A helper class to make requests to Amazon's product api.

Amazon.java

import org.joda.time.DateTime;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import javax.xml.bind.DatatypeConverter;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.Iterator;
import java.util.Map;
import java.util.SortedMap;

/**
 * @author Patrick Lorio
 */
public class Amazon {
    private static final String UTF8 = "UTF-8";
    private static final String HMAC_SHA256 = "HmacSHA256";
    private static final String AMNZ_URI = "/onca/xml";

    private final String associateTag;
    private final String accessKeyId;

    private final Mac mac;

    public Amazon(String associateTag, String accessKeyId, String secretKey)
            throws NoSuchAlgorithmException, InvalidKeyException {
        this.associateTag = associateTag;
        this.accessKeyId = accessKeyId;

        mac = Mac.getInstance(HMAC_SHA256);
        mac.init(new SecretKeySpec(getByte(secretKey), HMAC_SHA256));
    }

    public String getUrl(String endpoint, SortedMap<String, String> params) {
        params.put("AWSAccessKeyId", accessKeyId);
        params.put("Timestamp", getTimestamp());
        params.put("AssociateTag", associateTag);

        String paramStr = getParams(params);
        String sign = String.format("GET\n%s\n%s\n%s", endpoint, AMNZ_URI, paramStr);
        String signature = DatatypeConverter.printBase64Binary(mac.doFinal(getByte(sign)));

        return String.format("http://%s%s?%s&Signature=%s", endpoint, AMNZ_URI, paramStr, encodeRFC3986(signature));
    }

    private String getParams(SortedMap<String, String> params) {
        StringBuilder sb = new StringBuilder();
        Iterator<Map.Entry<String, String>> iterator = params.entrySet().iterator();

        while (iterator.hasNext()) {
            Map.Entry<String, String> item = iterator.next();
            sb.append(encodeRFC3986(item.getKey())).append("=").append(encodeRFC3986(item.getValue()));
            if (iterator.hasNext()) {
                sb.append('&');
            }
        }

        return sb.toString();
    }

    private String encodeRFC3986(String str) {
        try {
            return URLEncoder.encode(str, UTF8)
                    .replace("+", "%20")
                    .replace("*", "%2A")
                    .replace("%7E", "~");
        } catch (UnsupportedEncodingException ignored) {
            return str;
        }
    }

    private String getTimestamp() {
        return new DateTime().toString("yyyy-MM-dd'T'HH:mm:ss'Z'");
    }

    private byte[] getByte(String str) {
        try {
            return str.getBytes(UTF8);
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(UTF8 + " is not supported.", e);
        }
    }
}