netty之EventLoopGroup

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() * 2
        DEFAULT_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.
     *
     * 
 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. 
     *
     * @return  the maximum number of processors available to the virtual
     *          machine; never smaller than one
     * @since 1.4
     */
    public native int availableProcessors();