WARNING: sendResponseHeaders: being invoked with a content length for a HEAD request

[yakirgb]

Hi,
I'm using JAR: jmx_prometheus_httpserver-0.16.1-jar-with-dependencies.jar to monitor HDFS NameNodes
Every 60 seconds i'm getting this warning:

Aug 16 12:34:59 nn00103 java[22008]: Aug 16, 2021 12:34:59 PM sun.net.httpserver.ExchangeImpl sendResponseHeaders
Aug 16 12:34:59 nn00103 java[22008]: WARNING: sendResponseHeaders: being invoked with a content length for a HEAD request

The exporter is working without any problem, only printing warning.
Thank you,Yakir

[dhoard]

Created PR ( #688 ) to resolve the issue. @fstab please review.

[fstab]

Sorry for the delay. Could you let me know which Java version you are using or what you did to see this warning? I cannot reproduce this.

To me it looks like the warning is wrong, according to to the HTTP/1.1 Spec there must be a Content-length header.

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

[dhoard]

I'm running Azul's OpenJDK version 11...

openjdk version "11.0.12" 2021-07-20 LTS
OpenJDK Runtime Environment Zulu11.50+19-CA (build 11.0.12+7-LTS)
OpenJDK 64-Bit Server VM Zulu11.50+19-CA (build 11.0.12+7-LTS, mixed `mode)`

-

The warning is in the OpenJDK 11 though OpenJDK 13 codebase, so can't explain why you're not seeing it...

https://github.com/openjdk/jdk11u/blob/63fc945c1d4cdc893d89796804acfe0ce9341860/src/jdk.httpserver/share/classes/sun/net/httpserver/ExchangeImpl.java#L238

https://github.com/openjdk/jdk17/blob/a37254c79fa5973465d90f4b52ab88fe68016c9f/src/jdk.httpserver/share/classes/sun/net/httpserver/ExchangeImpl.java#L238

-

The change was made based Java bug #6886723 and the proposed workaround to remove the warning...

https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6886723

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I did some further debugging and determined the correct way to resolve this warning is to set the Content-Length explicitly and then use -1 in the sendResponseHeaders() method call if it's a HEAD request. This keeps the correct Content-Length header vallue in the response and removes the warning.

long contentLength = response.size();
t.getResponseHeaders().set("Content-Length", String.valueOf(contentLength));
if (t.getRequestMethod().equals("HEAD")) {
    contentLength = -1;
 }
t.sendResponseHeaders(HttpURLConnection.HTTP_OK, contentLength);
response.writeTo(t.getResponseBody());

The change is trivial, but requires a refactor of the TestHTTPServerJUnit test class to capture both the content-length and response body for testing.

I will update the PR with these changes for you to review.

[fstab]

Thanks for the explanation, and thanks for your fix. It's merged now. The warning will be gone with the next release of client_java.