HMAC stands for Hash Message Authentication Code and it is used for verifying the integrity of data exchanged between two applications etc.In general,any hash function -such as MD5 or SHA1- can be used in order to create an HMAC signature but for this tutorial we will use hash function SHA256.

In order to create the HMAC we will need a key and javax.crypto.Mac class.

package com.javaonly.hmac.test;

import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import org.apache.commons.codec.binary.Hex;
import javax.crypto.Mac;

public class HMACTest {

	public static void main(String args[]) throws NoSuchAlgorithmException,
			KeyManagementException,
			 InvalidKeyException,
			IllegalBlockSizeException, BadPaddingException {

		
		
		String macKey = "The HMAC key";
		String macData ="the data string"
		System.out.println("MACDATA:"+macData);

		Mac mac = Mac.getInstance("HmacSHA256");
                //get the bytes of the hmac key and data string
		byte[] secretByte = macKey.getBytes("UTF-8");
		byte[] dataBytes = macData.getBytes("UTF-8");
		SecretKey secret = new SecretKeySpec(secretByte, "HMACSHA256");

		mac.init(secret);
		byte[] doFinal = mac.doFinal(dataBytes);
		byte[] hexB = new Hex().encode(doFinal);
		String checksum = Hex.encodeHex(doFinal);
		
	}
}

 In the above code snippet Mac is the class that will create the HMAC signature.It is initialized with a SecretKeySpec object that implements SecretKey interface.SecretKeySepc object is used to create a key in a vendor-independent way.Finally,The HMAC signature's bytes are encoded into a Hexademical String through the org.apache.commons.codec.binary.Hex class