Netty：EventLoop、Pipeline、ByteBuf。一条连接一辈子归一个线程，字节沿一串处理器流过，缓冲区用完要自己还

EventLoopGroup boss    = new MultiThreadIoEventLoopGroup(1, NioIoHandler.newFactory());
EventLoopGroup workers = new MultiThreadIoEventLoopGroup(NioIoHandler.newFactory()); // 默认 2 × 核
new ServerBootstrap()
    .group(boss, workers)
    .channel(NioServerSocketChannel.class)
    .childHandler(new ChannelInitializer<SocketChannel>() {   // 每条新连接调一次
        protected void initChannel(SocketChannel ch) {
            ch.pipeline()
              .addLast(new LengthFieldBasedFrameDecoder(1024, 0, 4, 0, 4)) // 入站：拆帧
              .addLast(new LengthFieldPrepender(4))                       // 出站：补长度
              .addLast(new ReserveHandler());                             // 入站：业务
        }
    })
    .bind(9090).sync();
// 处理器：继承 SimpleChannelInboundHandler<ByteBuf>，实现 channelRead0，读完自动 release
// 写响应：ctx.writeAndFlush(ctx.alloc().buffer().writeByte(1).writeInt(left));

<dependency>
  <groupId>io.netty</groupId>
  <artifactId>netty-codec-base</artifactId>   <!-- 带上 transport、buffer、common；长度帧编解码器在这里 -->
  <version>4.2.19.Final</version>
</dependency>

// 服务端 Pipeline 见 ④-5：frame → prepender → reserve。worker 线程 2 个。
static class ReserveHandler extends SimpleChannelInboundHandler<ByteBuf> {
    public void channelActive(ChannelHandlerContext ctx) {
        log("reserve", "新连接 " + ctx.channel().remoteAddress() + " 绑定到这个线程");
    }
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf frame) {  // frame 已去掉长度头
        log("reserve", "readerIndex=" + frame.readerIndex() + " writerIndex=" + frame.writerIndex());
        long sku = frame.readLong();
        int count = frame.readInt();
        boolean[] hit = {false};                     // 多条连接可能在不同 worker 上，扣减要原子
        int left = STOCK.compute(sku, (k, v) -> {
            int cur = v == null ? 0 : v;
            if (cur >= count) { hit[0] = true; return cur - count; }
            return cur;
        });
        log("reserve", "预占 sku=" + sku + " x" + count + " → " + (hit[0] ? "成功" : "库存不够"));
        ctx.writeAndFlush(ctx.alloc().buffer(5).writeByte(hit[0] ? 1 : 0).writeInt(left));
    }                                                // 返回后父类 release(frame)
}

// 客户端（订单服务）：库存 5 件，连发 3 个“预占 2 件”，塞进同一个 ByteBuf 一次写出
ByteBuf out = ch.alloc().buffer();
for (int count : new int[]{2, 2, 2}) out.writeInt(12).writeLong(88L).writeInt(count);
ch.writeAndFlush(out);    // main 线程调用 → 变成任务交给这条连接的 EventLoop

cd study-labs/java
mvn -q -pl network/netty-eventloop-pipeline-bytebuf compile exec:java \
  -Dexec.mainClass=labs.network.netty_eventloop_pipeline_bytebuf.InventoryNetty