how to use writeUTF() method the right way?

Question

I am trying to make simple client and server that exchanges files, so, I am trying to use the writeUtf() method to send the file name as a String, but it is not working at all.

 public class Client {
    public static void main(String[] args) throws Exception {
    @SuppressWarnings("resource")
    Socket client = new Socket("localhost", 1241);
Path currentRelativePath = Paths.get("");
String currentPath = currentRelativePath.toAbsolutePath().toString();
DataOutputStream outToServer = new DataOutputStream(client.getOutputStream());
DataInputStream dataInFromServer = new DataInputStream(client.getInputStream());
Scanner keyboardInput=new Scanner(System.in);
System.out.println("Enter Operation Type(PUt, Get, Or Exit)");
String OperationType=keyboardInput.nextLine();
switch(OperationType){
case "PUT":
    outToServer.writeChar('P');
    System.out.println("File Or Directory name:");
    String FileOrDirName=keyboardInput.nextLine();
    //outToServer.writeUTF(FileOrDirName);
    //outToServer.write((short)FileOrDirName.length());
        outToServer.writeUTF(FileOrDirName);
    File fileOrDir=new File(FileOrDirName);
    if(fileOrDir.getParent()==currentPath || fileOrDir.getParent()==null){
    if(fileOrDir.isDirectory()){
        outToServer.writeInt(1);
        FileOperations.zipDir(fileOrDir);
        outToServer.write(FileOperations.readFileAsByteArray(FileOrDirName+".zip"));

        File tmp=new File(FileOrDirName+".zip");
        tmp.delete();
    }
    else {
        outToServer.writeInt(0);

        byte toSend[]=FileOperations.readFileAsByteArray(FileOrDirName);
        outToServer.writeDouble(toSend.length);
        for(int i=0; i<toSend.length; i++)
                outToServer.writeByte(toSend[i]);
    //  outToServer.write(ToSend,0,ToSend.length);
        //outToServer.write(FileOperations.readFileAsByteArray(FileOrDirName));
}
    outToServer.flush();
    outToServer.close();
    keyboardInput.close();
    client.close();
}   
    }

}
    }

and here is the Server:

        import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.IOException;
import java.net.Socket;
import java.nio.file.Path;
import java.nio.file.Paths;

class TcpFileServerThread extends Thread{
    private Socket client;
    public TcpFileServerThread(Socket client) {
        this.client = client;
    }
    public void run() {
        try {
            DataOutputStream outToClient = new DataOutputStream(client.getOutputStream());
            DataInputStream dataInFromClient = new DataInputStream(client.getInputStream()) ;
            Path currentRelativePath = Paths.get("");
            String currentPath = currentRelativePath.toAbsolutePath().toString();
            //File f;
            //System.out.println(dataInFromClient.readInt());
            /** client request PUT **/
            switch(dataInFromClient.readChar()){
            case 'P':
            int lengthRecvd= (int)dataInFromClient.readDouble();
            byte[] toRecv=new byte[lengthRecvd];

            for(int i=0; i<lengthRecvd; i++){
                toRecv[i]=dataInFromClient.readByte();
            }

        //  dataInFromClient.read(file_or_dir_from_client);
            //FileOrDirName="uu.txt";
            String FileOrDirName=dataInFromClient.readUTF();
        //  int arrLen=dataInFromClient.readShort();
        /** for(int i=0; i<arrLen; i++){
                char o=dataInFromClient.readChar();
                System.out.println(o);
            }**/

            System.out.println(FileOrDirName);
            File f=new File(FileOrDirName);
            if(f.getParent()==null || f.getParent()==currentPath){

            FileOperations.writeFromByteArrayToFile(toRecv, FileOrDirName); 
            if(dataInFromClient.readInt()==1){
                File zipFl=new File(FileOrDirName+".zip");
            FileOperations.unzip(zipFl);
                zipFl.delete();
            }   
            }

            }



        }
        catch(IOException e){
            e.printStackTrace();
        }
    }
    }

I get FileNotFoundException and the output for System.out.println(FileOrDirName) in server is a blank line as it was empty.

Answer 1

There is only one way to use it, and this isn't it.

int lengthRecvd= (int)dataInFromClient.readDouble();

Here you are reading a double that hasn't been written. So you will get whatever the double value of the next 8 bytes in the input is.

        byte[] toRecv=new byte[lengthRecvd];
        for(int i=0; i<lengthRecvd; i++){
            toRecv[i]=dataInFromClient.readByte();
        }

Here you are reading that many bytes of input, which could be anything, and which also hasn't been written.

Remove all this.

        String FileOrDirName=dataInFromClient.readUTF();

Here you are finally reading the data, which is the only thing that has actually been written.

It is surprising that this code doesn't throw an IOException of some kind.

You can only read with DataInputStream data that has actually been written by the corresponding method of DataOutputStream, apart from mere byte arrays.