Java DataInputStream长度

Question

I am creating a file server application for school assignment. What I currently have is a simple Client class that sends an image through TCP and a Server class that receives it and writes it to the file.

this is my client code

import java.io.*;
import java.net.*;

class Client {
    public static void main(String args[]) throws Exception {
        long start = System.currentTimeMillis();
        Socket clientSocket = new Socket("127.0.0.1", 6789);
        DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

        File file = new File("hot.jpg");
        FileInputStream fin = new FileInputStream(file);
        byte sendData[] = new byte[(int)file.length()];
        fin.read(sendData);

        outToServer.write(sendData, 0, sendData.length);
        clientSocket.close();

        long end = System.currentTimeMillis();
        System.out.println("Took " + (end - start) + "ms");
    }
}

and this is my server code.

import java.io.*;
import java.net.*;

class Server {
    public static void main(String args[]) throws Exception {
        ServerSocket serverSocket = new ServerSocket(6789);
        Socket connectionSocket = serverSocket.accept();
        DataInputStream dis = new DataInputStream(connectionSocket.getInputStream());

        byte[] receivedData = new byte[61500]; // <- THIS NUMBER

        for(int i = 0; i < receivedData.length; i++)
            receivedData[i] = dis.readByte();

        connectionSocket.close();
        serverSocket.close();

        FileOutputStream fos = new FileOutputStream("received.jpg");
        fos.write(receivedData);
        fos.close();
    }
}

My question is how to get the size of the file that is being sent. If you check the Server code you'll see that I've hardcoded the number ie 61500 at the moment. How can I retrieve this number dynamically?

Or, am I doing this the wrong way? What an alternative solution would be?

Answer 1

Add one "length field" before sending the file. (Note that since you read the file to memory the maximum size of the file can be ~2GB.)

---

Before sending the file write the length of the file:

  outToServer.writeInt(sendData.length);

And when receiving read the length first and use it as a length:

  int dataLength = dis.readInt()
  byte[] receivedData = new byte[dataLength];

---

A better way would be not to read the file into memory first but to transfer it directly from the FileInputStream - then you could transfer bigger files!

Answer 2

If you know the length, using readFully() is much more efficient than reading a byte at a time.

In this case, you don't need to know the length, you can write the loop to read/write as much data as you get.

InputStream is = connectionSocket.getInputStream();

byte[] bytes = new byte[8192];
int len;
while((len = is.read(bytes)) > 0)
    fos.write(bytes, 0, len);

You can avoid reading the whole file into memory by copying the data as you read it.

FileInputStream fis = new FileInputStream(filename);
OutputStream os = socket.getOutputStream();

byte[] bytes = new byte[8192];
int len;
while((len = fis.read(bytes)) > 0)
    os.write(bytes, 0, len);

You can use Apache IOUtils .copy() to perform the copy from one stream to another if you wish.

This approach has the advantage that the file can be any size (greater than 2 GB). Using an array is limited to 2 GB (and uses more memory)