To check for file existence and permissions over an SFTP connection using JSch in Java, you need to use the ChannelSftp class provided by the JSch library. Here’s how you can do it step by step:

Steps:

1. Establish an SFTP connection using the JSch class.
2. Open an SFTP channel (ChannelSftp).
3. Use ChannelSftp.lstat() to check the existence and permissions of a file.

Example Code:

package org.kodejava.jsch;

import com.jcraft.jsch.*;

public class SFTPFileCheck {
   public static void main(String[] args) {
      String username = "username";
      String host = "example.com";
      int port = 22; // Default SFTP port
      String privateKey = "/path/to/private/key";
      String filePath = "/path/to/remote/file";

      JSch jsch = new JSch();
      Session session = null;
      ChannelSftp channelSftp = null;

      try {
         // Set up authentication with SSH private key
         jsch.addIdentity(privateKey);
         session = jsch.getSession(username, host, port);

         // Disable strict host key checking for simplicity
         session.setConfig("StrictHostKeyChecking", "no");

         // Connect to the SFTP server
         session.connect();

         // Open an SFTP channel
         channelSftp = (ChannelSftp) session.openChannel("sftp");
         channelSftp.connect();

         // Check if the file exists and get its attributes
         try {
            SftpATTRS attrs = channelSftp.lstat(filePath);

            // File exists, print permissions
            System.out.println("File exists: " + filePath);
            System.out.println("Permissions: " + attrs.getPermissionsString());
            System.out.println("Size: " + attrs.getSize() + " bytes");
         } catch (SftpException e) {
            if (e.id == ChannelSftp.SSH_FX_NO_SUCH_FILE) {
               // File does not exist
               System.out.println("File does not exist: " + filePath);
            } else {
               // Other SFTP error
               e.printStackTrace();
            }
         }

      } catch (Exception e) {
         e.printStackTrace();
      } finally {
         // Disconnect from SFTP
         if (channelSftp != null && channelSftp.isConnected()) {
            channelSftp.disconnect();
         }
         if (session != null && session.isConnected()) {
            session.disconnect();
         }
      }
   }
}

Explanation:

1. Session Setup:
   • jsch.addIdentity(privateKey) is used to authenticate using an SSH private key; replace this with setPassword() if you’re using a username/password.
2. File Check:
   • channelSftp.lstat(filePath) is used to get file attributes. If the file does not exist, it throws an SftpException with the SSH_FX_NO_SUCH_FILE error code.
3. Permissions:
   • attrs.getPermissionsString() provides the permissions in a Unix-style format (e.g., -rw-r--r--).
4. Error Handling:
   • Catch SftpException to handle specific cases, such as file not found or other SFTP-related errors.
5. Cleanup:
   • Disconnect the SFTP channel and session when done to free up resources.

Notes:

• Make sure you have the jsch-<version>.jar file added to your project’s classpath.
• Ensure network connectivity, appropriate SSH access, and file permissions on the remote server.
• For large-scale applications, consider using a logging framework (e.g., SLF4J) rather than System.out.

This example provides the basic workflow for checking file existence and retrieving permissions over SFTP using JSch.

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Maven Dependencies

<dependency>
    <groupId>com.jcraft</groupId>
    <artifactId>jsch</artifactId>
    <version>0.1.55</version>
</dependency>