Tcp和IP协议是很重要的内容，但是要理解这些协议最好的方式就是自己根据业务写一些demo，加深理解。

业务需求是：通过电脑端（网页或客户端形式）发送文件到Android的客户端，下面是使用UDP实现的一个简单的文件传输Demo，因UDP为不可靠传输，可能会丢包。

• 服务器端发送本地文件，代码如下：

package client;

import server.udp.UDPUtils;

import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.InetSocketAddress;

public class UDPClient {

    private static final String SEND_FILE_PATH = "/Users/wangjie/Documents/123.mp4";

    public static void main(String[] args){
        long startTime = System.currentTimeMillis();

        byte[] buf = new byte[UDPUtils.BUFFER_SIZE];
        byte[] receiveBuf = new byte[1];

        RandomAccessFile accessFile = null;
        DatagramPacket dpk = null;
        DatagramSocket dsk = null;
        int readSize = -1;
        try {
            accessFile = new RandomAccessFile(SEND_FILE_PATH,"r");
//            dpk = new DatagramPacket(buf, buf.length,new InetSocketAddress(InetAddress.getByName("localhost"), UDPUtils.PORT + 1));
            dpk = new DatagramPacket(buf, buf.length,new InetSocketAddress(InetAddress.getByName("192.168.1.119"), UDPUtils.PORT + 1));
            dsk = new DatagramSocket(UDPUtils.PORT);
            int sendCount = 0;
            while((readSize = accessFile.read(buf,0,buf.length)) != -1){
                System.out.println("readSize:"+readSize);
                dpk.setData(buf, 0, readSize);
                dsk.send(dpk);
                // wait server response
                {
                    while(true){
                        dpk.setData(receiveBuf, 0, receiveBuf.length);
                        dsk.receive(dpk);

                        // confirm server receive
                        if(!UDPUtils.isEqualsByteArray(UDPUtils.successData,receiveBuf,dpk.getLength())){
                            System.out.println("resend ...");
                            dpk.setData(buf, 0, readSize);
                            dsk.send(dpk);
                        }else
                            break;
                    }
                }

                System.out.println("send count of "+(++sendCount)+"!");
            }
            // send exit wait server response
            while(true){
                System.out.println("client send exit message ....");
                dpk.setData(UDPUtils.exitData,0,UDPUtils.exitData.length);
                dsk.send(dpk);

                dpk.setData(receiveBuf,0,receiveBuf.length);
                dsk.receive(dpk);
                // byte[] receiveData = dpk.getData();
                if(!UDPUtils.isEqualsByteArray(UDPUtils.exitData, receiveBuf, dpk.getLength())){
                    System.out.println("client Resend exit message ....");
                    dsk.send(dpk);
                }else
                    break;
            }
        }catch (Exception e) {
            e.printStackTrace();
        } finally{
            try {
                if(accessFile != null)
                    accessFile.close();
                if(dsk != null)
                    dsk.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        long endTime = System.currentTimeMillis();
        System.out.println("time:"+(endTime - startTime));
    }
}

• 客户端接收文件存到本地，代码如下：

package cn.studyou.androidsocket;


import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.InetSocketAddress;

public class UDPReceiveFileActivity extends Activity {

    private String localUrl;
    private static final String FILE_DIR = Environment.getExternalStorageDirectory().getAbsolutePath() + "/VideoCache/";
    private static final String SAVE_FILE_PATH = Environment.getExternalStorageDirectory().getAbsolutePath() + "/VideoCache/";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_udpfile_mplayer);

        Log.e("FILE_DIR", FILE_DIR);
        new Thread(new Runnable() {
            @Override
            public void run() {
                receiveFile();
            }
        }).start();
    }
    public void receiveFile() {

        byte[] buf = new byte[UDPUtils.BUFFER_SIZE];

        DatagramPacket dpk = null;
        DatagramSocket dsk = null;
        BufferedOutputStream bos = null;
        try {
            InetAddress loc = InetAddress.getByName("192.168.1.107");

            dpk = new DatagramPacket(buf, buf.length, new InetSocketAddress(loc, UDPUtils.PORT));
            dsk = new DatagramSocket(UDPUtils.PORT + 1);

            if (localUrl == null) {
                localUrl = SAVE_FILE_PATH + "1235.mp4";
            }
            File cacheFile = new File(localUrl);

            if (!cacheFile.exists()) {
                cacheFile.getParentFile().mkdirs();
                cacheFile.createNewFile();
            }
            bos = new BufferedOutputStream(new FileOutputStream(localUrl));
            System.out.println("wait client ....");
            dsk.receive(dpk);
            System.out.println("wait clientq ....");

            int readSize = 0;
            int readCount = 0;
            int flushSize = 0;
            while ((readSize = dpk.getLength()) != 0) {
                // validate client send exit flag
                if (UDPUtils.isEqualsByteArray(UDPUtils.exitData, buf, readSize)) {
                    System.out.println("server exit ...");
                    // send exit flag
                    dpk.setData(UDPUtils.exitData, 0, UDPUtils.exitData.length);
                    dsk.send(dpk);
                    break;
                }

                bos.write(buf, 0, readSize);
                if (++flushSize % 1000 == 0) {
                    flushSize = 0;
                    bos.flush();
                }
                dpk.setData(UDPUtils.successData, 0, UDPUtils.successData.length);
                dsk.send(dpk);

                dpk.setData(buf, 0, buf.length);
                System.out.println("receive count of " + (++readCount) + " !");
                dsk.receive(dpk);
            }

            // last flush
            bos.flush();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (bos != null)
                    bos.close();
                if (dsk != null)
                    dsk.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }


    }

}