参考

1、Android Socket通信--UdpClient
2、Android UDP
3、Android Socket通信（一）--基于UDP协议通信

截图

1、Android 客户端

client.png

2、PC服务端：用的是SocketTool软件模拟

server.png

流程

1、连接DatagramSocket的服务端（ip和port）：开启异步线程和socket
2、发送数据（DatagramPacket）：异步
3、接收数据（DatagramPacket）：注意连接状态，异步读取
4、关闭连接：关闭DatagramSocket和对应线程

注意

1、异常：android.os.NetworkOnMainThreadException。 socket需要在线程中使用
2、前后端统一传输或者接收协议 [requestcode size d1 d2 d3 ... ]，在解析时候用得到
3、实施监控socket的连接状态，还是用心跳包发过去，然后返回数据，一段时间没有的话则代表socket连接失败。
4、注意receive接收数据后的有效长度（一个是预存的buffer，一个是有效结果buffer）
5、客户端连上去后不知道为何一定要先发送一次，才能接收？
6、UDP不安全，有长度限制64K

代码

1、UdpClient.java：udp-socket的客户端，略微做了通用封装，主要是连接，发送，接收，然后设置监听

/**
 * Created by wujn on 2019/2/15.
 * Version : v1.0
 * Function: udp client 64k限制
 */
public class UdpClient {


    /**
     * single instance UdpClient
     * */
    private static UdpClient mSocketClient = null;
    private UdpClient(){}
    public static UdpClient getInstance(){
        if(mSocketClient == null){
            synchronized (UdpClient.class) {
                mSocketClient = new UdpClient();
            }
        }
        return mSocketClient;
    }


    String TAG_log = "Socket";
    private DatagramSocket mSocket;
    private DatagramPacket sendPacket;    //发送
    private DatagramPacket receivePacket; //接受

//  private OutputStream mOutputStream;
//  private InputStream mInputStream;

    private SocketThread mSocketThread;
    private boolean isStop = false;//thread flag


    /**
     * 128 - 数据按照最长接收，一次性
     * */
    private class SocketThread extends Thread {

        private String ip;
        private int port;
        public SocketThread(String ip, int port){
            this.ip = ip;
            this.port = port;
        }

        @Override
        public void run() {
            Log.d(TAG_log,"SocketThread start ");
            super.run();

            //connect ...
            try {
                if (mSocket != null) {
                    mSocket.close();
                    mSocket = null;
                }

                InetAddress ipAddress = InetAddress.getByName(ip);
                mSocket = new DatagramSocket();
                mSocket.connect(ipAddress, port); //连接

                //设置timeout
                //mSocket.setSoTimeout(3000);
                Log.d(TAG_log,"udp connect = "+isConnect());

                if(isConnect()){
                    isStop = false;
                    uiHandler.sendEmptyMessage(1);
                }
                /* 此处这样做没什么意义不大，真正的socket未连接还是靠心跳发送，等待服务端回应比较好，一段时间内未回应，则socket未连接成功 */
                else{
                    uiHandler.sendEmptyMessage(-1);
                    Log.e(TAG_log,"SocketThread connect fail");
                    return;
                }

            }
            catch (IOException e) {
                uiHandler.sendEmptyMessage(-1);
                Log.e(TAG_log,"SocketThread connect io exception = "+e.getMessage());
                e.printStackTrace();
                return;
            }
            Log.d(TAG_log,"SocketThread connect over ");

            //发送一次，否则不发送则收不到，不知道为啥。。。
            sendByteCmd(new byte[]{00},-1);//send once

            //read ...
            while (isConnect() && !isStop && !isInterrupted()) {

                int size;
                try {
                    byte[] preBuffer = new byte[4 * 1024];//预存buffer
                    receivePacket = new DatagramPacket(preBuffer, preBuffer.length);
                    mSocket.receive(receivePacket);

                    if (receivePacket.getData() == null) return;
                    size = receivePacket.getLength();     //此为获取后的有效长度，一次最多读64k，预存小的话可能分包
                    Log.d(TAG_log, "pre data size = "+receivePacket.getData().length + ", value data size = "+size);
                    byte[] dataBuffer = Arrays.copyOf(preBuffer, size);

                    if (size > 0) {
                        Message msg = new Message();
                        msg.what = 100;
                        Bundle bundle = new Bundle();
                        bundle.putByteArray("data",dataBuffer);
                        bundle.putInt("size",size);
                        bundle.putInt("requestCode",requestCode);
                        msg.setData(bundle);
                        uiHandler.sendMessage(msg);
                    }
                    Log.i(TAG_log, "SocketThread read listening");
                    //Thread.sleep(100);//log eof
                }
                catch (IOException e) {
                    uiHandler.sendEmptyMessage(-1);
                    Log.e(TAG_log,"SocketThread read io exception = "+e.getMessage());
                    e.printStackTrace();
                    return;
                }
            }
        }
    }



    //==============================socket connect============================
    private String ip;
    private int port;
    /**
     * connect socket in thread
     * Exception : android.os.NetworkOnMainThreadException
     * */
    public void connect(String ip, int port){
        this.ip = ip;
        this.port = port;
        mSocketThread = new SocketThread(ip, port);
        mSocketThread.start();
    }

