Netty学习之旅------图说Netty线程模型

1、图说Netty线程模型

提到Netty的线程模型，我们不得不再重复提到主从Reactor线程模型，Netty线程模型基本上基于主从Reactor模型的实现方式，Netty线程模型将从如下两个图进行展开：

 

Reactor 主从线程模型

Netty服务端示例代码。

关于Reactor主从多线程模型的讲解，请重点关注Netty 线程模型前置篇，Reactor主从模式源码实现，从Netty服务端示例代码可以看出，Netty的线程模型，必然与EventLoopGroup脱离不了干系。不错，上文中的bossGroup就是相当与Reactor模式中的Main Reactor,而workGroup,则相当于SubReactor。

Netty线程模型类层次结构：

Netty线程模型类继承图：

从如上图可以知道，Netty的线程模型，中有4个基础接口，它们分别是EventLoopGroup、EventLoop、EventExecuteGroup、EventExecute。其中EventExecute扩展自java.util.concurrent.ScheduledExecutorService接口，类似与线程池（执行）的职责，而EventLoop首先继承自EventExecute，并主要扩展了register方法,就是将通道Channel注册到Selector的方法。

NioEventLoop,就是基于Nio的实现。在这个类中有一个亮点，就是规避了JDK nio的一个bug,Selector select方法的空轮询,核心思想是，如果连续多少次（默认为512）在没有超时的情况就返回，并且已经准备就绪的键的数量为0，则认为发生了空轮询，如果发生了空轮询，就新建一个新的Selector,并重新将通道，关心的事件注册到新的Selector,并关闭旧的Selector,其源码实现如下：

private void select(boolean oldWakenUp) throws IOException {
        Selector selector = this.selector;
        try {
            int selectCnt = 0;
            long currentTimeNanos = System.nanoTime();
            long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
            for (;;) {
                long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
                if (timeoutMillis <= 0) {
                    if (selectCnt == 0) {
                        selector.selectNow();
                        selectCnt = 1;
                    }
                    break;
                }

                int selectedKeys = selector.select(timeoutMillis);
                selectCnt ++;

                if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
                    // - Selected something,
                    // - waken up by user, or
                    // - the task queue has a pending task.
                    // - a scheduled task is ready for processing
                    break;
                }
                if (Thread.interrupted()) {
                    // Thread was interrupted so reset selected keys and break so we not run into a busy loop.
                    // As this is most likely a bug in the handler of the user or it's client library we will
                    // also log it.
                    //
                    // See https://github.com/netty/netty/issues/2426
                    if (logger.isDebugEnabled()) {
                        logger.debug("Selector.select() returned prematurely because " +
                                "Thread.currentThread().interrupt() was called. Use " +
                                "NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
                    }
                    selectCnt = 1;
                    break;
                }

                long time = System.nanoTime();
                if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
                    // timeoutMillis elapsed without anything selected.
                    selectCnt = 1;
                } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
                        selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
                    // The selector returned prematurely many times in a row.
                    // Rebuild the selector to work around the problem.
                    logger.warn(
                            "Selector.select() returned prematurely {} times in a row; rebuilding selector.",
                            selectCnt);

                    rebuildSelector();
                    selector = this.selector;

                    // Select again to populate selectedKeys.
                    selector.selectNow();
                    selectCnt = 1;
                    break;
                }

                currentTimeNanos = time;
            }

            if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Selector.select() returned prematurely {} times in a row.", selectCnt - 1);
                }
            }
        } catch (CancelledKeyException e) {
            if (logger.isDebugEnabled()) {
                logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector - JDK bug?", e);
            }
            // Harmless exception - log anyway
        }
    }


/**
     * Replaces the current {@link Selector} of this event loop with newly created {@link Selector}s to work
     * around the infamous epoll 100% CPU bug.
     */
    public void rebuildSelector() {
        if (!inEventLoop()) {
            execute(new Runnable() {
                @Override
                public void run() {
                    rebuildSelector();
                }
            });
            return;
        }

        final Selector oldSelector = selector;
        final Selector newSelector;

        if (oldSelector == null) {
            return;
        }

        try {
            newSelector = openSelector();
        } catch (Exception e) {
            logger.warn("Failed to create a new Selector.", e);
            return;
        }

        // Register all channels to the new Selector.
        int nChannels = 0;
        for (;;) {
            try {
                for (SelectionKey key: oldSelector.keys()) {
                    Object a = key.attachment();
                    try {
                        if (!key.isValid() || key.channel().keyFor(newSelector) != null) {
                            continue;
                        }

                        int interestOps = key.interestOps();
                        key.cancel();
                        SelectionKey newKey = key.channel().register(newSelector, interestOps, a);
                        if (a instanceof AbstractNioChannel) {
                            // Update SelectionKey
                            ((AbstractNioChannel) a).selectionKey = newKey;
                        }
                        nChannels ++;
                    } catch (Exception e) {
                        logger.warn("Failed to re-register a Channel to the new Selector.", e);
                        if (a instanceof AbstractNioChannel) {
                            AbstractNioChannel ch = (AbstractNioChannel) a;
                            ch.unsafe().close(ch.unsafe().voidPromise());
                        } else {
                            @SuppressWarnings("unchecked")
                            NioTask task = (NioTask) a;
                            invokeChannelUnregistered(task, key, e);
                        }
                    }
                }
            } catch (ConcurrentModificationException e) {
                // Probably due to concurrent modification of the key set.
                continue;
            }

            break;
        }

        selector = newSelector;

        try {
            // time to close the old selector as everything else is registered to the new one
            oldSelector.close();
        } catch (Throwable t) {
            if (logger.isWarnEnabled()) {
                logger.warn("Failed to close the old Selector.", t);
            }
        }

        logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
    }