protectedcrc.java

real-time design patterns in Real-time java

package designPatterns;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.io.StreamCorruptedException;

public class ProtectedCrc {
	private int crc;
	private byte[] bytearr;

	public void storeObj(Serializable object) throws IOException {
		// Serialize to a byte array
		ObjectOutput out;
		ByteArrayOutputStream bos = new ByteArrayOutputStream() ;

		out = new ObjectOutputStream(bos) ;
		out.writeObject(object);
		out.close();
		bytearr = bos.toByteArray();
		crc = calcCrc();
	}

	public Serializable getObj() throws StreamCorruptedException, IOException, ClassNotFoundException {
		// Deserialize from a byte array
		ObjectInputStream in = new ObjectInputStream(new ByteArrayInputStream(bytearr));
        Serializable obj = (Serializable) in.readObject();
        in.close();
        return obj;
	}
	
	public boolean checkCrc() {
		return (calcCrc() == crc);
	}

	private int calcCrc() {
		int crc = 0x0;

		crc  = 0xFFFFFFFF;       // initial contents of shift register
		int poly = 0xEDB88320;   // polynomial

		for (int b = 0; b < bytearr.length; b++) {
			int temp = (crc ^ bytearr[b]) & 0xff;

			// read 8 bits one at a time
			for (int i = 0; i < 8; i++) {
				if ((temp & 1) == 1) temp = (temp >>> 1) ^ poly;
				else                 temp = (temp >>> 1);
			}
			crc = (crc >>> 8) ^ temp;
		}
		crc = crc ^ 0xffffffff;

		return crc;
	}
}