根据Filip Hanik（一个Tomcat提交程序），它意味着***"模拟阻塞"**。（参考：Tomcat User Mailing list post）*

这只是一个猜测，但它可能代表模拟阻塞，意味着一个阻塞API包裹在底层的非阻塞nio api周围。

源代码：https://github.com/apache/tomcat/blob/8.5.x/java/org/apache/tomcat/util/net/NioEndpoint.java

/**
         * NioEndpoint.NioSocketWrapper.fillReadBuffer() 用于直接读取内容到 传入的任意 ByteBuffer 中
         *
         * @param block  是否阻塞读
         * @param buffer 待接收数据的buffer
         * @return 读取到的字节数
         * @throws IOException
         */
        private int fillReadBuffer(boolean block, ByteBuffer buffer) throws IOException {
            int n = 0;
            if (getSocket() == NioChannel.CLOSED_NIO_CHANNEL) {
                throw new ClosedChannelException();
            }
            if (block) { // if no readListener. block variable is true.
                long timeout = getReadTimeout();
                long startNanos = 0;
                do {
                    if (startNanos > 0) {
                        long elapsedMillis = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startNanos);
                        if (elapsedMillis == 0) {
                            elapsedMillis = 1;
                        }
                        timeout -= elapsedMillis;
                        if (timeout <= 0) {
                            throw new SocketTimeoutException();
                        }
                    }
                    n = getSocket().read(buffer);
                    if (n == -1) {
                        throw new EOFException();
                    } else if (n == 0) {
                        if (!readBlocking) {
                            readBlocking = true;
                            registerReadInterest(); // <=============== (向关联的Poller)注册读事件.
                        }
                        synchronized (readLock) {
                            if (readBlocking) {
                                try {
                                    if (timeout > 0) {
                                        startNanos = System.nanoTime();
                                        readLock.wait(timeout); // block itself here < ===================
                                    } else {
                                        readLock.wait();
                                    }
                                } catch (InterruptedException e) {
                                    // Continue
                                }
                            }
                        }
                    }
                } while (n == 0); // TLS needs to loop as reading zero application bytes is possible
            } else {
                n = getSocket().read(buffer);
                if (n == -1) {
                    throw new EOFException();
                }
            }
            return n;
        }