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Pure Java implementation for JSON Web Token (JWT)
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package com.rongjih.jwt;

import javax.json.Json;
import javax.json.JsonException;
import javax.json.JsonObject;
import javax.json.JsonObjectBuilder;
import java.io.ByteArrayInputStream;
import java.util.Base64;

/**
 * JWT Header.
 *
 * @author RJ
 * @since JDK1.8
 */
public final class Header extends Base {
	public static final String DEFAULT_TYPE = "JWT";
	public static final Algorithm DEFAULT_ALGORITHM = Algorithm.HS256;
	public static final Header DEFAULT = new Header(DEFAULT_TYPE, DEFAULT_ALGORITHM);

	/**
	 * The JWT Header type
	 */
	public String type;

	/**
	 * The JWT Header algorithm
	 */
	public Algorithm algorithm;

	public Header() {
		super();
	}

	public Header(Algorithm algorithm) {
		super();
		this.algorithm = algorithm;
	}

	public Header(String type, Algorithm algorithm) {
		super();
		this.type = type;
		this.algorithm = algorithm;
	}

	@Override
	public JsonObject toJson() {
		JsonObjectBuilder builder = Json.createObjectBuilder();
		if (type != null && !type.isEmpty()) builder.add("typ", type);
		if (algorithm != null) builder.add("alg", algorithm.name());
		return builder.build();
	}

	/**
	 * Decode the base64-string to Header.
	 *
	 * @param header the base64-string to be decoded
	 * @return the header instance
	 * @throws DecodeException if header is not a standard jwt header
	 */
	public static Header decode(String header) {
		// verify to json with JAX-RS standard
		JsonObject json;
		try {
			json = Json.createReader(new ByteArrayInputStream(Base64.getUrlDecoder().decode(header))).readObject();
		} catch (JsonException e) {
			throw new DecodeException("Failed convert to JsonObject", e);
		} catch (IllegalArgumentException e) {
			throw new DecodeException("The header is not in valid Base64 scheme", e);
		}

		// validate the typ parameter
		String type = json.containsKey("typ") ? json.getString("typ") : null;
		if (type != null && !DEFAULT_TYPE.equals(type))
			throw new DecodeException("The header type '" + type + "' is not support.");

		// validate the alg parameter
		String algorithm = json.containsKey("alg") ? json.getString("alg") : null;
		if (algorithm != null && !Algorithm.HS256.name().equals(algorithm))
			throw new DecodeException("The header algorithm '" + algorithm + "' is not support.");

		// construct a header instance
		if (DEFAULT_TYPE.equals(type) && Algorithm.HS256.name().equals(algorithm)) {
			// optimise the default standard header to always the same instance
			return Header.DEFAULT;
		} else {
			// create a whole new instance
			Header h = new Header();
			h.type = type;
			h.algorithm = (algorithm != null ? Algorithm.valueOf(algorithm) : null);
			return h;
		}
	}
}