Netty组件详解

太过爱你忘了你带给我的痛 2022-02-09 09:57 305阅读 0赞

> 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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25paHVpMTIz_size_16_color_FFFFFF_t_70]

### 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;
        }

## 总结 ##

1.  NioEventLoop组件
2.  Channel组件
3.  ByteBuf组件
4.  Pipeline组件
5.  ChannelHandler组件

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