dubbo的attachments機制用于在業(yè)務請求之外傳遞附加信息，在很多場景下都有應用戒职。本文嘗試從源碼角度解析rest協(xié)議下栗恩，attachments如何傳遞。

dubbo的attachments機制簡介

dubbo中有attachments機制洪燥，在傳遞業(yè)務參數(shù)之外還可以傳遞附加信息磕秤，可以理解為像http一樣，在報文傳送主要信息外蚓曼，還有報文頭可傳遞附加信息亲澡。
dubbo中attachments示例為钦扭，在consumer端放入：

RpcContext.getContext().setAttachment("test-attachment-key", "some-msg");

在provider端取出：

Map<String, String> attachments = RpcContext.getContext().getAttachments();

rest下如何傳遞

dubbo-protocol-rest可以讓dubbo使用rest方式提供和使用服務纫版，同樣也支持attachments機制，鍵值對位于http報文頭客情，日志形如：

DEBUG org.apache.http.headers - http-outgoing-0 >> GET /restService/sayHello?name=gxk HTTP/1.1
DEBUG org.apache.http.headers - http-outgoing-0 >> Accept: application/json;charset=UTF-8
DEBUG org.apache.http.headers - http-outgoing-0 >> Dubbo-Attachments: test-attachment-key=some-msg
DEBUG org.apache.http.headers - http-outgoing-0 >> Host: 124.126.21.38:8080
DEBUG org.apache.http.headers - http-outgoing-0 >> Connection: Keep-Alive
DEBUG org.apache.http.headers - http-outgoing-0 >> User-Agent: Apache-HttpClient/4.5.2 (Java/11.0.2)
DEBUG org.apache.http.headers - http-outgoing-0 >> Accept-Encoding: gzip,deflate

注意其中第3行即為使用http報文頭方式傳遞attachments

Dubbo-Attachments: test-attachment-key=some-msg

具體到代碼實現(xiàn)其弊，見com.alibaba.dubbo.rpc.protocol.rest.RpcContextFilter癞己，源碼片段如下：

public class RpcContextFilter implements ContainerRequestFilter, ClientRequestFilter {

    private static final String DUBBO_ATTACHMENT_HEADER = "Dubbo-Attachments";

    // 略

    @Override
    public void filter(ContainerRequestContext requestContext) throws IOException {
        // 略
        String headers = requestContext.getHeaderString(DUBBO_ATTACHMENT_HEADER);
        if (headers != null) {
            for (String header : headers.split(",")) {
                int index = header.indexOf("=");
                if (index > 0) {
                    String key = header.substring(0, index);
                    String value = header.substring(index + 1);
                    if (!StringUtils.isEmpty(key)) {
                        RpcContext.getContext().setAttachment(key.trim(), value.trim());
                    }
                }
            }
        }
    }

    @Override
    public void filter(ClientRequestContext requestContext) throws IOException {
        int size = 0;
        for (Map.Entry<String, String> entry : RpcContext.getContext().getAttachments().entrySet()) {
            // 略
            String attachments = entry.getKey() + "=" + entry.getValue();
            requestContext.getHeaders().add(DUBBO_ATTACHMENT_HEADER, attachments);
        }
    }
}

兩個函數(shù)依次為作為服務端和客戶端如何處理attachments，可見rest方式下梭伐，attachments主要和RpcContext打交道：

1. 作為服務端痹雅，從http報文頭中取出Dubbo-Attachments的內(nèi)容，將形如a=b,c=d的內(nèi)容放入RpcContext
2. 作為客戶端糊识，從RpcContext中取出所有鍵值對绩社，放入http報文頭的Dubbo-Attachments

另外，在com.alibaba.dubbo.rpc.filter.ContextFilter中赂苗，可見將Invocation中的attachments放入了RpcContext愉耙，代碼片段如下：

public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
    Map<String, String> attachments = invocation.getAttachments();
    // 略
    // mreged from dubbox
    // we may already added some attachments into RpcContext before this filter (e.g. in rest protocol)
    if (attachments != null) {
        if (RpcContext.getContext().getAttachments() != null) {
            RpcContext.getContext().getAttachments().putAll(attachments);
        } else {
            RpcContext.getContext().setAttachments(attachments);
        }
    }
    // 略
}

上述兩個組件，RpcContextFilter作用于http請求拌滋，回調(diào)時機早于ContextFilter等dubbo的filter朴沿。