That's a pretty tough task. If you can ensure that the HTTP Content-Length and Content-Type headers are present in the upload request (or in the multipart body when it's a multipart/form-data request), then it would be an easy job with help of RandomAccessFile. The content length is mandatory so that the RandomAccessFile knows how long the file will be and write the character at the position you want it to be. The character encoding (which is usually present as an attribute of the content type header) is mandatory to know how many bytes a character will take into account (because RandomAccessFile is byte based and for example UTF-8 encoding is variable-byte-length).
Here's a kickoff example (leaving obvious exception handling aside):
package com.stackoverflow.q2725897;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.io.Reader;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.nio.charset.CharsetEncoder;
public class Test {
public static void main(String... args) throws Exception {
// Stub input. You need to gather it yourself from your sources.
File file = new File("/file.txt");
long length = file.length(); // Get it from HTTP request header using file upload API in question (Commons FileUpload?).
String encoding = "UTF-8"; // Get it from HTTP request header using file upload API in question (Commons FileUpload?).
InputStream content = new FileInputStream(file); // Get it from HTTP request body using file upload API in question (Commons FileUpload?).
// Now the real job.
Reader input = new InputStreamReader(content, encoding);
RandomAccessFile output = new RandomAccessFile(new File("/filereversed.txt"), "rwd");
CharsetEncoder encoder = Charset.forName(encoding).newEncoder();
for (int data; (data = input.read()) != -1;) {
ByteBuffer bytes = encoder.encode(CharBuffer.wrap(new char[] { (char) data }));
length -= bytes.limit();
output.seek(length);
output.write(bytes.array());
}
// Should actually be done in finally.
input.close();
output.close();
}
}
If those headers are not present (especially Content-length is important), then you'll really need to store it on disk first until end of stream and then re-read and reverse it the same way with help of RandomAccessFile.
Update: it would actually be tougher than it look like. Is the character encoding of the input always guaranteed to be the same? If so, what one would it be? Also, what would you like to do with for example surrogate characters and newlines? The above example doesn't take that into account correctly. But it at least gives the base idea.