ResolvConf Class — netty Architecture

Source Code

resolver-dns/src/main/java/io/netty/resolver/dns/ResolvConf.java lines 32–112

final class ResolvConf {
    private final List<InetSocketAddress> nameservers;

    /**
     * Reads from the given reader and extracts the {@code nameserver}s using the syntax of the
     * {@code /etc/resolv.conf} file, see {@code man resolv.conf}.
     *
     * @param reader contents of {@code resolv.conf} are read from this {@link BufferedReader},
     *              up to the caller to close it
     */
    static ResolvConf fromReader(BufferedReader reader) throws IOException {
        return new ResolvConf(reader);
    }

    /**
     * Reads the given file and extracts the {@code nameserver}s using the syntax of the
     * {@code /etc/resolv.conf} file, see {@code man resolv.conf}.
     */
    static ResolvConf fromFile(String file) throws IOException {
        // Use 1 MB to be a bit conservative
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(
                new BoundedInputStream(new FileInputStream(file), 1024 * 1024)))) {
            return fromReader(reader);
        }
    }

    /**
     * Returns the {@code nameserver}s from the {@code /etc/resolv.conf} file. The file is only read once
     * during the lifetime of this class.
     */
    static ResolvConf system() {
        ResolvConf resolvConv = ResolvConfLazy.machineResolvConf;
        if (resolvConv != null) {
            return resolvConv;
        }
        throw new IllegalStateException("/etc/resolv.conf could not be read");
    }

    private ResolvConf(BufferedReader reader) throws IOException {
        List<InetSocketAddress> nameservers = new ArrayList<>();
        String ln;
        while ((ln = reader.readLine()) != null) {
            ln = ln.trim();
            if (ln.isEmpty()) {
                continue;
            }

            if (ln.startsWith("nameserver")) {
                ln = ln.substring("nameserver".length());
                int cIndex = ln.indexOf('#');
                if (cIndex != -1) {
                    ln = ln.substring(0, cIndex);
                }
                ln = ln.trim();
                if (ln.isEmpty()) {
                    continue;
                }
                nameservers.add(new InetSocketAddress(ln, 53));
            }
        }
        this.nameservers = Collections.unmodifiableList(nameservers);
    }

    List<InetSocketAddress> getNameservers() {
        return nameservers;
    }

    private static final class ResolvConfLazy {
        static final ResolvConf machineResolvConf;

        static {
            ResolvConf resolvConf;
            try {
                resolvConf = ResolvConf.fromFile("/etc/resolv.conf");
            } catch (IOException | SecurityException e) {
                resolvConf = null;
            }
            machineResolvConf = resolvConf;
        }
    }
}