netty 是事件驱动的，这里面有两个含义，一是 netty 接收到 socket 数据后，会产生事件，事件在 pipeline 上传播，二是事件由特定的线程池处理。
NioEventLoop 轮询网络事件
// io.netty.channel.nio.NioEventLoop#processSelectedKey private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) { 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 registered 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) { // 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); // 建立连接，深层会调用 fireChannelActive 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 if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { // 读数据，在流水线上传播读事件和连接关闭事件 unsafe.read(); } } catch (CancelledKeyException ignored) { unsafe.close(unsafe.voidPromise()); } } // io.netty.channel.nio.AbstractNioByteChannel.NioByteUnsafe#read public final void read() { final ChannelConfig config = config(); if (shouldBreakReadReady(config)) { clearReadPending(); return; } final ChannelPipeline pipeline = pipeline(); final ByteBufAllocator allocator = config.getAllocator(); final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle(); allocHandle.reset(config); ByteBuf byteBuf = null; boolean close = false; try { do { byteBuf = allocHandle.allocate(allocator); allocHandle.lastBytesRead(doReadBytes(byteBuf)); if (allocHandle.lastBytesRead() <= 0) { // nothing was read. release the buffer. byteBuf.release(); byteBuf = null; close = allocHandle.lastBytesRead() < 0; if (close) { // There is nothing left to read as we received an EOF. readPending = false; } break; } allocHandle.incMessagesRead(1); readPending = false; // 触发 ChannelRead pipeline.fireChannelRead(byteBuf); byteBuf = null; } while (allocHandle.continueReading()); allocHandle.readComplete(); // 触发 ChannelReadComplete pipeline.fireChannelReadComplete(); if (close) { // 触发 ChannelInactive 和 ChannelUnregister closeOnRead(pipeline); } } catch (Throwable t) { handleReadException(pipeline, byteBuf, t, close, allocHandle); } 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(); } } }
HandlerContext 中有一个整数 executionMask，不同的 bit 位表示不同的事件，为 1 表示可以处理该事件。
// io.netty.channel.AbstractChannelHandlerContext private final int executionMask; final class ChannelHandlerMask { // Using to mask which methods must be called for a ChannelHandler. static final int MASK_EXCEPTION_CAUGHT = 1; static final int MASK_CHANNEL_REGISTERED = 1 << 1; static final int MASK_CHANNEL_UNREGISTERED = 1 << 2; static final int MASK_CHANNEL_ACTIVE = 1 << 3; static final int MASK_CHANNEL_INACTIVE = 1 << 4; static final int MASK_CHANNEL_READ = 1 << 5; static final int MASK_CHANNEL_READ_COMPLETE = 1 << 6; static final int MASK_USER_EVENT_TRIGGERED = 1 << 7; static final int MASK_CHANNEL_WRITABILITY_CHANGED = 1 << 8; static final int MASK_BIND = 1 << 9; static final int MASK_CONNECT = 1 << 10; static final int MASK_DISCONNECT = 1 << 11; static final int MASK_CLOSE = 1 << 12; static final int MASK_DEREGISTER = 1 << 13; static final int MASK_READ = 1 << 14; static final int MASK_WRITE = 1 << 15; static final int MASK_FLUSH = 1 << 16; private static final int MASK_ALL_INBOUND = MASK_EXCEPTION_CAUGHT | MASK_CHANNEL_REGISTERED | MASK_CHANNEL_UNREGISTERED | MASK_CHANNEL_ACTIVE | MASK_CHANNEL_INACTIVE | MASK_CHANNEL_READ | MASK_CHANNEL_READ_COMPLETE | MASK_USER_EVENT_TRIGGERED | MASK_CHANNEL_WRITABILITY_CHANGED; private static final int MASK_ALL_OUTBOUND = MASK_EXCEPTION_CAUGHT | MASK_BIND | MASK_CONNECT | MASK_DISCONNECT | MASK_CLOSE | MASK_DEREGISTER | MASK_READ | MASK_WRITE | MASK_FLUSH; }
以 ChannelActive 为例，通过比较 bit 位上的值，判断该 HandlerContext 是否处理 ChannelActive 事件
// io.netty.channel.AbstractChannelHandlerContext#fireChannelActive public ChannelHandlerContext fireChannelActive() { invokeChannelActive(findContextInbound(MASK_CHANNEL_ACTIVE)); return this; } // io.netty.channel.AbstractChannelHandlerContext#findContextInbound private AbstractChannelHandlerContext findContextInbound(int mask) { AbstractChannelHandlerContext ctx = this; do { ctx = ctx.next; } while ((ctx.executionMask & mask) == 0); return ctx; }
如何使用 UserEvent？
首先让自己的 handler 实现 userEventTriggered 方法
class MyInboundHandler extends SimpleChannelInboundHandler<Object> { @Override public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception { // 处理事件，简单打印 System.out.println(evt); // 从当前 HandlerContext 向后传播 evt，如果没有这行代码，则不会向后传播事件了 super.userEventTriggered(ctx, evt); } @Override protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception { super.channelRead0(ctx, msg); } }
通过 pipeline 传播事件，从 HeadContext 向后传播事件
channel.pipeline().fireUserEventTriggered("i am an event");
read 事件，是从 HeadContext 开始向后传播
write 操作，是从 TailContext 开始向前传播
