GoogleAuthenticator.java

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;
import java.util.Optional;
import java.util.function.Consumer;

import static java.lang.Integer.parseInt;
import static java.lang.String.format;
import static java.lang.System.currentTimeMillis;
import static java.net.URLEncoder.encode;
import static java.nio.charset.StandardCharsets.UTF_8;
import static java.util.Optional.empty;
import static java.util.Optional.of;
import static java.util.concurrent.TimeUnit.SECONDS;
import static spark.Spark.*;

public class GoogleAuthenticator {
	private static final String FORMAT_AUTH_REQUEST = "https://accounts.google.com/o/oauth2/v2/auth" +
			"?redirect_uri=%s" +
			"&client_id=%s" +
			"&login_hint=%s" +
			"&prompt=%s" +
			"&state=%s" +
			"&scope=%s";
	private static final String
			PARAM_STATE = "state",
			PARAM_ACCESS_TOKEN = "access_token",
			PARAM_EXPIRES_IN = "expires_in";
	
	private final Map<String, Consumer<Optional<Token>>> listeners = new HashMap<>();
	private final String redirectUrl;
	
	public GoogleAuthenticator(String redirectUrl) throws IOException {
		this(new URL(redirectUrl));
	}
	
	public GoogleAuthenticator(URL redirectUrl) throws UnsupportedEncodingException {
		this.redirectUrl = encode(redirectUrl.toString(), UTF_8.name());
		init();
		port(redirectUrl.getPort());
		ipAddress(redirectUrl.getHost());
		get(redirectUrl.getPath(), (request, response) -> {
			final String state = request.queryParams(PARAM_STATE);
			if (state == null)
				throw new IllegalStateException("No state was returned!");
			final String token = request.queryParams(PARAM_ACCESS_TOKEN);
			final Consumer<Optional<Token>> listener = listeners.remove(state);
			if (listener != null)
				listener.accept(token == null ? empty() : of(new Token(
						token,
						parseInt(request.queryParams(PARAM_EXPIRES_IN))
				)));
			return response;
		});
	}
	
	public URL authenticate(
			Object clientId,
			String state,
			String scope,
			String hint,
			Prompt prompt,
			Consumer<Optional<Token>> callback
	) {
		try {
			listeners.put(state, callback);
			return new URL(format(
					FORMAT_AUTH_REQUEST,
					redirectUrl,
					clientId,
					hint,
					prompt,
					state,
					scope
			));
		} catch (MalformedURLException reason) {
			throw new RuntimeException(reason);
		}
	}
	
	
	public enum Prompt {
		NONE, CONSENT, SELECT_ACCOUNT;
		
		private final String name = name().toLowerCase();
		
		@Override
		public String toString() { return name; }
	}
	
	public static class Token {
		public final String token;
		public final long expiration;
		
		Token(String token, int expiresIn) {
			this.token = token;
			this.expiration = currentTimeMillis() + SECONDS.toMillis(expiresIn);
		}
	}
}