从 ftpClient的官方网的FAQ里面看到，实现这个需要用一个 自定义的SocketFactory
然后实现里面的 createSocket方法，有好多个。
http://wiki.apache.org/jakarta-commons/Net/FrequentlyAskedQuestions
原文如下：
Q: How can I set a connection timeout?
A: setDefaultTimeout does not set the connect
timeout. It provides a default socket timeout. Only in J2SE 1.4 was the
ability to specify a timeout on connect added to the Socket API. Since
Commons Net 1.2.x has a J2SE 1.2 compatibility requirement, the ability
to specify a connect timeout is not included. The way to workaround
this is to implement your own SocketFactory and set it with SocketClient.setSocketFactory (FTPClient is a subclass of SocketClient). When you implement the SocketFactory,
add a setConnectTimeout method or some such. Inside of the createSocket
methods, use the J2SE 1.4 connect method with the timeout. We could
actually provide socket factory that subclasses DefaultSocketFactory
to do this without breaking backward compatibility, but that would have
to be discussed further. The way to do it is to compile it only if J2SE
>= 1.4 is being used. SocketClient could check for
availability of the J2SE 1.4 connect method and instantiate the J2SE
>= 1.4 factory if available (using Class.forName and newInstance).
The setDefaultTimeout method could then be changed to also set the
connect timeout in the new factory if being used. If users want this
functionality enough, the best chance of getting it implemented soon is
to submit a patch.
不过我看了 ftpClient 2.0的源代码。
ftpClient.connect(hostName, 21);
看看 connect的代码如下
public void connect(String hostname, int port)
throws SocketException, IOException
{
_socket_= _socketFactory_.createSocket();
_socket_.connect(new InetSocketAddress(hostname, port), connectTimeout);
_connectAction_();
}
其中有一个connectTimeout正是我们需要的连接超时，我们看看它的定义
/** The socket's connect timeout (0 = infinite timeout) */
private static final int DEFAULT_CONNECT_TIMEOUT = 0;
protected int connectTimeout = DEFAULT_CONNECT_TIMEOUT;
可见默认是不限超时的。再搜索一下，找到如下这个方法
/**
* Sets the connection timeout in milliseconds, which will be passed to the {@link Socket} object's
* connect() method.
* @param connectTimeout The connection timeout to use (in ms)
* @since 2.0
*/
public void setConnectTimeout(int connectTimeout) {
this.connectTimeout = connectTimeout;
}
/**
* Get the underlying socket connection timeout.
* @return
* @since 2.0
*/
public int getConnectTimeout() {
return connectTimeout;
}
可见，我们完全可以直接设置超时时间就行了。具体代码如下：
FTPClient ftpClient = new FTPClient();
ftpClient.setConnectTimeout(1000); // 一秒钟，如果超过就判定超时了
ftpClient.connect(hostName, 21);
估计2,0这个版本对这个问题进行了完善和增强，可以这么简单的实现了。
