Android Messenger realizes two way communication between processes

Brief introduction

Messenger is the simplest way of Android inter-process communication (IPC), which can realize two-way communication between processes. See official website for details

Code implementation

Server application implementation

MessengerService receives messages from clients:

package com.test.messengerservice;
import android.app.Service;
import android.content.Intent;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;
import androidx.annotation.NonNull;

public class MessengerService extends Service {

    // Message type of receiving client 
    private static final int SEND_MESSENGER = 0;
    private static final int CONFIG_NET = 1;
    private static final int CANCEL = 2;

    // Type of message sent to client 
    private static final int FIND_DEVICE = 10;

    public MessengerService() {
    }

    private Messenger messenger = new Messenger(new ServiceHandler());
    private static Messenger mClient;

    public class ServiceHandler extends Handler {

        @Override
        public void handleMessage(@NonNull Message msg) {
            //  Processing messages 
            switch(msg.what){
                case SEND_MESSENGER:
                    Log.d("service", "receive messenger");
                    mClient = msg.replyTo;
                    break;
                case CONFIG_NET:
                    Log.d("service", "config net task");
                    mClient = msg.replyTo;
                    break;
                case CANCEL:
                    Log.d("service", "cancel task");
                    mClient = msg.replyTo;
                    break;
                default:
                    break;
            }
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        Log.i("service", "service bind");
        return messenger.getBinder();
    }

    @Override
    public boolean onUnbind(Intent intent) {
        Log.i("service", "service unbind");
        mClient = null;
        return super.onUnbind(intent);
    }

 // Send a message to the client 
    public static void sendMessage() {
        if (null == mClient) {
            Log.d("service", "client is null");
            return;
        }
        try {
            Message message = Message.obtain(null, FIND_DEVICE);
            mClient.send(message);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }
}

Register the messenger service in AndroidManifest. xml:

<service
    android:name=".MessengerService"
    android:enabled="true"
    android:exported="true">
    <intent-filter>
        <action android:name="android.intent.action.MESSENGER"/>
    </intent-filter>
</service>

The setup buttons in MainActivity are used to actively send messages to the client:

package com.test.messengerservice;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = findViewById(R.id.findDeviceButton);
        button.setOnClickListener( new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                MessengerService.sendMessage();
            }
        });
    }
}

Client application implementation

Bind the server's service in MainActivity and set the button to send messages to the client:

package com.test.messengerclient;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    //  Server-side Messenger
    private Messenger mService;
    //  Client's Messenger
    private Messenger mMessenger;

    private Button buttonConfigNet;
    private Button buttonCancel;

    // Type of message sent to server 
    private static final int SEND_MESSENGER = 0;
    private static final int CONFIG_NET = 1;
    private static final int CANCEL = 2;

    // Receive the message type of the server 
    private static final int FIND_DEVICE = 10;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Client Messenger
        mMessenger = new Messenger(new ClientHandler());
        // Binding service 
        bindServiceInvoked();

  // Set click events 
        buttonConfigNet = findViewById(R.id.buttonConfigNet);
        buttonCancel = findViewById(R.id.buttonCancel);
        
        buttonConfigNet.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(mService != null) {
                    try {
                        Log.d("client", "send config net");
                        Message message = Message.obtain(null, CONFIG_NET);
                        message.replyTo = mMessenger;
                        mService.send(message);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                }
            }
        });

        buttonCancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(mService != null) {
                    try {
                        Log.d("client", "send cancel");
                        Message message = Message.obtain(null, CANCEL);
                        message.replyTo = mMessenger;
                        mService.send(message);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
    }

 // Receive service connection and disconnection messages 
    ServiceConnection serviceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            Log.i("client", "service connected");
            mService = new Messenger(service);

   // Because the server does not have the client's after binding Messenger  After binding, put the client first Messenger Send to the server 
            if(mService != null) {
                try {
                    Log.d("client", "send messenger");
                    Message message = Message.obtain(null, SEND_MESSENGER);
                    message.replyTo = mMessenger;
                    mService.send(message);
                } catch (RemoteException e) {
                    e.printStackTrace();
                }
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            Log.i("client", "service disconnected");
            mService = null;
        }
    };

 // Receive messages from the server 
    public class ClientHandler extends Handler {

        @Override
        public void handleMessage(@NonNull Message msg) {
            switch (msg.what){
                case FIND_DEVICE:
                    Log.i("client", "find device");
                    break;
            }
        }
    }

 // Bind server-side service
    private void bindServiceInvoked()
    {
        Intent intent = new Intent();
        intent.setAction("android.intent.action.MESSENGER");
        intent.setPackage("com.test.messengerservice");
        bindService(intent, serviceConnection, Context.BIND_AUTO_CREATE);
    }
}