Absolute minimum code to get a valid oauth_signature populated in Java or Groovy?

For Twitter oAuth: In case anyone needs to generate oAuth signature and header to connect to Twitter API, here is the code. This requires minimum of Java 8 and NO 3rd party library.

import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;
import java.util.Date;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Random;
import java.util.stream.Collectors;

import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;

/**
 * Class to generate Oauth 1.0a header for Twitter
 *
 */
public class TwitterOauthHeaderGenerator {

    private String consumerKey;
    private String consumerSecret;
    private String signatureMethod;
    private String token;
    private String tokenSecret;
    private String version;

    public TwitterOauthHeaderGenerator(String consumerKey, String consumerSecret, String token, String tokenSecret) {
        this.consumerKey = consumerKey;
        this.consumerSecret = consumerSecret;
        this.token = token;
        this.tokenSecret = tokenSecret;
        this.signatureMethod = "HMAC-SHA1";
        this.version = "1.0";
    }

    private static final String oauth_consumer_key = "oauth_consumer_key";
    private static final String oauth_token = "oauth_token";
    private static final String oauth_signature_method = "oauth_signature_method";
    private static final String oauth_timestamp = "oauth_timestamp";
    private static final String oauth_nonce = "oauth_nonce";
    private static final String oauth_version = "oauth_version";
    private static final String oauth_signature = "oauth_signature";
    private static final String HMAC_SHA1 = "HmacSHA1";

    /**
     * Generates oAuth 1.0a header which can be passed as Authorization header
     * 
     * @param httpMethod
     * @param url
     * @param requestParams
     * @return
     */
    public String generateHeader(String httpMethod, String url, Map<String, String> requestParams) {
        StringBuilder base = new StringBuilder();
        String nonce = getNonce();
        String timestamp = getTimestamp();
        String baseSignatureString = generateSignatureBaseString(httpMethod, url, requestParams, nonce, timestamp);
        String signature = encryptUsingHmacSHA1(baseSignatureString);
        base.append("OAuth ");
        append(base, oauth_consumer_key, consumerKey);
        append(base, oauth_token, token);
        append(base, oauth_signature_method, signatureMethod);
        append(base, oauth_timestamp, timestamp);
        append(base, oauth_nonce, nonce);
        append(base, oauth_version, version);
        append(base, oauth_signature, signature);
        base.deleteCharAt(base.length() - 1);
        System.out.println("header : " + base.toString());
        return base.toString();
    }

    /**
     * Generate base string to generate the oauth_signature
     * 
     * @param httpMethod
     * @param url
     * @param requestParams
     * @return
     */
    private String generateSignatureBaseString(String httpMethod, String url, Map<String, String> requestParams, String nonce, String timestamp) {
        Map<String, String> params = new HashMap<>();
        requestParams.entrySet().forEach(entry -> {
            put(params, entry.getKey(), entry.getValue());
        });
        put(params, oauth_consumer_key, consumerKey);
        put(params, oauth_nonce, nonce);
        put(params, oauth_signature_method, signatureMethod);
        put(params, oauth_timestamp, timestamp);
        put(params, oauth_token, token);
        put(params, oauth_version, version);
        Map<String, String> sortedParams = params.entrySet().stream().sorted(Map.Entry.comparingByKey())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new));
        StringBuilder base = new StringBuilder();
        sortedParams.entrySet().forEach(entry -> {
            base.append(entry.getKey()).append("=").append(entry.getValue()).append("&");
        });
        base.deleteCharAt(base.length() - 1);
        String baseString = httpMethod.toUpperCase() + "&" + encode(url) + "&" + encode(base.toString());
        return baseString;
    }

    private String encryptUsingHmacSHA1(String input) {
        String secret = new StringBuilder().append(encode(consumerSecret)).append("&").append(encode(tokenSecret)).toString();
        byte[] keyBytes = secret.getBytes(StandardCharsets.UTF_8);
        SecretKey key = new SecretKeySpec(keyBytes, HMAC_SHA1);
        Mac mac;
        try {
            mac = Mac.getInstance(HMAC_SHA1);
            mac.init(key);
        } catch (NoSuchAlgorithmException | InvalidKeyException e) {
            e.printStackTrace();
            return null;
        }
        byte[] signatureBytes = mac.doFinal(input.getBytes(StandardCharsets.UTF_8));
        return new String(Base64.getEncoder().encode(signatureBytes));
    }

