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Eclipse Ditto is a framework for creating and managing digital twins in the IoT.
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/*
 * Copyright (c) 2019 Contributors to the Eclipse Foundation
 *
 * See the NOTICE file(s) distributed with this work for additional
 * information regarding copyright ownership.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0
 *
 * SPDX-License-Identifier: EPL-2.0
 */
package org.eclipse.ditto.connectivity.model;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.function.Function;

import org.eclipse.ditto.json.JsonFieldDefinition;
import org.eclipse.ditto.json.JsonObject;
import org.eclipse.ditto.json.JsonParseException;

/**
 * Credentials of a connection. Credential types are extensible. To add a new credential type, implement this
 * interface and {@link CredentialsVisitor} and their serialization.
 */
public interface Credentials {

    /**
     * Evaluate this object by a visitor.
     *
     * @param visitor the visitor.
     * @param  type of the evaluation result.
     * @return evaluation result.
     */
     T accept(CredentialsVisitor visitor);

    /**
     * Serialize as JSON.
     *
     * @return JSON representation of this object.
     */
    JsonObject toJson();

    /**
     * Register a deserializer for a subclass. Each subclass should call this method at least once before
     * they are considered for deserialization.
     * 
     * Warning: Putting deserializer registration in static blocks of subclasses does not guarantee registration
     * because static blocks are executed only if classes are loaded for some other reason.
     * 
     *
     * @param type the credential type.
     * @param deserializer deserializer of the subclass.
     */
    static void registerDeserializer(final String type,
            final Function deserializer) {

        JsonFields.DESERIALIZER_MAP.put(type, deserializer);
    }

    /**
     * Deserialize credentials from JSON.
     *
     * @param jsonObject credentials in JSON format.
     * @return deserialized credentials.
     */
    static Credentials fromJson(final JsonObject jsonObject) {
        final String type = jsonObject.getValueOrThrow(JsonFields.TYPE);
        final Function deserializer = JsonFields.DESERIALIZER_MAP.get(type);
        if (deserializer == null) {
            throw JsonParseException.newBuilder()
                    .message(String.format("Unknown credential type <%s>", type))
                    .description("Original JSON: " + jsonObject.toString())
                    .build();
        }
        return deserializer.apply(jsonObject);
    }


    /**
     * JSON fields common to all subclasses.
     */
    abstract class JsonFields {

        private static final ConcurrentMap> DESERIALIZER_MAP =
                new ConcurrentHashMap<>();

        /**
         * JSON field definition of the credential type identifier. All subclasses should identify themselves by this
         * field in their serialization.
         */
        public static final JsonFieldDefinition TYPE = JsonFieldDefinition.ofString("type");

        static {
            // load subclasses in this package.
            registerDeserializer(ClientCertificateCredentials.TYPE, ClientCertificateCredentials::fromJson);
            registerDeserializer(SshPublicKeyCredentials.TYPE, SshPublicKeyCredentials::fromJson);
            registerDeserializer(UserPasswordCredentials.TYPE, UserPasswordCredentials::fromJson);
            registerDeserializer(HmacCredentials.TYPE, HmacCredentials::fromJson);
            registerDeserializer(OAuthClientCredentials.TYPE, OAuthClientCredentials::fromJson);
        }
    }
}