Netty组件详解-蒲公英云

之前在做项目的时候使用到了Netty这个网络框架，对于Java中的IO模型有了进一步的了解，熟悉的NIO非阻塞的模式。而Netty就是对于Java NIO 的高级封装。这篇文章就是个人根据Netty4.1.6的源码，进行了总结。

Netty组件

NioEventLoop

对于Netty中的NioEventLoop这个组件来说，它就是类似于写的普通网络编程中的通过创建一个新的线程Thread来实现对于客户端的监听，这个监听如果放到主线程中会导致主线程阻塞，所以在实现的时候通过创建要给新线程的方式来实现。从这个角度上理解，这个NioEventLoop组件，主要做的两件事情，第一，就是建立客户端和服务器端段的连接，保证连接正常。第二，实现客户端和服务器端的数据的交互。
但是在使用Thread实现上面两个功能的时候要保证监听过程处于一个持续监听的状态，也就是说在其中要实现一个死循环。通过这个死循环来持续监听连接在Netty中也提供了这样的一个方法。那就是下面这个方法

@Override
    protected void run() { 
        //实现一个死循环不断去绑定对应的监听状态
        for (;;) { 
            try { 
                //根据不同的装响应实现不同的操作
                switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) { 
                    case SelectStrategy.CONTINUE:
                        continue;
                    case SelectStrategy.SELECT:
                        select(wakenUp.getAndSet(false));
                        // 'wakenUp.compareAndSet(false, true)' is always evaluated
                        // before calling 'selector.wakeup()' to reduce the wake-up
                        // overhead. (Selector.wakeup() is an expensive operation.)
                        //
                        // However, there is a race condition in this approach.
                        // The race condition is triggered when 'wakenUp' is set to
                        // true too early.
                        //
                        // 'wakenUp' is set to true too early if:
                        // 1) Selector is waken up between 'wakenUp.set(false)' and
                        // 'selector.select(...)'. (BAD)
                        // 2) Selector is waken up between 'selector.select(...)' and
                        // 'if (wakenUp.get()) { ... }'. (OK)
                        //
                        // In the first case, 'wakenUp' is set to true and the
                        // following 'selector.select(...)' will wake up immediately.
                        // Until 'wakenUp' is set to false again in the next round,
                        // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
                        // any attempt to wake up the Selector will fail, too, causing
                        // the following 'selector.select(...)' call to block
                        // unnecessarily.
                        //
                        // To fix this problem, we wake up the selector again if wakenUp
                        // is true immediately after selector.select(...).
                        // It is inefficient in that it wakes up the selector for both
                        // the first case (BAD - wake-up required) and the second case
                        // (OK - no wake-up required).
                        if (wakenUp.get()) { 
                            selector.wakeup();
                        }
                    default:
                        // fallthrough
                }
                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;
                //开始处理每个链接
                if (ioRatio == 100) { 
                    try { 
                        processSelectedKeys();
                    } finally { 
                        // Ensure we always run tasks.
                        runAllTasks();
                    }
                } else { 
                    final long ioStartTime = System.nanoTime();
                    try { 
                        processSelectedKeys();
                    } finally { 
                        // Ensure we always run tasks.
                        final long ioTime = System.nanoTime() - ioStartTime;
                        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                    }
                }
            } catch (Throwable t) { 
                handleLoopException(t);
            }
            // Always handle shutdown even if the loop processing threw an exception.
            try { 
                if (isShuttingDown()) { 
                    closeAll();
                    if (confirmShutdown()) { 
                        return;
                    }
                }
            } catch (Throwable t) { 
                handleLoopException(t);
            }
        }
    }

从代码中可以看出来真正实现对于数据读写操作就是从processSelectedKeys()方法开始的，而这个方法processSelectedKeys()。主要的作用是什么呢？

