Socket也称为套接字，可以用来实现进程间的通信。Socket本身支持传输任意字节流。

demo实现了简单的跨进程的聊天程序。

声明权限

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

服务端

public class TCPServerService extends Service {

    private boolean mIsServiceDestoryed = false;

    private String[] mDefinedMessages = new String[]{
            "你好啊~~",
            "我叫TCP，你呢",
            "今天刮台风了，OMG",
            "我是人工智能(障)噢",
    };

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public void onCreate() {
        new Thread(new TcpServer()).start();
        super.onCreate();
    }

    @Override
    public void onDestroy() {
        mIsServiceDestoryed = true;
        super.onDestroy();
    }

    private class TcpServer implements Runnable{

        @SuppressWarnings("resource")
        @Override
        public void run() {
            ServerSocket serverSocket = null;
            try {
                serverSocket = new ServerSocket(8688);
            } catch (IOException e) {
                System.err.println("establish tcp server failed,port:8688");
                e.printStackTrace();
            }
            while (!mIsServiceDestoryed){
                try {
                    final Socket client = serverSocket.accept();//接收客户端的数据
                    System.out.println("accept");
                    new Thread(){
                        @Override
                        public void run() {
                            try {
                                responseClient(client);
                            } catch (IOException e) {
                                e.printStackTrace();
                            }
                        }
                    }.start();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    private void responseClient(Socket client) throws IOException {
        //接收客户端消息
        BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
        //向客户端发送消息
        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(client.getOutputStream())),true);
        out.println("欢迎来到聊天室");//向客户端发送消息
        while (!mIsServiceDestoryed){
            String str = in.readLine();//接收到的来自客户端的消息
            System.out.println("msg from client:" + str);
            if(str ==null){
                break;//断开连接
            }
            int i = new Random().nextInt(mDefinedMessages.length);
            String msg = mDefinedMessages[i];
            out.println(msg);
            System.out.println("send:" + msg);
        }
        System.out.println("client quit.:");
        //关闭流
        out.close();
        in.close();
        client.close();
    }
}

客户端

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    private static final int MESSAGE_RECEIVE_NEW_MSG = 1;
    private static final int MESSAGE_SOCKET_CONNECTED = 2;

    private Button mSendButton;
    private TextView mMessageTextView;
    private EditText mMessageEditText;

    private PrintWriter mPrintWriter;
    private Socket mClientSocket;

    @SuppressLint("HandlerLeak")
    private Handler mHandler = new Handler(){
        @Override@SuppressLint("HandlerLeak")
        public void handleMessage(Message msg) {
            switch (msg.what){
                case MESSAGE_RECEIVE_NEW_MSG:
                    mMessageTextView.setText(mMessageTextView.getText() + (String)msg.obj);
                    break;
                case MESSAGE_SOCKET_CONNECTED:
                    mSendButton.setEnabled(true);
                    break;
                default:
                    break;
            }
        }
    };

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

    private void initView(){
        mMessageTextView = (TextView) findViewById(R.id.textView);
        mSendButton = (Button) findViewById(R.id.button);
        mSendButton.setOnClickListener(this);
        mMessageEditText = (EditText) findViewById(R.id.editText);
        Intent service = new Intent(this, TCPServerService.class);
        startService(service);
        new Thread(){
            @Override
            public void run() {
                connectTCPServer();
            }
        }.start();
    }

    @Override
    protected void onDestroy() {
        //Activity退出时，关闭当前的Socket
        if(mClientSocket != null){
            try {
                mClientSocket.shutdownInput();
                mClientSocket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        super.onDestroy();
    }

    @Override
    public void onClick(View v) {
        if(v.getId() ==R.id.button){
            final String msg = mMessageEditText.getText().toString();
            if(!TextUtils.isEmpty(msg) && mPrintWriter != null){//isEmpty() null或""都返回true
                mPrintWriter.println(msg);
                mMessageEditText.setText("");
                String time = formatDateTime(System.currentTimeMillis());
                final String showedMsg = "self" + time + ":" + msg + "\n";
                mMessageTextView.setText(mMessageTextView.getText() + showedMsg);
            }
        }
    }

    private String formatDateTime(long time) {
        return new SimpleDateFormat("(HH:mm:ss)").format(new Date(time));
    }

    private void connectTCPServer(){
        Socket socket = null;
        //超时重连策略
        while(socket == null){
            try {
                socket = new Socket("localhost",8688);//指定服务器端的ip地址和端口号
                mClientSocket = socket;
                mPrintWriter = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())),true);
                mHandler.sendEmptyMessage(MESSAGE_SOCKET_CONNECTED);
                System.out.println("connect server success");
            } catch (IOException e) {
                SystemClock.sleep(1000);//延迟一秒后重连
                e.printStackTrace();
            }
        }

        try {
            BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            while (!MainActivity.this.isFinishing()){
                String msg = br.readLine();
                System.out.println("receive :" + msg);
                if (msg != null){
                    String time = formatDateTime(System.currentTimeMillis());
                    final String showedMsg = "server " + time + ":" + msg + "\n";
                    mHandler.obtainMessage(MESSAGE_RECEIVE_NEW_MSG,showedMsg).sendToTarget();
                }
            }
            System.out.println("quit...");
            mPrintWriter.close();
            br.close();
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}