Fixing jcraft JSch Throws InputStream closed Exception Downloading Large Files
When downloading large files (2GB or more) from a remote FTP site using the jcraft JSch library, you may encounter an InputStream closed Exception. This article will cover the key concepts, applications, and significance of this issue and provide a detailed solution.
Understanding the Issue
The jcraft JSch library is a popular Java implementation of the SSH2 protocol. It is often used for FTP file transfers, including downloading files from a remote FTP site. However, when downloading large files, you may encounter an InputStream closed Exception, which prevents the file from being fully downloaded.
The root cause of this issue is the default buffer size of the InputStream used by the JSch library. The buffer size is set to 32KB, which is not sufficient for handling large files. As a result, when the buffer becomes full, the InputStream is closed, and the file transfer is interrupted.
Solution: Increasing the Buffer Size
To fix the InputStream closed Exception, you need to increase the buffer size used by the JSch library. This can be done by creating a custom InputStream class that extends the default InputStream class and overrides the read() method to use a larger buffer size.
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
public class LargeBufferInputStream extends BufferedInputStream {
private static final int BUFFER_SIZE = 1024 * 1024; // 1MB buffer size
public LargeBufferInputStream(InputStream in) {
super(in, BUFFER_SIZE);
}
@Override
public int read() throws IOException {
byte[] buffer = new byte[BUFFER_SIZE];
int bytesRead = super.read(buffer, 0, BUFFER_SIZE);
if (bytesRead == -1) {
return -1;
}
return buffer[0];
}
@Override
public int read(byte[] buffer, int offset, int length) throws IOException {
int bytesToRead = length;
int bytesRead = 0;
while (bytesToRead > 0) {
int bytesReadThisTime = super.read(buffer, offset + bytesRead, bytesToRead);
if (bytesReadThisTime == -1) {
break;
}
bytesRead += bytesReadThisTime;
bytesToRead -= bytesReadThisTime;
}
return bytesRead;
}
}
To use the LargeBufferInputStream class, you need to modify the code that downloads the file from the remote FTP site. Instead of using the default InputStream class, you need to create a LargeBufferInputStream instance and use it to read the file data.
import com.jcraft.jsch.Channel;
import com.jcraft.jsch.ChannelSftp;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import java.io.FileOutputStream;
import java.io.InputStream;
public class FtpDownloader {
public static void main(String[] args) {
String host = "example.com";
String username = "username";
String password = "password";
String remoteFilePath = "/path/to/remote/file.txt";
String localFilePath = "/path/to/local/file.txt";
try {
JSch jsch = new JSch();
Session session = jsch.getSession(username, host, 22);
session.setPassword(password);
session.connect();
Channel channel = session.openChannel("sftp");
channel.connect();
ChannelSftp sftpChannel = (ChannelSftp) channel;
InputStream inputStream = new LargeBufferInputStream(sftpChannel.get(remoteFilePath));
FileOutputStream outputStream = new FileOutputStream(localFilePath);
byte[] buffer = new byte[1024];
int bytesRead;
while ((bytesRead = inputStream.read(buffer)) != -1) {
outputStream.write(buffer, 0, bytesRead);
}
inputStream.close();
outputStream.close();
sftpChannel.exit();
session.disconnect();
} catch (Exception e) {
e.printStackTrace();
}
}
}
Significance
Increasing the buffer size used by the JSch library is a simple yet effective solution to the InputStream closed Exception issue. This solution ensures that large files can be downloaded from a remote FTP site without interruption, which is essential for many applications, including data backup, migration, and synchronization.