Threads

PayloadCompressorUtility.java

package com.cinfin.sharedsvcs.ems.payload;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.zip.*;

/**
 * Singleton class for performing compression/decompression operations on
 * payload byte arrays.
 * 
 * @author Michael Yockey
 * 
 */
final class PayloadCompressionUtility {


	private static final int DEFLATER_BUFFER_SIZE_INCREMENT = 1024 * 3;

	PayloadCompressionUtility() {
	}

	byte[] compressPayload(byte[] payload) {
		Deflater deflater = new Deflater();
		deflater.setInput(payload);
		deflater.finish();
		byte[] buf = new byte[DEFLATER_BUFFER_SIZE_INCREMENT];
		int offset = 0;
		while (!deflater.finished()) {
			deflater.deflate(buf, offset, DEFLATER_BUFFER_SIZE_INCREMENT);
			byte[] tmp = buf.clone();
			buf = new byte[buf.length + DEFLATER_BUFFER_SIZE_INCREMENT];
			System.arraycopy(tmp, 0, buf, 0, tmp.length);
			offset += DEFLATER_BUFFER_SIZE_INCREMENT;
		}
		return buf;
	}

	byte[] uncompressPayload(byte[] payload, int size)
			throws DataFormatException {
		byte[] buf = new byte[size];
		Inflater inflater = new Inflater();
		inflater.setInput(payload);
		inflater.inflate(buf);
		return buf;
	}

	String calculateChecksum(byte[] payload) {
		try {
			MessageDigest md = MessageDigest.getInstance("SHA-256");
			synchronized (md) {
				byte[] hash = md.digest(payload);

				StringBuilder sb = new StringBuilder();
				for (byte b : hash) {
					sb.append(String.format("%02x", b));
				}
				return sb.toString();
			}
		} catch (NoSuchAlgorithmException e) {
			// This checked exception must be handled to avoid compiler errors.
			// This catch block needs no handling code as long as the hashing
			// algorithm is dictated by EMS and not by the user.
		}
		return null;
	}
}

static PayloadCompressionUtility getInstance() {
if (singletonInstance == null) {
singletonInstance = new PayloadCompressionUtility();
}
return synchronized(singletonInstance);
}

No more singleton. Made the constructor package-accessible and synchronized only the operations around MessageDigest.digest(). Most web discussions about synchronizing calls to java.* classes focus on what's written in the Javadoc, ie. never assuming that a class is thread-safe unless it's specified as thread-safe in the Javadoc. I'm going to play with MD a little to see if I can provoke it to behave badly with threads.