/*

 * Copyright (c) 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 jdk.dns.conf;

import java.util.LinkedList;

import java.util.List;

import java.util.StringTokenizer;

public class DnsResolverConfiguration {

    // Lock held whilst loading configuration or checking

    private static ReentrantLock lock = new ReentrantLock();

    // Addresses have changed

    private static boolean changed = false;

    // Time of last refresh.

    private static long lastRefresh = -1;

    // or configured as preference in the future.

    // DNS suffix list and name servers populated by native method

    private static String os_searchlist;

    private static String os_nameservers;

    // Cached lists

    private static LinkedList<String> searchlist;

    private static LinkedList<String> nameservers;

    // Parse string that consists of token delimited by space or commas

    // and return LinkedHashMap

    private LinkedList<String> stringToList(String str) {

        LinkedList<String> ll = new LinkedList<>();

        // comma and space are valid delimiters

        StringTokenizer st = new StringTokenizer(str, ", ");

        while (st.hasMoreTokens()) {

            String s = st.nextToken();

            if (!ll.contains(s)) {

                ll.add(s);

            }

        }

        return ll;

    }

    // Load DNS configuration from OS

    private void loadConfig() {

        assert lock.isHeldByCurrentThread();

        // if address have changed then DNS probably changed as well;

        // otherwise check if cached settings have expired.

        //

        if (changed) {

            changed = false;

        } else {

            if (lastRefresh >= 0) {

                if ((currTime - lastRefresh) < TIMEOUT) {

                    return;

                }

            }

        }

        // load DNS configuration, update timestamp, create

        // new HashMaps from the loaded configuration

        //

        loadDNSconfig0();

        searchlist = stringToList(os_searchlist);

        nameservers = stringToList(os_nameservers);

        os_searchlist = null;                       // can be GC'ed

        os_nameservers = null;

    }

    }

    @SuppressWarnings("unchecked") // clone()

    public List<String> searchlist() {

        lock.lock();

        try {

            loadConfig();

            // List is mutable so return a shallow copy

            return (List<String>) searchlist.clone();

        } finally {

            lock.unlock();

        }

    }

    @SuppressWarnings("unchecked") // clone()

    public List<String> nameservers() {

        try {

            loadConfig();

            // List is mutable so return a shallow copy

            return (List<String>) nameservers.clone();

        } finally {

            lock.unlock();

        }

    }

    // --- Address Change Listener

    static class AddressChangeListener extends Thread {

        public void run() {

            for (; ; ) {

                // wait for configuration to change

                if (notifyAddrChange0() != 0)

                    return;

                lock.lock();

                try {

                    changed = true;

                } finally {

                    lock.unlock();

                }

            }

        }

    }

    // --- Native methods --

    static native void init0();

    static native void loadDNSconfig0();

    static native int notifyAddrChange0();

    static {

        System.loadLibrary("resolver");

        init0();

        // start the address listener thread

        AddressChangeListener thr = new AddressChangeListener();

        thr.setDaemon(true);

        thr.start();

    }

}