如何从外部调用方法到AsyncTask类中？

Question

I have two simple classes:

public class MainActivity extends Activity {
    NetworkTask task;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        [...]       
        task = new NetworkTask();
        task.execute();
    }

public void myClickHandler(View view) {
    switch(view.getId()) {
        case R.id.button1:
            // Why this line crash?
            task.connection("127.0.0.1");
        break;
        }
    }
}

and

public class NetworkTask extends AsyncTask<String, Void, String> {
    Socket sock;
    volatile boolean running = true;

    public int connection(String url){
        try{
            sock = new Socket(url, 4567)
        }
        catch (IOException ex){
            Logger.getLogger(NetworkTask.class.getName()).log(Level.SEVERE, null, ex);
            return -1;
        }
    }

    public String doInBackground(String... strings) {

        // If I do this, it works well
        //connection(127.0.0.1);

        while(running)
        {
            [...]
        }

        return null;
    }   
}

As I commented when I call connection method from outside of the AsyncTask method, the app crashes more particulary « sock = new Socket(...) » line. But when connection call is done inside the AsynTask method socket is created. I don't understand why.

What's happening?

Thanks.

Answer 1

It's because when you do

task.connection("127.0.0.1");

You are still working in the main (UI) Thread - you're not using the AsyncTask properly. Instead you're using it like a normal class, and so, you get a NetworkOnMainThreadException on the new Android versions.

However when you call from doInBackground() , it means you started the AsyncTask via execute and the work is done in a separate Thread, letting everything work as it should.

Keep in mind that if you are doing non-network stuff, you can still call from outside. However, I'd recommend keeping your AsyncTask depend on the outside as little as possible, since AsyncTasks only run once. You then have to make a new instance if you want to do more work, which means if you depend on setter methods or similar, you have to make sure you call those methods again, which makes this simple class more complex than needed.

For a good, to the point explanation of how to use an AsyncTask, this is a pretty good source. And of course the official documentation .