Socket SoLinger

   public int getSoLinger() throws SocketException {
        if (isClosed())
            throw new SocketException("Socket is closed");
        Object o = getImpl().getOption(SocketOptions.SO_LINGER);
        if (o instanceof Integer) {
            return ((Integer) o).intValue();
        } else {
            return -1;
        }
    }

    public void setSoLinger(boolean on, int linger) throws SocketException {
        if (isClosed())
            throw new SocketException("Socket is closed");
        if (!on) {
            getImpl().setOption(SocketOptions.SO_LINGER, new Boolean(on));
        } else {
            if (linger < 0) {
                throw new IllegalArgumentException("invalid value for SO_LINGER");
            }
            if (linger > 65535)
                linger = 65535;
            getImpl().setOption(SocketOptions.SO_LINGER, new Integer(linger));
        }
    }

  /**
     * Specify a linger-on-close timeout.  This option disables/enables
     * immediate return from a <B>close()</B> of a TCP Socket.  Enabling
     * this option with a non-zero Integer <I>timeout</I> means that a
     * <B>close()</B> will block pending the transmission and acknowledgement
     * of all data written to the peer, at which point the socket is closed
     * <I>gracefully</I>.  Upon reaching the linger timeout, the socket is
     * closed <I>forcefully</I>, with a TCP RST. Enabling the option with a
     * timeout of zero does a forceful close immediately. If the specified
     * timeout value exceeds 65,535 it will be reduced to 65,535.
     * <P>
     * Valid only for TCP: SocketImpl
     *
     * @see Socket#setSoLinger
     * @see Socket#getSoLinger
     */
    @Native public final static int SO_LINGER = 0x0080;

在Java Socket中，当我们调用Socket的close方法时，默认的行为是当底层网卡所有数据都发送完毕后，关闭连接
通过setSoLinger方法，我们可以修改close方法的行为

1. setSoLinger(true, 0)
   1. 当网卡收到关闭连接请求后，无论数据是否发送完毕，立即发送RST包关闭连接
2. setSoLinger(true, delay_time)

当网卡收到关闭连接请求后，等待delay_time
如果在delay_time过程中数据发送完毕，正常四次挥手关闭连接
如果在delay_time过程中数据没有发送完毕，发送RST包关闭连接