/*

 * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package com.sun.jndi.ldap;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.util.*;

import javax.naming.NamingException;

import com.sun.jndi.ldap.spi.LdapDnsProvider;

import com.sun.jndi.ldap.spi.LdapDnsProviderResult;

import sun.security.util.SecurityConstants;

/**

 * The {@code LdapDnsProviderService} is responsible for creating and providing

 * access to the registered {@code LdapDnsProvider}s. The {@link ServiceLoader}

 * is used to find and register any implementations of {@link LdapDnsProvider}.

 *

 * <p> Instances of this class are safe for use by multiple threads.

 */

final class LdapDnsProviderService {

    private static volatile LdapDnsProviderService service;

    private static final Object LOCK = new int[0];

    private final ServiceLoader<LdapDnsProvider> providers;

    /**

     * Creates a new instance of LdapDnsProviderService

     */

    private LdapDnsProviderService() {

        SecurityManager sm = System.getSecurityManager();

        if (sm == null) {

            providers = ServiceLoader.load(

                    LdapDnsProvider.class,

                    ClassLoader.getSystemClassLoader());

        } else {

            final PrivilegedAction<ServiceLoader<LdapDnsProvider>> pa =

                    () -> ServiceLoader.load(

                            LdapDnsProvider.class,

                            ClassLoader.getSystemClassLoader());

            providers = AccessController.doPrivileged(

                pa,

                null,

                new RuntimePermission("ldapDnsProvider"),

                SecurityConstants.GET_CLASSLOADER_PERMISSION);

        }

    }

    /**

     * Retrieves the singleton instance of LdapDnsProviderService.

     */

    static LdapDnsProviderService getInstance() {

        if (service != null) return service;

        synchronized (LOCK) {

            if (service != null) return service;

            service = new LdapDnsProviderService();

        }

        return service;

    }

    /**

     * Retrieves result from the first provider that successfully resolves

     * the endpoints. If no results are found when calling installed

     * subclasses of {@code LdapDnsProvider} then this method will fall back

     * to the {@code DefaultLdapDnsProvider}.

     *

     * @throws NamingException if the {@code url} in not valid or an error

     *                         occurred while performing the lookup.

     */

    LdapDnsProviderResult lookupEndpoints(String url, Hashtable<?,?> env)

        throws NamingException

    {

        LdapDnsProviderResult result = null;

        Hashtable<?, ?> envCopy = new Hashtable<>(env);

        synchronized (LOCK) {

            Iterator<LdapDnsProvider> iterator = providers.iterator();

            while (result == null && iterator.hasNext()) {

                result = iterator.next().lookupEndpoints(url, envCopy)

                        .filter(r -> !r.getEndpoints().isEmpty())

                        .orElse(null);

            }

        }

        if (result == null) {

            return new DefaultLdapDnsProvider().lookupEndpoints(url, env)

                .orElse(new LdapDnsProviderResult("", Collections.emptyList()));

        }

        return result;

    }

}