Netty ByteBuf釋放時機

ByteBuf釋放不當容易造成內存泄漏。

 

一般情況下，業務handler中使用到的ByteBuf可以分為兩類，請求ByteBuf和響應ByteBuf。如下：

public class MyServerHandler extends SimpleChannelInboundHandler<ByteBuf>{
    
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
        //服務器回送數據給客戶端, 回送一個隨機id ,
        ByteBuf responseByteBuf = Unpooled.copiedBuffer(UUID.randomUUID().toString() + " ", Charset.forName("utf-8"));//這里UnpooledHeapByteBuf
        ctx.writeAndFlush(responseByteBuf);
    }
}

msg即為請求ByteBuf，而responseByteBuf即為響應ByteBuf。

 

響應ByteBuf（如writeAndFlush(buf)）由netty釋放，例如在將HeapBuf轉換為DirectBuf的時候釋放。

    protected final ByteBuf newDirectBuffer(ByteBuf buf) {
        final int readableBytes = buf.readableBytes();
        if (readableBytes == 0) {
            ReferenceCountUtil.safeRelease(buf);
            return Unpooled.EMPTY_BUFFER;
        }

        final ByteBufAllocator alloc = alloc();
        if (alloc.isDirectBufferPooled()) {
            ByteBuf directBuf = alloc.directBuffer(readableBytes);
            directBuf.writeBytes(buf, buf.readerIndex(), readableBytes);
            ReferenceCountUtil.safeRelease(buf);//將作為HeapByteBuf的buf引用計數-1，更新狀態
            return directBuf;
        }

        final ByteBuf directBuf = ByteBufUtil.threadLocalDirectBuffer();
        if (directBuf != null) {
            directBuf.writeBytes(buf, buf.readerIndex(), readableBytes);
            ReferenceCountUtil.safeRelease(buf);
            return directBuf;
        }

        // Allocating and deallocating an unpooled direct buffer is very expensive; give up.
        return buf;
    }

請求ByteBuf，Handler繼承SimpleChannelInboundHandler可釋放；

    @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);//業務handler只需重寫channelRead0
            } else {
                release = false;
                ctx.fireChannelRead(msg);
            }
        } finally {
            if (autoRelease && release) {
                ReferenceCountUtil.release(msg);//引用計數-1
            }
        }
    }

請求ByteBuf，fireChannelRead()，由TailContext釋放。

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {onUnhandledInboundMessage(ctx, msg);}

    protected void onUnhandledInboundMessage(ChannelHandlerContext ctx, Object msg) {
        onUnhandledInboundMessage(msg);
        if (logger.isDebugEnabled()) {
            logger.debug("Discarded message pipeline : {}. Channel : {}.",
                         ctx.pipeline().names(), ctx.channel());
        }
    }

    protected void onUnhandledInboundMessage(Object msg) {
        try {
            logger.debug(
                    "Discarded inbound message {} that reached at the tail of the pipeline. " +
                            "Please check your pipeline configuration.", msg);
        } finally {
            ReferenceCountUtil.release(msg);//引用計數-1
        }
    }

對於請求ByteBuf，當業務handler沒有繼承SimpleChannelInboundHandler，沒有fireChannelRead，也沒有ReferenceCountUtil.release的話，這時就要考慮內存溢出的情況了。

 

所以應該時刻記着由最后一個使用的人釋放（ReferenceCountUtil.release(byteBuf)）。