Android AsyncTask - avoid multiple instances running

Question

I have AsyncTask that processes some background HTTP stuff. AsyncTask runs on schedule (Alarms/service) and sometime user executes it manually.

I process records from SQLite and I noticed double-posts on server which tells me that sometime scheduled task runs and at the same time user runs it manually causing same record to be read and processed from DB twice. I remove records after they processed but still get this.

How should I handle it ? Maybe organize some kind of queing?

Answer 1

You can execute your AsyncTask's on an Executor using executeOnExecutor()

To make sure that the threads are running in a serial fashion please use: SERIAL_EXECUTOR.

Misc: How to use an Executor

If several activities are accessing your DB why don't create a sort of gateway database helper and use the synchronized block to ensure only one thread has access to it at an instant

Answer 2

Or, you can try this to see if the Task is currently running or not:

if (katitsAsyncTask.getStatus().equals(AsyncTask.Status.FINISHED))
     katitsAsyncTask.execute();
else
     // wait until it's done.

Answer 3

Initialize the AsyncTask to null. Only create a new one if it is null. In onPostExecute, set it to null again, at the end. Do the same in onCancelled, in case the user cancels this. Here's some untested code to illustrate the basic idea.

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class FooActivity extends Activity {

    private class MyAsyncTask extends AsyncTask<Foo, Foo, Foo> {
        @Override
        protected void onPostExecute(Foo foo) {
                    // do stuff
            mMyAsyncTask = null;
        }

        @Override
        protected void onCancelled() {
            // TODO Auto-generated method stub
            mMyAsyncTask = null;
        }

        @Override
        protected Foo doInBackground(Foo... params) {
                    try {
                         // dangerous stuff                         
                    } catch (Exception e) {
                        // handle. Now we know we'll hit onPostExecute()
                    }

            return null;
        }
    }

    private MyAsyncTask mMyAsyncTask = null;

    @Override
    public void onCreate(Bundle bundle) {
        Button button = (Button) findViewById(R.id.b2);
        button.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                if (mMyAsyncTask == null) {
                    mMyAsyncTask = new MyAsyncTask();
                    mMyAsyncTask.execute(null);
                }
            }

        });
    }

}