How to patch binary files with Java

Recently, I wanted to create an “intelligent” binary patcher, which not only replaces some chunk of binary data at a file’s predefined offset, but instead performs search and replace. Here is the Java class I came up with.

import java.io.*;
import java.math.BigInteger;

public class BinaryPatcher {
	private String content;

	private final String filename;
	private final String searchPattern;
	private final String replacePattern;

	public BinaryPatcher(String filename, String searchPattern,
			String replacePattern) throws IOException {

		this.filename = filename;
		this.searchPattern = convertHexStringToBinaryString(searchPattern);
		this.replacePattern = convertHexStringToBinaryString(replacePattern);

		initializeContent();
	}

	public boolean isPatchable() {
		return getContent().contains(searchPattern);
	}

	public void patch() throws IOException {
		if (!isPatchable()) {
			throw new IOException("Search pattern not found");
		}

		String originalContent = getContent();
		String patchedContent = originalContent.replace(searchPattern,
				replacePattern);

		saveStringToFile(patchedContent, filename);
	}

	public void createBackup(String filename) throws IOException {
		saveStringToFile(getContent(), filename);
	}

	private static String convertHexStringToBinaryString(String theHexString) {
		byte[] byteSequence = new BigInteger(theHexString, 16).toByteArray();
		return new String(byteSequence);
	}

	private void initializeContent() throws IOException {
		byte[] buffer = new byte[(int) new File(filename).length()];
		FileInputStream in = new FileInputStream(filename);
		in.read(buffer);
		in.close();
		content = new String(buffer);
	}

	private void saveStringToFile(String content, String filename)
			throws IOException {
		FileOutputStream out = new FileOutputStream(filename);
		out.write(content.getBytes());
		out.close();
	}

	private String getContent() {
		return content;
	}
}

The usage is pretty straight forward. Let’s have a look at an basic example:

class Tester {
	public static void main(String[] args) {
		try {
			String filename = "patchme";
			BinaryPatcher p = new BinaryPatcher(filename, "AB00FF14", "AB11FF14");

			if (p.isPatchable()) {
				p.createBackup(filename + ".org");
				p.patch();
			}
		} catch(IOException e) {
			System.out.println("ERROR: " + e.getMessage());
		}
	}
}

We try to patch the file patchme. The search and replace binary data is given by hex sequences, in this example AB00FF14 is replaced by AB11FF14. If the file is patchable, i.e. it contains the search pattern, a backup named patchme.org is created an finally, the original file is modified.

That’s all. Pretty simple but useful. You can download the BinaryPatcher class here.