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() {