    /**
     * Percentage encode String as per RFC 3986, Section 2.1
     * 
     * @param value
     * @return
     */
    private String encode(String value) {
        String encoded = "";
        try {
            encoded = URLEncoder.encode(value, "UTF-8");
        } catch (Exception e) {
            e.printStackTrace();
        }
        String sb = "";
        char focus;
        for (int i = 0; i < encoded.length(); i++) {
            focus = encoded.charAt(i);
            if (focus == '*') {
                sb += "%2A";
            } else if (focus == '+') {
                sb += "%20";
            } else if (focus == '%' && i + 1 < encoded.length() && encoded.charAt(i + 1) == '7' && encoded.charAt(i + 2) == 'E') {
                sb += '~';
                i += 2;
            } else {
                sb += focus;
            }
        }
        return sb.toString();
    }

    private void put(Map<String, String> map, String key, String value) {
        map.put(encode(key), encode(value));
    }

    private void append(StringBuilder builder, String key, String value) {
        builder.append(encode(key)).append("=\"").append(encode(value)).append("\",");
    }

    private String getNonce() {
        int leftLimit = 48; // numeral '0'
        int rightLimit = 122; // letter 'z'
        int targetStringLength = 10;
        Random random = new Random();

        String generatedString = random.ints(leftLimit, rightLimit + 1).filter(i -> (i <= 57 || i >= 65) && (i <= 90 || i >= 97)).limit(targetStringLength)
                .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append).toString();
        return generatedString;

    }

    private String getTimestamp() {
        return Math.round((new Date()).getTime() / 1000.0) + "";
    }

}

Sample usage using Spring RestTemplate to get details of a Twitter user:

TwitterOauthHeaderGenerator generator = new TwitterOauthHeaderGenerator("consumerKey", "consumerSecret", "token", "tokenSecret");
Map<String, String> requestParams = new HashMap<>();
requestParams.put("usernames", "some_handle");
String header = generator.generateHeader("GET", "https://api.twitter.com/labs/1/users", requestParams);
HttpHeaders headers = new HttpHeaders();
headers.add("Authorization", header);
HttpEntity<String> httpEntity = new HttpEntity<String>("body", headers);
ResponseEntity<SomeModel> someModelEntity= restTemplate.exchange("https://api.twitter.com/labs/1/users?usernames=some_handle",
                HttpMethod.GET, httpEntity, SomeModel.class);
System.out.println(someModelEntity.getBody());

Complete code and working demo available at Twitter-Play

For OAuth 1.0a with JIRA. I was able to benefit from the Twitter example above, except that in case of JIRA I had to digitally sign the generatedSignatureBaseString with RSA-SHA1 instead of HMACSHA1.

Below are the snippets to do the signing with RSA-SHA1. This is in Groovy and uses Java 8 without any external libraries:

import java.security.SignatureException;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import java.security.Signature;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;


//-------- generate RSA-SHA1 signature from request data
def encryptUsingRSASHA1(String data, String key) throws 
java.security.SignatureException
{
String result
try {
    // get an rsa_sha1 key from the raw key bytes
    //SHA1withRSA
    Signature signature = Signature.getInstance("SHA1withRSA");
    signature.initSign(getPrivateKey(key));
    signature.update(data.getBytes("UTF-8"));
    byte[] rawRSAsigned = signature.sign();
    result= rawRSAsigned.encodeBase64()
    
} catch (Exception e) {
    throw new SignatureException("Failed to generate Signature : " + 
e.getMessage());
}
return result
} 

//get PrivateKey from key string
def getPrivateKey(String privateKey) throws NoSuchAlgorithmException, 
InvalidKeySpecException {
    String privateKeyPEM = privateKey.replace("-----BEGIN PRIVATE KEY-----\n", 
"").replace("-----END PRIVATE KEY-----", "").replaceAll("\n","");

    byte[] privateKeyBytes = privateKeyPEM.decodeBase64();
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
    KeyFactory kf = KeyFactory.getInstance("RSA");
    return kf.generatePrivate(keySpec);
}