.

Of course i know about the protected void onPostExecute() in the AsyncTask, but does that mean that all the code that follows in my main Thread has to be in that function in the parallel Thread? It all depends on the outcome of the URL check.

I know about the .get() method, but that freezes my MainThread and the UI, so that's not really an option for me.

Is there any other way of making the Main thread wait for the AsyncTask? I know the defeats the purpose of the AsyncTask, but all I want is to check if the URL exists and I can only do checks in an AsyncTask.

Answer 1

You don't need multithread for your task. All what you need is to check that url is available. Right? Of course it may take some time and you have to wait this time in your main thread. That's right. But you still need to wait. Don't create unnecessary complexity. Just read url, get response code, validate it, check for timeout exception and you are good to go.

Answer 2

We need to look at the specification of AsyncTask. Typically in any asynchronous method you have a callback function that lets you know the background operation has completed. In the case of Android it is onPostExecute()

It can be found here. http://developer.android.com/reference/android/os/AsyncTask.html

When an asynchronous task is executed, the task goes through 4 steps:

1. onPreExecute() , invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.

2. doInBackground(Params...) , invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.

3. onProgressUpdate(Progress...) , invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.

4. onPostExecute(Result) , invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

For your specific case you might want to do something like this..

yourAsyncTask{
   @Override
   protected Boolean doInBackground(Void... params){
        // CHECK YOUR URL HERE
   }
   .
   .
   .
   @Override
   public void onPostExecute(Result result){
      if(isTrue) {
        myButton.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            //Do Something
         }
        });
      }
   }
}