How to edit JSch SCP to send one file from a remote host to another remote host?

Question

I am trying to use the JSch library to send a file between two remote host. I am currently using the ScpFrom class to accomplish this. However, I was trying to modify the code to allow me to connect two remote host and send the file, though it looks like it only allows me to send it from one remote host to my local machine. I already tried SFTP class in JSch, but to no luck. So please do not suggest SFTP class. I will list the code I used below:

Code:

/* -*-mode:java; c-basic-offset:2; indent-tabs-mode:nil -*- */
/**
 * This program will demonstrate the file transfer from remote to local
 *   $ CLASSPATH=.:../build javac ScpFrom.java
 *   $ CLASSPATH=.:../build java ScpFrom user@remotehost:file1 file2
 * You will be asked passwd. 
 * If everything works fine, a file 'file1' on 'remotehost' will copied to
 * local 'file1'.
 *
 */
import com.jcraft.jsch.*;
import java.awt.*;
import javax.swing.*;
import java.io.*;

public class ScpFrom{
  public static void main(String[] args){

    String array [] = {"pf2rr1@paris44app1.linux.seis.com:/home/pf2rr1/test.txt","/home/pf2rr1/test"};
    if(array.length!=2){
      System.err.println("usage: java ScpFrom user@remotehost:file1 file2");
      System.exit(-1);
    }      

    FileOutputStream fos=null;
    try{

      String user=array[0].substring(0, array[0].indexOf('@'));
      array[0]=array[0].substring(array[0].indexOf('@')+1);
      String host=array[0].substring(0, array[0].indexOf(':'));
      String rfile=array[0].substring(array[0].indexOf(':')+1);
      String lfile=array[1];

      String prefix=null;
      if(new File(lfile).isDirectory()){
        prefix=lfile+File.separator;
      }

      JSch jsch=new JSch();
      Session session=jsch.getSession(user, host, 22);

      // username and password will be given via UserInfo interface.
      UserInfo ui=new MyUserInfo();
      session.setUserInfo(ui);
      session.connect();

      // exec 'scp -f rfile' remotely
      String command="scp -f "+rfile;
      System.out.println(command);
      Channel channel=session.openChannel("exec");
      ((ChannelExec)channel).setCommand(command);

      // get I/O streams for remote scp
      OutputStream out=channel.getOutputStream();
      InputStream in=channel.getInputStream();

      channel.connect();

      byte[] buf=new byte[1024];

      // send '\0'
      buf[0]=0; out.write(buf, 0, 1); out.flush();

      while(true){
    int c=checkAck(in);
        if(c!='C'){
      break;
    }

        // read '0644 '
        in.read(buf, 0, 5);

        long filesize=0L;
        while(true){
          if(in.read(buf, 0, 1)<0){
            // error
            break; 
          }
          if(buf[0]==' ')break;
          filesize=filesize*10L+(long)(buf[0]-'0');
        }

        String file=null;
        for(int i=0;;i++){
          in.read(buf, i, 1);
          if(buf[i]==(byte)0x0a){
            file=new String(buf, 0, i);
            break;
      }
        }

    //System.out.println("filesize="+filesize+", file="+file);

        // send '\0'
        buf[0]=0; out.write(buf, 0, 1); out.flush();

        // read a content of lfile
        fos=new FileOutputStream(prefix==null ? lfile : prefix+file);
        int foo;
        while(true){
          if(buf.length<filesize) foo=buf.length;
      else foo=(int)filesize;
          foo=in.read(buf, 0, foo);
          if(foo<0){
            // error 
            break;
          }
          fos.write(buf, 0, foo);
          filesize-=foo;
          if(filesize==0L) break;
        }
        fos.close();
        fos=null;

    if(checkAck(in)!=0){
      System.exit(0);
    }

        // send '\0'
        buf[0]=0; out.write(buf, 0, 1); out.flush();
      }

      session.disconnect();