//调用上面的处理方法
private void processSelectedKeys() { 
    //如果SelectionKey为空
    if (selectedKeys != null) { 
       processSelectedKeysOptimized(selectedKeys.flip());
    } else { 
       processSelectedKeysPlain(selector.selectedKeys());
    }
}
private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) { 
        for (int i = 0;; i ++) { 
            final SelectionKey k = selectedKeys[i];
            if (k == null) { 
                break;
            }
            // null out entry in the array to allow to have it GC'ed once the Channel close
            // See https://github.com/netty/netty/issues/2363
            selectedKeys[i] = null;
            final Object a = k.attachment();
            if (a instanceof AbstractNioChannel) { 
                processSelectedKey(k, (AbstractNioChannel) a);
            } else { 
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }
            if (needsToSelectAgain) { 
                // null out entries in the array to allow to have it GC'ed once the Channel close
                // See https://github.com/netty/netty/issues/2363
                for (;;) { 
                    i++;
                    if (selectedKeys[i] == null) { 
                        break;
                    }
                    selectedKeys[i] = null;
                }
                selectAgain();
                // Need to flip the optimized selectedKeys to get the right reference to the array
                // and reset the index to -1 which will then set to 0 on the for loop
                // to start over again.
                //
                // See https://github.com/netty/netty/issues/1523
                selectedKeys = this.selectedKeys.flip();
                i = -1;
            }
        }
    }
private void processSelectedKeysPlain(Set<SelectionKey> selectedKeys) { 
        // check if the set is empty and if so just return to not create garbage by
        // creating a new Iterator every time even if there is nothing to process.
        // See https://github.com/netty/netty/issues/597
        if (selectedKeys.isEmpty()) { 
            return;
        }
        Iterator<SelectionKey> i = selectedKeys.iterator();
        for (;;) { 
            final SelectionKey k = i.next();
            final Object a = k.attachment();
            i.remove();
            if (a instanceof AbstractNioChannel) { 
                processSelectedKey(k, (AbstractNioChannel) a);
            } else { 
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }
            if (!i.hasNext()) { 
                break;
            }
            if (needsToSelectAgain) { 
                selectAgain();
                selectedKeys = selector.selectedKeys();
                // Create the iterator again to avoid ConcurrentModificationException
                if (selectedKeys.isEmpty()) { 
                    break;
                } else { 
                    i = selectedKeys.iterator();
                }
            }
        }
    }

可以看到上面的processSelectedKeys()方法通过对于SelectionKey的判断实现了两种不同的逻辑。但是这两个逻辑最为核心的操作就是AbstractNioChannel类的存在，可以继续跟进源码会发现这个类其实就是一个Channel，在Java NIO中我们知道一个Channel就类似于普通网络编程中的一个Socket。所以说这里的AbstractNioChannel是对于Channel在Netty NIO实现的基础上做了进一步的封装。

Channel

在上面提到的概念就是Channel可以理解为一个Socket那么既然是用来做客户端和服务器端的数据传递工作，首先需要搞清楚的是Channel是什么时候创建？第二点就是Channel底层实现也是通过Socket来实现的又对Socket做了哪些优化呢？
在NioEventLoop类中有如下的一个方法

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) { 
        //获取到unsafe
        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) { 
            final EventLoop eventLoop;
            try { 
                eventLoop = ch.eventLoop();
            } catch (Throwable ignored) { 
                // If the channel implementation throws an exception because there is no event loop, we ignore this
                // because we are only trying to determine if ch is registered to this event loop and thus has authority
                // to close ch.
                return;
            }
            // Only close ch if ch is still registerd to this EventLoop. ch could have deregistered from the event loop
            // and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
            // still healthy and should not be closed.
            // See https://github.com/netty/netty/issues/5125
            if (eventLoop != this || eventLoop == null) { 
                return;
            }
            // close the channel if the key is not valid anymore
            unsafe.close(unsafe.voidPromise());
            return;
        }
        try { 
            int readyOps = k.readyOps();
            // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
            // the NIO JDK channel implementation may throw a NotYetConnectedException.
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) { 
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                // See https://github.com/netty/netty/issues/924
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);
                unsafe.finishConnect();
            }
            // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
            if ((readyOps & SelectionKey.OP_WRITE) != 0) { 
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }
            // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
            // to a spin loop
            //这里有一个对于OP_ACCEPT的判断。
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { 
                unsafe.read();
                if (!ch.isOpen()) { 
                    // Connection already closed - no need to handle write.
                    return;
                }
            }
        } catch (CancelledKeyException ignored) { 
            unsafe.close(unsafe.voidPromise());
        }
    }

在上面这个方法中有一块逻辑是指定注意的

if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { 
     unsafe.read();
     if (!ch.isOpen()) { 
          // Connection already closed - no need to handle write.
                    return;
     }
 }

