Summary
-------

Add static factory creation methods to `java.nio.channels.SocketChannel` and `java.nio.channels.ServerSocketChannel` which take a `java.net.ProtocolFamily`. These will allow channels to be created IPv4 only or IPv4/IPv6 regardless of the VM wide setting of the `"java.net.preferIPv4Stack"` system property. It will also facilitate the support of future non-IP protocol families as anticipated in [JEP 380](https://openjdk.java.net/jeps/380).

Problem
-------

SocketChannel and ServerSocketChannel do not allow to specify a protocol family when opening a new channel. `DatagramChannel` has had this capability since Java 1.7. For consistency, it should be available for TCP sockets as well as UDP sockets.

Solution
--------

The solution is straightforward and basically identical to that previously added to `DatagramChannel`, namely the addition of one new static factory method to `SocketChannel` and `ServerSocketChannel`, and the equivalent supporting methods to `SelectorProvider`.

Specification
-------------

Add the following new method to java.nio.channels.SocketChannel

    /**
     * Opens a socket channel. The {@code family} parameter specifies the
     * {@link ProtocolFamily protocol family} of the channel's socket.
     *
     * <p> The new channel is created by invoking the {@link
     * java.nio.channels.spi.SelectorProvider#openSocketChannel(ProtocolFamily)
     * openSocketChannel(ProtocolFamily)} method of the system-wide default.
     * {@link java.nio.channels.spi.SelectorProvider} object. </p>
     *
     * @param   family
     *          The protocol family
     *
     * @return  A new socket channel
     *
     * @throws  UnsupportedOperationException
     *          If the specified protocol family is not supported. For example,
     *          suppose the parameter is specified as {@link
     *          java.net.StandardProtocolFamily#INET6 StandardProtocolFamily.INET6}
     *          but IPv6 is not enabled on the platform.
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @since 15
     */

    public static SocketChannel open(ProtocolFamily family) throws IOException {}

Add the following new method to java.nio.channels.ServerSocketChannel

    /**
     * Opens a server-socket channel.The {@code family} parameter specifies the
     * {@link ProtocolFamily protocol family} of the channel's socket.
     *
     * <p> The new channel is created by invoking the {@link
     * java.nio.channels.spi.SelectorProvider#openServerSocketChannel(ProtocolFamily)
     * openServerSocketChannel(ProtocolFamily)} method of the system-wide default {@link
     * java.nio.channels.spi.SelectorProvider} object. </p>
     *
     * @param   family
     *          The protocol family
     *
     * @return  A new socket channel
     *
     * @throws  UnsupportedOperationException
     *          If the specified protocol family is not supported. For example,
     *          suppose the parameter is specified as {@link
     *          java.net.StandardProtocolFamily#INET6 StandardProtocolFamily.INET6}
     *          but IPv6 is not enabled on the platform.
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @since 15
     */
    public static ServerSocketChannel open(ProtocolFamily family) throws IOException {}

Add the following methods to java.nio.channels.spi.SelectorProvider

    /**
     * Opens a socket channel.
     *
     * @implSpec The default implementation of this method first checks that
     *          the given protocol {@code family} is not {@code null},
     *          then throws {@link UnsupportedOperationException}.
     *
     * @param   family
     *          The protocol family
     *
     * @return  The new channel
     *
     * @throws  UnsupportedOperationException
     *          If the specified protocol family is not supported
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @since 15
     */
    public SocketChannel openSocketChannel(ProtocolFamily family) throws IOException {}

    /**
     * Opens a server-socket channel.
     *
     * @implSpec The default implementation of this method first checks that
     *          the given protocol {@code family} is not {@code null},
     *          then throws {@link UnsupportedOperationException}.
     *
     * @param   family
     *          The protocol family
     *
     * @return  The new channel
     *
     * @throws  UnsupportedOperationException
     *          If the specified protocol family is not supported
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @since 15
     */
    public ServerSocketChannel openServerSocketChannel(ProtocolFamily family) throws IOException {}