如何通过套接字发送文件？ 爪哇

Question

I am trying to send a file through sockets, currently from client to server. The file to be written is created, but when I open it, it's empty. Also, If I send a string to the server after sending the file, what I send gets written onto the text file that was supposed to be received.

I am testing with text files only.

Why does this happen? How can I correct it?

jbutton.addActionListener(
                new ActionListener() {
                    public void actionPerformed(ActionEvent event) {
                        JFileChooser fc = new JFileChooser();
                        fc.setCurrentDirectory(new File(System.getProperty("user.home")));
                        FileNameExtensionFilter filter = new FileNameExtensionFilter("*.Images", "jpg","gif","png","txt");
                        fc.addChoosableFileFilter(filter);
                        int result = fc.showSaveDialog(null);
                         if(result == JFileChooser.APPROVE_OPTION){
                             File selectedFile = fc.getSelectedFile();
                             String path = selectedFile.getAbsolutePath();
                            try {
                                userText.setText("Sending File: " + selectedFile);
                                file = new FileInputStream(path);
                                byte b[] = new byte[30000];
                                file.read(b,0,b.length);
                                os = connection.getOutputStream();
                                os.write(b,0,b.length);
                                
                            } catch (FileNotFoundException e) {
                                e.printStackTrace();
                            }
                            catch (IOException e) {
                                e.printStackTrace();
                            }
                            
                         }
                         else if(result == JFileChooser.CANCEL_OPTION){
                             System.out.println("No File Select");
                         }  
                    }
                    

//get stream to send and receive data 
    private void setupStreams() throws IOException{
        output = new ObjectOutputStream(socket.getOutputStream());
        output.flush();
        input = new ObjectInputStream(socket.getInputStream());
        showMessage("\n Streams Are now Set up \n");
    }
    
    //during the chat conversation
    private void whileChatting() throws IOException{
        String message = "You are now connected ";
        showMessage(message);
        ableToType(true);
        do {
            try {
                message = (String) input.readObject();
                showMessage("\n" + message);
                
                byte b[] = new byte[30000];
                InputStream is = socket.getInputStream();
                FileOutputStream fr = new FileOutputStream("C:\\Users\\Documents\\TestingFileClientToServer.txt");
                is.read(b,0,b.length);
                fr.write(b,0,b.length);
            
                
            }catch(ClassNotFoundException classNotFoundException) {
                showMessage("\n Error on input \n");
            }
            
        }while(!message.equals("CLIENT - END"));
    }

Answer 1

So this is wrong (in two places):

            fr.write(b,0,b.length);

You should get the number of bytes actually read in the read call and use that size in your write call.

This is also a problem:

            message = (String) input.readObject();

don't use this unless you really are sending objects over streams... which you shouldn't

As to answering your question about why the file has no content: You need to close the socket on the sending side after all data has been sent and flushed. Then on the receiving side, you need to catch SocketException for Socket closure, then close the file you are writing to. Not closing the file is why you are not seeing any content.

Another thing is that you are creating Object streams for no reason. Don't use Object streams and don't use Data streams. Just use raw Input/Output streams. You don't need buffers as you are doing efficient reading and writing.