    /**
     * socket is connect
     * */
    public boolean isConnect(){
        boolean flag = false;
        if (mSocket != null) {
            flag = mSocket.isConnected();
        }
        return flag;
    }
    
    /**
     * socket disconnect
     * */
    public void disconnect() {
        isStop = true;
        if (mSocket != null) {
            mSocket.close();
            mSocket = null;
        }
        if (mSocketThread != null) {
            mSocketThread.interrupt();//not intime destory thread,so need a flag
        }
    }



    /**
     * send byte[] cmd
     * Exception : android.os.NetworkOnMainThreadException
     * */
    public void sendByteCmd(final byte[] mBuffer,int requestCode) {
        this.requestCode = requestCode;

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    InetAddress ipAddress = InetAddress.getByName(ip);
                    sendPacket = new DatagramPacket(mBuffer, mBuffer.length, ipAddress, port);
                    mSocket.send(sendPacket);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }).start();

    }




    Handler uiHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch(msg.what){
                //connect error
                case -1:
                    if (null != onDataReceiveListener) {
                        onDataReceiveListener.onConnectFail();
                        disconnect();
                    }
                    break;

                //connect success
                case 1:
                    if (null != onDataReceiveListener) {
                        onDataReceiveListener.onConnectSuccess();
                    }
                    break;

                //receive data
                case 100:
                    Bundle bundle = msg.getData();
                    byte[] buffer = bundle.getByteArray("data");
                    int size = bundle.getInt("size");
                    int mequestCode = bundle.getInt("requestCode");
                    if (null != onDataReceiveListener) {
                        onDataReceiveListener.onDataReceive(buffer, size, mequestCode);
                    }
                    break;
            }
        }
    };

    
    /**
     * socket response data listener
     * */
    private OnDataReceiveListener onDataReceiveListener = null;
    private int requestCode = -1;
    public interface OnDataReceiveListener {
        public void onConnectSuccess();
        public void onConnectFail();
        public void onDataReceive(byte[] buffer, int size, int requestCode);
    }
    public void setOnDataReceiveListener(
            OnDataReceiveListener dataReceiveListener) {
        onDataReceiveListener = dataReceiveListener;
    }
    

}

MainActivity.java：使用，简单明了

private void initListener(){
        //socket connect
        btn_connect.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String ip = et_ip.getText().toString();
                String port = et_port.getText().toString();

                if(TextUtils.isEmpty(ip)){
                    Toast.makeText(UdpActivity.this,"IP地址为空",Toast.LENGTH_SHORT).show();
                    return;
                }
                if(TextUtils.isEmpty(port)){
                    Toast.makeText(UdpActivity.this,"端口号为空",Toast.LENGTH_SHORT).show();
                    return;
                }

                connect(ip, Integer.parseInt(port));
            }
        });

        //socket disconnect
        btn_disconnect.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                disconnect();
            }
        });

        //socket send
        btn_send.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (UdpClient.getInstance().isConnect()) {
                    byte[] data = et_send.getText().toString().getBytes();
                    send(data);
                } else {
                    Toast.makeText(UdpActivity.this,"尚未连接，请连接Socket",Toast.LENGTH_SHORT).show();
                }
            }
        });

        //clear receive
        btn_clear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                tv_receive.setText("");
            }
        });
    }

    /**
     * socket data receive
     * */
    private void initDataReceiver(){
        UdpClient.getInstance().setOnDataReceiveListener(dataReceiveListener);
    }

    /**
     * socket connect
     * */
    private void connect(String ip, int port){
        UdpClient.getInstance().connect(ip, port);
    }

    /**
     * socket disconnect
     * */
    private void disconnect(){
        UdpClient.getInstance().disconnect();
        tv_state.setText("未连接");
    }

    /**
     * socket send
     * */
    private void send(byte[] data){
        String ip = et_ip.getText().toString();
        String port = et_port.getText().toString();

        UdpClient.getInstance().sendByteCmd(data,1001);
    }


    /**
     * socket data receive
     * data(byte[]) analyze
     * */
    private UdpClient.OnDataReceiveListener dataReceiveListener = new UdpClient.OnDataReceiveListener() {
        @Override
        public void onConnectSuccess() {
            Log.i(TAG_log,"onDataReceive connect success");
            tv_state.setText("已连接");
        }

        @Override
        public void onConnectFail() {
            Log.e(TAG_log,"onDataReceive connect fail");
            tv_state.setText("未连接");
        }

        @Override
        public void onDataReceive(byte[] buffer, int size, int requestCode) {
            //获取有效长度的数据
            byte[] data = new byte[size];
            System.arraycopy(buffer, 0, data, 0, size);

            final String oxValue = Arrays.toString(HexUtil.Byte2Ox(data));
            Log.i(TAG_log,"onDataReceive requestCode = "+requestCode + ", content = "+oxValue);

            tv_receive.setText(tv_receive.getText().toString() + oxValue + "\n");

        }
    };



    @Override
    protected void onDestroy() {
        UdpClient.getInstance().disconnect();
        super.onDestroy();
    }

2019 ^{(*￣(oo)￣)} 诸事顺利！