      System.exit(0);
    }
    catch(Exception e){
      System.out.println(e);
      try{if(fos!=null)fos.close();}catch(Exception ee){}
    }
  }

  static int checkAck(InputStream in) throws IOException{
    int b=in.read();
    // b may be 0 for success,
    //          1 for error,
    //          2 for fatal error,
    //          -1
    if(b==0) return b;
    if(b==-1) return b;

    if(b==1 || b==2){
      StringBuffer sb=new StringBuffer();
      int c;
      do {
    c=in.read();
    sb.append((char)c);
      }
      while(c!='\n');
      if(b==1){ // error
    System.out.print(sb.toString());
      }
      if(b==2){ // fatal error
    System.out.print(sb.toString());
      }
    }
    return b;
  }

  public static class MyUserInfo implements UserInfo, UIKeyboardInteractive{
    public String getPassword(){ return passwd; }
    public boolean promptYesNo(String str){
      Object[] options={ "yes", "no" };
      int foo=JOptionPane.showOptionDialog(null, 
             str,
             "Warning", 
             JOptionPane.DEFAULT_OPTION, 
             JOptionPane.WARNING_MESSAGE,
             null, options, options[0]);
       return foo==0;
    }

    String passwd;
    JTextField passwordField=(JTextField)new JPasswordField(20);

    public String getPassphrase(){ return null; }
    public boolean promptPassphrase(String message){ return true; }
    public boolean promptPassword(String message){
      Object[] ob={passwordField}; 
      int result=
      JOptionPane.showConfirmDialog(null, ob, message,
                    JOptionPane.OK_CANCEL_OPTION);
      if(result==JOptionPane.OK_OPTION){
    passwd=passwordField.getText();
    return true;
      }
      else{ return false; }
    }
    public void showMessage(String message){
      JOptionPane.showMessageDialog(null, message);
    }
    final GridBagConstraints gbc = 
      new GridBagConstraints(0,0,1,1,1,1,
                             GridBagConstraints.NORTHWEST,
                             GridBagConstraints.NONE,
                             new Insets(0,0,0,0),0,0);
    private Container panel;
    public String[] promptKeyboardInteractive(String destination,
                                              String name,
                                              String instruction,
                                              String[] prompt,
                                              boolean[] echo){
      panel = new JPanel();
      panel.setLayout(new GridBagLayout());

      gbc.weightx = 1.0;
      gbc.gridwidth = GridBagConstraints.REMAINDER;
      gbc.gridx = 0;
      panel.add(new JLabel(instruction), gbc);
      gbc.gridy++;

      gbc.gridwidth = GridBagConstraints.RELATIVE;

      JTextField[] texts=new JTextField[prompt.length];
      for(int i=0; i<prompt.length; i++){
        gbc.fill = GridBagConstraints.NONE;
        gbc.gridx = 0;
        gbc.weightx = 1;
        panel.add(new JLabel(prompt[i]),gbc);

        gbc.gridx = 1;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.weighty = 1;
        if(echo[i]){
          texts[i]=new JTextField(20);
        }
        else{
          texts[i]=new JPasswordField(20);
        }
        panel.add(texts[i], gbc);
        gbc.gridy++;
      }

      if(JOptionPane.showConfirmDialog(null, panel, 
                                       destination+": "+name,
                                       JOptionPane.OK_CANCEL_OPTION,
                                       JOptionPane.QUESTION_MESSAGE)
         ==JOptionPane.OK_OPTION){
        String[] response=new String[prompt.length];
        for(int i=0; i<prompt.length; i++){
          response[i]=texts[i].getText();
        }
    return response;
      }
      else{
        return null;  // cancel
      }
    }
  }
}

Answer 1

SCP protocol (ScpFrom class) cannot be used to transfer between two remote hosts directly.

To SCP files between two remote hosts, simply execute scp command on one host to transfer the file to other host. That's what scp tool does internally, when you execute it with syntax:

scp user@host1:file user@host2:dest

It connect to host1 and there it executes:

scp file user@host2:dest

So while session is connected to host1 do:

String command = "scp file user@host2:dest";
Channel channel = session.openChannel("exec");
((ChannelExec)channel).setCommand(command);
channel.connect();
// ...