原帖：http://www.blogjava.net/adapterofcoms/archive/2010/03/02/314242.html
有人说java nio在多线程环境下编程简直就是个恶梦,其实你如果能把握住java nio API的要领,你就可以将之驾驭.
0. 一个 channal 对应一个SelectionKey in the same selector.
e.g:
SelectionKey sk=sc.register(selector, SelectionKey.OP_READ, handler);
sk==sc.register(selector, SelectionKey.OP_WRITE, handler) true?
selector.select() 每次返回的对同一channal的sk是否相同?
1.channel.register(...) may block if invoked concurrently with another registration[another.register(...)] or selection operation[selector.select(...)] involving *****the same selector*****.
这个是register方法jdk src上的原文,
e.g:
如果一个selection thread已经在select方法上等待ing,那么这个时候如果有另一条线程调用channal.register方法的话,那么它将被blocking.
2.selectionKey.cancel() : The key will be removed from all of the selector's key sets during *****the next selection operation[selector.select(...)]*****.
may block briefly if invoked concurrently with a cancellation[cancel()] or selection operation[select(...)] involving ***the same selector***.
这个也是cancel方法jdk src上的原文,
e.g:
你先将一个selectionKey.cancel(),然后随即再channel.register to the same selector,
在cancel和register之间,如果没有线程(包括当前线程)进行select操作的话,
那么 throws java.nio.channels.CancelledKeyException.
所以 cancel-->select-->re-register.
3.if don't remove the current selectedKey from selector.selectedKeys()[Set] 将导致 selector.select(...) not block [may be cpu 100%,specially when client cut the current channel(connection)].
e.g:
Iterator<SelectionKey> it=selector.selectedKeys().iterator();
...for/while it.hasNext()...
it.remove();<------*****must do it. or Keys' Set.clear() finally;
if remove the current selectedKey from selector.selectedKeys()[Set] but don't sk.interestOps(sk.interestOps()& (~sk.readyOps()));将导致 selector.select(...) not block [select() not block several times, or excepted exception]
4.op_write should not be registered to the selector. [may be cpu100%]
5. if involving wakeup() before select() [wakeup called several times >=1],the next select() not block [not block just once].
尽管以前有些人分析了nio的wakeup性能及not block in linux的bug,但是java nio依然是高效的,那些c/c++的牛人们去看看jre/bin目录下的nio.dll/nio.so吧,java nio是基于select模型(这个是c/c++中常用网络编程模型之一)的.
基于java nio的服务器:mina,girzzly[glassfish],jetty(基于girzzly),tomcat6[可以配置Http11NioProtocol]...
其中从本人对girzzly,tomcat6的源码分析来看,它们都还没有真正发挥出nio异步处理请求的优点,它们的读写还都是blocking的虽然使用了selectorPool,此外tomcat6要剥离出socket通信还要花费一定的功夫.而mina却是名不符其实,还有bug哦.