ServerSocket 限制来访 Ip

作为 Socket 的服务器端，按照道理，一般是“来者通吃”，不管是谁来访，我都愿意跟你建立连接。但如果我只想跟指定的家伙建立关系呢？

答案就是利用 ServerSocket 的第三个参数：InetAddress bindAddr。

怎么用呢？先来看看源码是怎么写的。

/**
 * Create a server with the specified port, listen backlog, and
 * local IP address to bind to.  The <i>bindAddr</i> argument
 * can be used on a multi-homed host for a ServerSocket that
 * will only accept connect requests to one of its addresses.
 * If <i>bindAddr</i> is null, it will default accepting
 * connections on any/all local addresses.
 * The port must be between 0 and 65535, inclusive.
 * A port number of {@code 0} means that the port number is
 * automatically allocated, typically from an ephemeral port range.
 * This port number can then be retrieved by calling
 * {@link #getLocalPort getLocalPort}.
 *
 * <P>If there is a security manager, this method
 * calls its {@code checkListen} method
 * with the {@code port} argument
 * as its argument to ensure the operation is allowed.
 * This could result in a SecurityException.
 *
 * The {@code backlog} argument is the requested maximum number of
 * pending connections on the socket. Its exact semantics are implementation
 * specific. In particular, an implementation may impose a maximum length
 * or may choose to ignore the parameter altogther. The value provided
 * should be greater than {@code 0}. If it is less than or equal to
 * {@code 0}, then an implementation specific default will be used.
 * <P>
 * @param port  the port number, or {@code 0} to use a port
 *              number that is automatically allocated.
 * @param backlog requested maximum length of the queue of incoming
 *                connections.
 * @param bindAddr the local InetAddress the server will bind to
 *
 * @throws  SecurityException if a security manager exists and
 * its {@code checkListen} method doesn't allow the operation.
 *
 * @throws  IOException if an I/O error occurs when opening the socket.
 * @exception  IllegalArgumentException if the port parameter is outside
 *             the specified range of valid port values, which is between
 *             0 and 65535, inclusive.
 *
 * @see SocketOptions
 * @see SocketImpl
 * @see SecurityManager#checkListen
 * @since   JDK1.1
 */
public ServerSocket(int port, int backlog, InetAddress bindAddr) throws IOException {
    setImpl();
    if (port < 0 || port > 0xFFFF)
        throw new IllegalArgumentException(
                   "Port value out of range: " + port);
    if (backlog < 1)
      backlog = 50;
    try {
        bind(new InetSocketAddress(bindAddr, port), backlog);
    } catch(SecurityException e) {
        close();
        throw e;
    } catch(IOException e) {
        close();
        throw e;
    }
}

这么一大段，最重要的一句是什么，就是写在注释里面的那几句。大意就是：“bindAddr 这个参数，如果有的话，那么 ServerSOcket 将只会 access 从这个地址发过来的连接请求，如果没有的话，那么它将允许所有地址发过来的请求”。