在上面这段逻辑中，可以看到在SelectionKey判断的时候会有一个OP_ACCEPT事件，这里就会有一个unsafe.read的方法。可进入NioMessageUnsafe中查看read方法如下。而这个NioMessageUnsafe就是对于连接处理的一个类。

private final class NioMessageUnsafe extends AbstractNioUnsafe { 
        private final List<Object> readBuf = new ArrayList<Object>();
        @Override
        public void read() { 
            assert eventLoop().inEventLoop();
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();
            final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
            allocHandle.reset(config);
            boolean closed = false;
            Throwable exception = null;
            try { 
                try { 
                    do { 
                        int localRead = doReadMessages(readBuf);
                        if (localRead == 0) { 
                            break;
                        }
                        if (localRead < 0) { 
                            closed = true;
                            break;
                        }
                        allocHandle.incMessagesRead(localRead);
                    } while (allocHandle.continueReading());
                } catch (Throwable t) { 
                    exception = t;
                }
                int size = readBuf.size();
                for (int i = 0; i < size; i ++) { 
                    readPending = false;
                    pipeline.fireChannelRead(readBuf.get(i));
                }
                readBuf.clear();
                allocHandle.readComplete();
                pipeline.fireChannelReadComplete();
                if (exception != null) { 
                    closed = closeOnReadError(exception);
                    pipeline.fireExceptionCaught(exception);
                }
                if (closed) { 
                    inputShutdown = true;
                    if (isOpen()) { 
                        close(voidPromise());
                    }
                }
            } finally { 
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) { 
                    removeReadOp();
                }
            }
        }
    }

在上面逻辑中值得关注一个方法便是doReadMessages(readBuf)方法，查看这个方法的实现会发现，是由NioServerSocketChannel实现。到这里终于是接近Java NIO的底层了。

@Override
    protected int doReadMessages(List<Object> buf) throws Exception { 
        SocketChannel ch = javaChannel().accept();
        try { 
            if (ch != null) { 
                buf.add(new NioSocketChannel(this, ch));
                return 1;
            }
        } catch (Throwable t) { 
            logger.warn("Failed to create a new channel from an accepted socket.", t);
            try { 
                ch.close();
            } catch (Throwable t2) { 
                logger.warn("Failed to close a socket.", t2);
            }
        }
        return 0;
    }

关于Java底层NIO的实现,而这里所对应的SocketChannel就是对应的NIO模型中的SocketChannel。通过获取到对应的ServerSocketChannel，进行的accept操作。在获取到对象之后将其添加到一个Object的列表中。而这个列表理解为获取到了一个Channel的列表。Netty直接把一个SocketChannel封装成了一个NioSocketChannel。然后通过一个List进行返回，最后只需要通过这个NioSocketChannel进行数据的读写操作就可以了。

@Override
    protected ServerSocketChannel javaChannel() { 
        return (ServerSocketChannel) super.javaChannel();
    }

ByteBuf

在Java实现的NIO编程中有一个ByteBuffer，这个ByteBuffer是对IO流的封装，在Netty中还有就是对于ByteBuffer的封装ByteBuf类，在这个类中定义了很多的read和write方法，而这些方法就是对一普通IO中的输入输出流。如下图
在这里插入图片描述

Pipline

Pipeline，对应的普通IO编程中的对于数据的处理操作。那么Netty是什么时候将Pipeline加入到对应的连接处理过程中的呢？深究一下会发现在AbstractChannel的构造方法中调用了newChannelPipeline方法。而这个方法创建了一个默认的ChannelPipeline。也就是说我们也可以自己实现这个Pipeline。

protected AbstractChannel(Channel parent) { 
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }
protected DefaultChannelPipeline newChannelPipeline() { 
    return new DefaultChannelPipeline(this);
}

通过下面的操作，最终将逻辑处理操作加载到Channel中。

protected DefaultChannelPipeline(Channel channel) { 
        this.channel = ObjectUtil.checkNotNull(channel, "channel");
        succeededFuture = new SucceededChannelFuture(channel, null);
        voidPromise =  new VoidChannelPromise(channel, true);
        tail = new TailContext(this);
        head = new HeadContext(this);
        head.next = tail;
        tail.prev = head;
    }

ChannelHandler

对应业务逻辑处理块，在ChannelPipeline中有很多的add方法、remove方法其实这些方法就是实现了对于处理逻辑的动态的处理。这里使用的一个策略模式 使用者可以根据不同的处理逻辑实现不同的Handler，但是整个的处理逻辑是不变的就是实现对于数据的读写操作。这里看一个比较常用的方法addLast()。无论传入什么样的Handler都不会改变这个方法的实现逻辑。而这方法就是将对应的处理逻辑加入到整个处理的最后。

@Override
    public final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) { 
        if (handlers == null) { 
            throw new NullPointerException("handlers");
        }
        for (ChannelHandler h: handlers) { 
            if (h == null) { 
                break;
            }
            addLast(executor, null, h);
        }
        return this;
    }