Netty ByteBuf的回收_netty自动回收bytebuf-优快云博客

本文链接：https://blog.youkuaiyun.com/kakaweb/article/details/119675574

本文详细解析了ByteBuf在Netty中的三种回收策略：ChannelOutboundHandler尾部处理、ByteToMessageDecoder处理和手动管理。重点介绍了如何确保正确释放ByteBuf，以及推荐的处理方式如配合ByteToMessageDecoder和SimpleChannelInboundHandler。

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以下三种情况下对于ByteBuf的回收策略：

ChannelOutboundHandler中write的ByteBuf，在pipeline中的tail handler将负责释放，详见AbstractChannelHandlerContext中的如下函数


    private void write(Object msg, boolean flush, ChannelPromise promise) {
        ObjectUtil.checkNotNull(msg, "msg");
        try {
            if (isNotValidPromise(promise, true)) {
                ReferenceCountUtil.release(msg);
                // cancelled
                return;
            }
        } catch (RuntimeException e) {
            ReferenceCountUtil.release(msg);
            throw e;
        }

        final AbstractChannelHandlerContext next = findContextOutbound(flush ?
                (MASK_WRITE | MASK_FLUSH) : MASK_WRITE);
        final Object m = pipeline.touch(msg, next);
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            if (flush) {
                next.invokeWriteAndFlush(m, promise);
            } else {
                next.invokeWrite(m, promise);
            }
        } else {
            final WriteTask task = WriteTask.newInstance(next, m, promise, flush);
            if (!safeExecute(executor, task, promise, m, !flush)) {
                // We failed to submit the WriteTask. We need to cancel it so we decrement the pending bytes
                // and put it back in the Recycler for re-use later.
                //
                // See https://github.com/netty/netty/issues/8343.
                task.cancel();
            }
        }
    }

ChannelInboundHandlerAdapter中接收到的ByteBuf，由最后一个接收到ByteBuf的handler负责释放，例如ByteToMessageDecoder处理过后的ByteBuf，由于向下游传递的类型不为ByteBuf，在ByteToMessageDecoder中即进行释放

    final void decodeRemovalReentryProtection(ChannelHandlerContext ctx, ByteBuf in, List<Object> out)
            throws Exception {
        decodeState = STATE_CALLING_CHILD_DECODE;
        try {
            decode(ctx, in, out);
        } finally {
            boolean removePending = decodeState == STATE_HANDLER_REMOVED_PENDING;
            decodeState = STATE_INIT;
            if (removePending) {
                fireChannelRead(ctx, out, out.size());
                out.clear();
                handlerRemoved(ctx);
            }
        }
    }


    @Override
    public final void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        if (decodeState == STATE_CALLING_CHILD_DECODE) {
            decodeState = STATE_HANDLER_REMOVED_PENDING;
            return;
        }
        ByteBuf buf = cumulation;
        if (buf != null) {
            // Directly set this to null so we are sure we not access it in any other method here anymore.
            cumulation = null;
            numReads = 0;
            int readable = buf.readableBytes();
            if (readable > 0) {
                ctx.fireChannelRead(buf);
                ctx.fireChannelReadComplete();
            } else {
                buf.release();
            }
        }
        handlerRemoved0(ctx);
    }

SimpleChannelInboundHandler只对ByteBuf类型的数据进行回收

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        boolean release = true;
        try {
            if (acceptInboundMessage(msg)) {
                @SuppressWarnings("unchecked")
                I imsg = (I) msg;
                channelRead0(ctx, imsg);
            } else {
                release = false;
                ctx.fireChannelRead(msg);
            }
        } finally {
            if (autoRelease && release) {
                ReferenceCountUtil.release(msg);
            }
        }
    }


    public static boolean release(Object msg) {
        if (msg instanceof ReferenceCounted) {
            return ((ReferenceCounted) msg).release();
        }
        return false;
    }

由此做出如下总结：

向外写数据时使用ByteBuf，保证能够在最后一个ChannelOutboundHandler时被回收，因为ReferenceCountUtil.release(msg)只回收实现了ReferenceCounted的类
业务处理handler使用非ByteBuf类型作为输入时，需要保证之前的ChannelInboundHandler将回收接收到的ByteBuf
ByteToMessageDecoder会回收ByteBuf，将生成的消息向下游传递
如果自行申请了ByteBuf，需要在使用完成后手动回收

最佳实践