在主线程上运行在android中意味着什么

Question

I'm trying to connect to a PC server (using Hercules) through TCP from an android app (client) but I'm really lost and don't know where to go. None of the tutorials are fully working for me, most of them can allow me to send messages from client to server but not vice versa.

I read about connections not supposed to be run from the "main thread" but what does that mean?

Also any examples of a TCP connection from android would be great.

Thanks!

Answer 1

I suspect that the "main thread" in this context means that thread that is managing the user interface. If you do anything much in this thread, you run the risk of Android killing your app because it appears to have hung.

Personally, I don't think it's a huge problem to have the user interface thread block for a few milliseconds to do a TCP operation. You'd need to make sure that you code the operation to have sensible timeouts, so you don't end with a dead app because the remote server takes too long to respond.

The official way to handle situations like this is to define the network operations in services, or in separate threads, and have the user interface thread communicate with those services or threads using short-lived operations.

This process is documented with examples here:

https://developer.android.com/training/basics/network-ops/connecting

Answer 2

Main thread in android is responsible to create and display UI on screen

to perform task related to connection strictly need to use background thread otherwise UI become laggy.

There are two method available to perform background Task

1. Runnable:

    new Thread(new Runnable() { public void run() { ..... // code here } }).start();

2. Android AsyncTask: https://developer.android.com/reference/android/os/AsyncTask

Answer 3

Like @Kevin Boone said, Main thread means UI thread in Android.

You can't do networking operations in the Main thread, otherwise you will get NetworkOnMainThreadException . You can create a new thread and then pass result back to the Main thread using Handler. Code could look like this:

public class MyActivity extends AppCompatActivity implements FetchDataUseCase.Listener {
    
    private FetchDataUseCase fetchDataUseCase;
    private TextView textView;
    private Button dataButton;

    public void onCreate() {
        textView = findViewById(R.id.textView);
        dataButton = findViewById(R.id.dataButton);
        dataButton.setOnClickListener(v -> getDataFromNetwork());

        fetchDataUseCase = new FetchDataUseCase(this);
    }

    void getDataFromNetwork() {
        fetchDataUseCase.fetchDataAndNotify();
        // start async operation! and receive result in onDataFetched()
    }

    @Override
    public void onDataFetched(String data) {
        // now you are in Main thread
        // do something with data

        textView.setText(data);
        textView.setVisibility(View.VISIBLE);
    }
    
    @Override
    public void onError() {
        textView.setText("ERROR!!!");
        textView.setVisibility(View.VISIBLE);
    }
}

public class FetchDataUseCase {
    public interface Listener {
        void onDataFetched(String data);
        void onError();
    }

    private final Listener listener;
    private final Handler handler = new Handler(Looper.getMainLooper());

    public FetchDataUseCase(Listener listener) {
        this.listener = listener;
    }

    public void fetchDataAndNotify() {
        new Thread(() -> {
            String myData = "";

            try {
                // your networking operation
                // where you receive some data
            } catch (Exception e) {
                handler.post(() -> listener.onError();
            } finally {
                // close stream, file, ...
            }

            // pass it back to Listener in Ui Thread
            handler.post(() -> listener.onDataFetched(myData);
        }).start();
    }
}

read ThreadPoster: Multi-Threading in Android

And don't use AsyncTask =)) Async task is deprecated

Also, you need to add permision to your AndroidManifest file.

<uses-permission android:name="android.permission.INTERNET"/>

Hope it will help you)) Good luck!