SelectionKey.attachment() returns null

写NIO程序的时候遇到了SelectionKey.attachment()为空的问题，最后解决了，这里简单介绍下，希望能对其他人有帮助：

 

SelectionKey key = socketChannel.register(selector, SelectionKey.OP_CONNECT);

key.attach(group);

...

client.register(selector, SelectionKey.OP_READ);

if(key.isReadable()){

ChannelWorkerGroup group = (ChannelWorkerGroup) key.attachment();

的时候却报group为空。

原因是第二次register为OP_READ的时候没有带上第三个参数，改成这样就好了：

client.register(selector, SelectionKey.OP_READ, group);

if(key.isReadable()){

ChannelWorkerGroup group = (ChannelWorkerGroup) key.attachment();