1.概述
Zookeeper作为一个服务器，需要与客户端进行网络通信，Zookeeper使用ServerCnxFactory管理与客户端的连接，其中有两个实现，一个是NIOServerCnxnFactory，使用java原生Nio实现，一个是NettyServerCnxnFactory，使用netty实现。
2.NIOServerCnxnFactory
使用的是java的NIO思路，1个accept Thread，该线程主要接收客户端的连接，并将其分配给selector thread; selector thread:该线程执行select()，由于在处理大量连接时，select()会成为性能瓶颈，因此启动多个selector thread，使用zookeeper.nio.numSelectorThreads来进行配置该类线程数，默认个数为核心数/2；worker thread：该线程执行基本的操作，可以理解为真正的处理线程，使用zookeeper.nio.numWorkerThreads来进行配置该线程的线程数，默认的线程数为：核心线程数*2；connection expiration thread：连接上的session过期，则关闭该连接。
上面是对这个NIOServerCnxnFactory类类上的注释说明
/**
* NIOServerCnxnFactory implements a multi-threaded ServerCnxnFactory using
* NIO non-blocking socket calls. Communication between threads is handled via
* queues.
*
* - 1 accept thread, which accepts new connections and assigns to a
* selector thread
* - 1-N selector threads, each of which selects on 1/N of the connections.
* The reason the factory supports more than one selector thread is that
* with large numbers of connections, select() itself can become a
* performance bottleneck.
* - 0-M socket I/O worker threads, which perform basic socket reads and
* writes. If configured with 0 worker threads, the selector threads
* do the socket I/O directly.
* - 1 connection expiration thread, which closes idle connections; this is
* necessary to expire connections on which no session is established.
*
* Typical (default) thread counts are: on a 32 core machine, 1 accept thread,
* 1 connection expiration thread, 4 selector threads, and 64 worker threads.
*/
public class NIOServerCnxnFactory extends ServerCnxnFactory 2.1 AcceptThread
下面是AcceptThread线程的源码，我先把里面的实现方法给删除了。该线程的执行流程：run方法执行selector.select()，并进行调用doAccept()接收客户端连接，因此我们需要重点关注doAccept()方法
private class AcceptThread extends AbstractSelectThread {
private final ServerSocketChannel acceptSocket;
private final SelectionKey acceptKey;
private final RateLogger acceptErrorLogger = new RateLogger(LOG);
private final Collection<SelectorThread> selectorThreads;
private Iterator<SelectorThread> selectorIterator;
private volatile boolean reconfiguring = false;
public AcceptThread(ServerSocketChannel ss, InetSocketAddress addr, Set<SelectorThread> selectorThreads) throws IOException {
}
public void run() {
}
public void setReconfiguring() {
reconfiguring = true;
}
private void select() {
}
/**
