创建EventLoopGroup使用无参构造方法时，发现会创建24个NioEventLoop（readonlyChildren.size=24）,经查，是取的java虚拟机的可用处理器数2倍。源码如下：

重点就是最后哪个方法的注释：Returns the number of processors available to the Java virtual machine.(返回Java虚拟机可用的处理器数。)

    /*** @see MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, Executor, Object...)*/protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {//如果没指定线程数，则取默认的DEFAULT_EVENT_LOOP_THREADS super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);}private static final int DEFAULT_EVENT_LOOP_THREADS;static {//核心逻辑就是看NettyRuntime.availableProcessors() * 2DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt("io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));if (logger.isDebugEnabled()) {logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);}}/*** Get the configured number of available processors. The default is {@link Runtime#availableProcessors()}.* This can be overridden by setting the system property "io.netty.availableProcessors" or by invoking* {@link #setAvailableProcessors(int)} before any calls to this method.** @return the configured number of available processors*/@SuppressForbidden(reason = "to obtain default number of available processors")synchronized int availableProcessors() {//如果availableProcessors =0，还未设置过，就取Runtime.getRuntime().availableProcessors()if (this.availableProcessors == 0) {final int availableProcessors =SystemPropertyUtil.getInt("io.netty.availableProcessors",Runtime.getRuntime().availableProcessors());setAvailableProcessors(availableProcessors);}return this.availableProcessors;}}/*** Returns the number of processors available to the Java virtual machine.** <p> This value may change during a particular invocation of the virtual* machine.  Applications that are sensitive to the number of available* processors should therefore occasionally poll this property and adjust* their resource usage appropriately. </p>** @return  the maximum number of processors available to the virtual*          machine; never smaller than one* @since 1.4*/public native int availableProcessors();