很长一段时间以来，我一直在努力在 Android 应用程序中处理 TFTP 协议(protocol)。其主要功能是从托管 TFTP 服务器的定制设计设备下载文件。
我正在浏览互联网，希望找到一些好的、已经编写好的实现。首先，我尝试使用 TFTP 库，它是 Apache Commons 的一部分。不幸的是没有运气 - 不断超时甚至完全卡住。经过进一步研究，我在 github 上找到了一些代码 - please take a look 。我已将代码采用到 Android，经过一些调整，我终于收到了一些文件。
该设备的创建者表示， block 大小应该正好是 1015 字节。所以我将包大小增加到1015并更新了创建读取请求包的方法:
DatagramPacket createReadRequestPacket(String strFileName) {
byte[] filename = strFileName.getBytes();
byte[] mode = currentMode.getBytes();
int len = rOpCode.length + filename.length + mode.length + 2;
ByteArrayOutputStream outputStream = new ByteArrayOutputStream(len);
try {
outputStream.write(rOpCode);
outputStream.write(filename);
byte term = 0;
outputStream.write(term);
outputStream.write(mode); // "octet"
outputStream.write(term);
outputStream.write("blksize".getBytes());
outputStream.write(term);
outputStream.write("1015".getBytes());
outputStream.write(term);
} catch (IOException e) {
e.printStackTrace();
}
byte[] readPacketArray = outputStream.toByteArray();
return new DatagramPacket(readPacketArray, readPacketArray.length, serverAddr, port);
}
正在下载 block ，但有一个主要问题 - 我下载的文件是分成几部分的，每个部分 512kB(除了最后一个)，而我在 Android 设备上收到的每个部分大约大 0.5kB。看起来好像每次多一个字节或者多一整个附加。显然我不完全理解它并且我错过了一些东西。
这是我的文件接收方法:
byte previousBlockNumber = (byte) -1;
try {
PktFactory pktFactory;
DatagramSocket clientSocket;
byte[] buf;
DatagramPacket sendingPkt;
DatagramPacket receivedPkt;
System.out.print(ftpHandle);
if (isConnected) {
System.out.println("You're already connected to " + hostname.getCanonicalHostName());
}
try {
hostname = InetAddress.getByName(host);
if (!hostname.isReachable(4000)) {
System.out.println("Hostname you provided is not responding. Try again.");
return false;
}
} catch (UnknownHostException e) {
System.out.println("tftp: nodename nor servname provided, or not known");
return false;
}
clientSocket = new DatagramSocket();
pktFactory = new PktFactory(PKT_LENGTH + 4, hostname, TFTP_PORT);
System.out.println("Connecting " +
hostname.getCanonicalHostName() + " at the port number " + TFTP_PORT);
isConnected = true;
ftpHandle = "tftp@" + hostname.getCanonicalHostName() + "> ";
System.out.println("mode " + PktFactory.currentMode);
if (!isConnected) {
System.out.println("You must be connected first!");
}
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
buf = new byte[PKT_LENGTH + 4];
/* Sending the reading request with the filename to the server. **/
try {
/* Sending a RRQ with the filename. **/
System.out.println("Sending request to server.");
sendingPkt = pktFactory.createReadRequestPacket(filename);
clientSocket.setSoTimeout(4500);
clientSocket.send(sendingPkt);
} catch (Exception e) {
e.printStackTrace();
System.out.println("Connection with server failed");
}
boolean receivingMessage = true;
while (true) {
try {
receivedPkt = new DatagramPacket(buf, buf.length);
clientSocket.setSoTimeout(10000);
clientSocket.receive(receivedPkt);
byte[] dPkt = receivedPkt.getData();
byte[] ropCode = pktFactory.getOpCode(dPkt);
/* rPkt either a DATA or an ERROR pkt. If an error then print the error message and
* terminate the program finish get command. **/
if (ropCode[1] == 5) {
String errorMsg = pktFactory.getErrorMessage(dPkt);
System.out.println(errorMsg);
return false;
}
if (receivedPkt.getLength() < PKT_LENGTH + 4 && ropCode[1] == 3) {
byte[] fileDataBytes = pktFactory.getDataBytes(dPkt);
outputStream.write(fileDataBytes);
if (isListFile) {
listBytes = outputStream.toByteArray();
} else {
FileOutputStream fstream = new FileOutputStream(Constants.EEG_DATA_PATH.concat("file.bin"), true);
// Let's get the last data pkt for the current transfering file.
fstream.write(outputStream.toByteArray());
fstream.close();
}
// It's time to send the last ACK message before Normal termination.
byte[] bNum = pktFactory.getBlockNum(dPkt);
DatagramPacket sPkt = pktFactory.createAckPacket(bNum, receivedPkt.getPort());
clientSocket.send(sPkt);
disconnect();
return true;
}
if (ropCode[1] == 3) {
if (receivingMessage) {
System.out.println("Receiving the file now..");
receivingMessage = false;
}
byte[] bNum = pktFactory.getBlockNum(dPkt);
//I've added this if and it reduces file size a little (it was more than 0,5kB bigger)
if (previousBlockNumber != bNum[1]) {
byte[] fileDataBytes = pktFactory.getDataBytes(dPkt);
previousBlockNumber = bNum[1];
outputStream.write(fileDataBytes);
}
/* For each received DATA pkt we need to send ACK pkt back. **/
DatagramPacket sPkt = pktFactory.createAckPacket(bNum, receivedPkt.getPort());
clientSocket.send(sPkt);
}
} catch (SocketTimeoutException e) {
disconnect();
System.out.println("Server didn't respond and timeout occured.");
return false;
}
}
} catch (Exception e) {
System.out.println(e.getMessage());
return false;
}
最佳答案
我知道出了什么问题。这种奇怪的行为是收到最后一个数据包时这一行的结果:
byte[] fileDataBytes = pktFactory.getDataBytes(dPkt);
返回的数组大小始终等于指定的数据包长度，即使接收到的数据较小。在我的例子中，最后一个数据包是 0 字节(tftp 为 +4 字节)，但即便如此，额外的 512 字节还是添加到了输出流中。
为了解决这个问题，我用额外的参数重载了提到的方法 - 当接收到的数据大小高于 4 字节且低于指定的数据包大小(512 字节)时，接收到的数据包的实际大小。此更改导致获得最后一个数据包的数组的正确大小，因此接收到的文件在操作结束时具有正确的大小。
关于java - 通过 TFTP 传输的文件与主机上传输的文件大小不同，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/58606025/