一文搞懂Netty发送数据全流程 | 你想知道的细节全在这里（三）

final class HeadContext extends AbstractChannelHandlerContext
            implements ChannelOutboundHandler, ChannelInboundHandler {
          
        @Override
        public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
            unsafe.write(msg, promise);
        }
 }
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protected abstract class AbstractUnsafe implements Unsafe {
        //待发送数据缓冲队列  Netty是全异步框架，所以这里需要一个缓冲队列来缓存用户需要发送的数据 
        private volatile ChannelOutboundBuffer outboundBuffer = new ChannelOutboundBuffer(AbstractChannel.this);
  
        @Override
        public final void write(Object msg, ChannelPromise promise) {
            assertEventLoop();
            //获取当前channel对应的待发送数据缓冲队列（支持用户异步写入的核心关键）
            ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;

            ..........省略..................

            int size;
            try {
                //过滤message类型 这里只会接受DirectBuffer或者fileRegion类型的msg
                msg = filterOutboundMessage(msg);
                //计算当前msg的大小
                size = pipeline.estimatorHandle().size(msg);
                if (size < 0) {
                    size = 0;
                }
            } catch (Throwable t) {
              ..........省略..................
            }
            //将msg 加入到Netty中的待写入数据缓冲队列ChannelOutboundBuffer中
            outboundBuffer.addMessage(msg, size, promise);
        }
    
 }
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 @Override
    protected final Object filterOutboundMessage(Object msg) {
        if (msg instanceof ByteBuf) {
            ByteBuf buf = (ByteBuf) msg;
            if (buf.isDirect()) {
                return msg;
            }

            return newDirectBuffer(buf);
        }

        if (msg instanceof FileRegion) {
            return msg;
        }

        throw new UnsupportedOperationException(
                "unsupported message type: " + StringUtil.simpleClassName(msg) + EXPECTED_TYPES);
    }
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public class DefaultChannelPipeline implements ChannelPipeline {
    //原子更新estimatorHandle字段
    private static final AtomicReferenceFieldUpdater<DefaultChannelPipeline, MessageSizeEstimator.Handle> ESTIMATOR =
            AtomicReferenceFieldUpdater.newUpdater(
                    DefaultChannelPipeline.class, MessageSizeEstimator.Handle.class, "estimatorHandle");

    //计算要发送msg大小的handler
    private volatile MessageSizeEstimator.Handle estimatorHandle;

    final MessageSizeEstimator.Handle estimatorHandle() {
        MessageSizeEstimator.Handle handle = estimatorHandle;
        if (handle == null) {
            handle = channel.config().getMessageSizeEstimator().newHandle();
            if (!ESTIMATOR.compareAndSet(this, null, handle)) {
                handle = estimatorHandle;
            }
        }
        return handle;
    }
}
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public final class DefaultMessageSizeEstimator implements MessageSizeEstimator {

    private static final class HandleImpl implements Handle {
        private final int unknownSize;

        private HandleImpl(int unknownSize) {
            this.unknownSize = unknownSize;
        }

        @Override
        public int size(Object msg) {
            if (msg instanceof ByteBuf) {
                return ((ByteBuf) msg).readableBytes();
            }
            if (msg instanceof ByteBufHolder) {
                return ((ByteBufHolder) msg).content().readableBytes();
            }
            if (msg instanceof FileRegion) {
                return 0;
            }
            return unknownSize;
        }
    }
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