作为课程作业的一部分，我们的任务是在 Java 提供的 UDP 层之上添加额外的可靠性层，以便发送大图片文件。这将使用 Go-Back-N 协议完成：http ://en.wikipedia.org/wiki/Go_back_N
据我了解，这个问题的症结在于能够发送数据包，同时检查是否有任何确认已进入旧数据包，这将允许您移动窗口。
我目前通过两个线程来做到这一点：一个在窗口中有空间时发送下一个数据包；一个持续监听任何传入的确认并做出适当反应的人。
我的问题是程序应该是线程化的，就好像这两个线程同时运行一样，但实际上，ACKReceiver 线程似乎获得了非常不成比例的时间。从线程转储来看，当发送线程到达 DataSocket.receive() 行时，它似乎“饿死”了一段时间，在此阻塞执行，同时不给其他线程运行的机会。
我看过以下问题，这似乎暗示问题与 DatagramSocket.receive 是同步的事实有关......但没有提供可用的问题解决方案：
这是我代码的发送方部分的代码，我相对确定另一端的接收方非常好（一方面，我不必使用任何线程来让它工作！）：
import java.io.*;
import java.net.*;
import java.nio.ByteBuffer;
public class Sender3 {
short base = 0;
short nextSeqNum = 0;
DatagramPacket[] packets;
ByteBuffer bb;
String endSys;
int portNum;
String fileName;
int retryTime;
int windowSize;
DatagramSocket clientSocket;
InetAddress IPAddress;
boolean timedOut = false;
public Sender3(String endSys, int portNum, String fileName, int retryTime, int windowSize){
this.endSys = endSys;
this.portNum = portNum;
this.fileName = fileName;
this.retryTime = retryTime;
this.windowSize = windowSize;
}
public static void main(String args[]) throws Exception{
//Check for current arguments and assign them
if(args.length != 5){
System.out.println("Invalid number of arguments. Please specify: <endSystem> <portNumber> <fileName> <retryTimeout><windowSize>");
System.exit(1);
}
Sender3 sendy = new Sender3(args[0], Integer.parseInt(args[1]), args[2], Integer.parseInt(args[3]), Integer.parseInt(args[4]));
sendy.go();
}
private void go() throws Exception{
clientSocket = new DatagramSocket();
bb = ByteBuffer.allocate(2);
byte[] picData = new byte[1021];
byte[] sendData = new byte[1024];
Thread.yield()
short seqNum = 0;
byte[] seqBytes = new byte[2];
byte EOFFlag = 0;
boolean acknowledged = false;
int lastPacketRetrys = 0;
int resends = 0;
IPAddress = InetAddress.getByName(endSys);
FileInputStream imReader = new FileInputStream(new File(fileName));
double fileSizeKb = imReader.available() / 1021.0; //We add 3 bytes to every packet, so dividing by 1021 will give us total kb sent.
int packetsNeeded = (int) Math.ceil(fileSizeKb);
packets = new DatagramPacket[packetsNeeded];
long startTime = System.currentTimeMillis();
long endTime;
double throughput;
//Create array of packets to send
for(int i = 0; i < packets.length; i++){
if(i == packets.length - 1){
EOFFlag = 1;
picData = new byte[imReader.available()];
sendData = new byte[picData.length + 3];
}
imReader.read(picData);
bb.putShort((short)i);
bb.flip();
seqBytes = bb.array();
bb.clear();
System.arraycopy(seqBytes, 0, sendData, 0, seqBytes.length);
sendData[2] = EOFFlag;
System.arraycopy(picData, 0, sendData, 3, picData.length);
packets[i] = new DatagramPacket((byte[])sendData.clone(), sendData.length, IPAddress, portNum);
}
ACKGetter ackGet = new ACKGetter();
Thread ackThread = new Thread(ackGet);
ackThread.start();
//System.out.println("timeout is: " + timedOut + " base is: " + base + " packet length is: " + packets.length + " nextSeqNum: " + nextSeqNum);
while(base != packets.length){
if(timedOut){
//System.out.println("Timed out waiting for acknowledgement, resending all unACKed packets in window");
clientSocket.setSoTimeout(retryTime);
resends++;
if(nextSeqNum == packets.length)
lastPacketRetrys++;
//Resend all packets in window
for (int i = base; i < nextSeqNum; i++){
// System.out.println("Resending packets with number: " + i);
clientSocket.send(packets[i]);
}
timedOut = false;
}
if(nextSeqNum - base < windowSize && nextSeqNum < packets.length){
//System.out.println("sending packet with seqNum: " + nextSeqNum);
clientSocket.send(packets[nextSeqNum]);
if(base == nextSeqNum){
clientSocket.setSoTimeout(retryTime);
}
nextSeqNum++;
}
else{
//Thread.yield();
}
}
if(lastPacketRetrys > 10){
System.out.println("Last packet ACK was lost (we think). So we just gave up, number of retransmissions will probably be higher");
}
endTime = System.currentTimeMillis();
throughput = 1000 * fileSizeKb / (endTime - startTime);
clientSocket.close();
imReader.close();
System.out.println("Number of retransmissions: " + resends);
System.out.println("Average throughput is: " + throughput + "Kb/s");
}
private class ACKGetter implements Runnable {
//Listen out for ACKs and update pointers accordingly
DatagramPacket ackPacket;
byte[] ackData = new byte[2];
public void run() {
while(base != packets.length){
if(base != nextSeqNum){
try{
ackPacket = new DatagramPacket(ackData, ackData.length);
clientSocket.receive(ackPacket);
ackData = ackPacket.getData();
bb.put(ackData[0]);
bb.put(ackData[1]);
bb.flip();
short ack = bb.getShort();
bb.clear();
if(base <= ack){
//System.out.println("acknowledgement for base num: " + base + "ack num:" + ack);
base = (short) (ack + 1);
//If theres nothing left in window, stop timing, otherwise restart the timer
if(base == nextSeqNum){
clientSocket.setSoTimeout(0);
}
else{
clientSocket.setSoTimeout(retryTime);
}
}
else{
//System.out.println("ACK didnt change anything: " + ack);
}
}
catch(Exception ex){
timedOut = true;
//System.out.println("Packet timed out...resending..");
}
}
Thread.yield();
}
}
}
}