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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
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 * 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.
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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 LdapDnsProviderService is responsible for creating and providing access to the registered LdapDnsProviders. The ServiceLoader is used to find and register any implementations of LdapDnsProvider. 
 Instances of this class are safe for use by multiple threads.
/**
 * 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
/**
     * 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.
/**
     * 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 LdapDnsProvider then this method will fall back to the DefaultLdapDnsProvider. 
Throws:
NamingException – if the url in not valid or an error occurred while performing the lookup./**
     * 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;
    }

}