如何安全地释放 netty 缓冲区？

Question

我正在使用 Netty 4.0，我已经阅读了描述正常行为的 Reference counted objects guide，但是当发生异常时我不知道如何释放 ByteBuf。

例如：

protected void encode(ChannelHandlerContext ctx, Object msg, List<Object> out) throws Exception {
    ByteBuf buf = null;
    if (msg instanceof HttpMessage) {
        if (state != ST_INIT) {
            throw new IllegalStateException("unexpected message type: " + StringUtil.simpleClassName(msg));
        }

        @SuppressWarnings({ "unchecked", "CastConflictsWithInstanceof" })
        H m = (H) msg;

        buf = ctx.alloc().buffer();
        // Encode the message.
        encodeInitialLine(buf, m);//
        encodeHeaders(m.headers(), buf);
        buf.writeBytes(CRLF);
        state = HttpHeaders.isTransferEncodingChunked(m) ? ST_CONTENT_CHUNK : ST_CONTENT_NON_CHUNK;
    }

    // Bypass the encoder in case of an empty buffer, so that the following idiom works:
    //
    //     ch.write(Unpooled.EMPTY_BUFFER).addListener(ChannelFutureListener.CLOSE);
    //
    // See https://github.com/netty/netty/issues/2983 for more information.

    if (msg instanceof ByteBuf && !((ByteBuf) msg).isReadable()) {
        out.add(EMPTY_BUFFER);
        return;
    }

    if (msg instanceof HttpContent || msg instanceof ByteBuf || msg instanceof FileRegion) {

        if (state == ST_INIT) {
            throw new IllegalStateException("unexpected message type: " + StringUtil.simpleClassName(msg));
        }

        final long contentLength = contentLength(msg);
        if (state == ST_CONTENT_NON_CHUNK) {
            if (contentLength > 0) {
                if (buf != null && buf.writableBytes() >= contentLength && msg instanceof HttpContent) {
                    // merge into other buffer for performance reasons
                    buf.writeBytes(((HttpContent) msg).content());
                    out.add(buf);
                } else {
                    if (buf != null) {
                        out.add(buf);
                    }
                    out.add(encodeAndRetain(msg));
                }
            } else {
                if (buf != null) {
                    out.add(buf);
                } else {
                    // Need to produce some output otherwise an
                    // IllegalStateException will be thrown
                    out.add(EMPTY_BUFFER);
                }
            }

            if (msg instanceof LastHttpContent) {
                state = ST_INIT;
            }
        } else if (state == ST_CONTENT_CHUNK) {
            if (buf != null) {
                out.add(buf);
            }
            encodeChunkedContent(ctx, msg, contentLength, out);
        } else {
            throw new Error();
        }
    } else {
        if (buf != null) {
            out.add(buf);
        }
    }
}

当encodeInitialLine(buf, m);抛出异常时，如何释放buf？

Answer 1

您应该将整个方法包装在一个 try-finally 块中，并且在 finally 块中，您应该检查 buf 是否为 null。如果它不为空，请调用 buf.release().

protected void encode(ChannelHandlerContext ctx, Object msg, List<Object> out) throws Exception {
    ByteBuf buf = null;
    try {
        ...
    } finally {
        if (buf != null) {
            buf.release();
        }
    }
}

如何安全地释放 netty 缓冲区？

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