src/jdk.naming.ldap/share/classes/com/sun/jndi/ldap/dns/LdapDnsProviderService.java

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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
 * 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.
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 * 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,
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package com.sun.jndi.ldap.dns;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.*;
import javax.naming.NamingException;

import com.sun.jndi.ldap.LdapCtxFactory;
import com.sun.jndi.ldap.LdapDnsProviderResultInternal;
import com.sun.jndi.ldap.LdapDnsProviderServiceInternal;
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 implements LdapDnsProviderServiceInternal {

    private static volatile LdapDnsProviderService service;
    private static final Object LOCK = new int[0];
    private final ServiceLoader<LdapDnsProvider> providers;

    static {
        LdapCtxFactory.registerLdapDnsProviderService(getInstance());
    }

    /**
     * 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);
            }
        }

        return result;
    }

    @Override
    public LdapDnsProviderResultInternal lookupEndpointsInternal(String url,
            Hashtable<?, ?> env) throws NamingException
    {
        LdapDnsProviderResult r = lookupEndpoints(url, env);
        return (r == null) ? null :
            new LdapDnsProviderResultInternal(r.getDomainName(), r.getEndpoints());
    }
}