RFR: 8320577: Improve MessageHeader's toString() function to make HttpURLConnection's debug log readable

[Daniel Jeliński]

On Fri, 10 Nov 2023 14:23:19 GMT, Oliver Lockwood <duke at openjdk.org> wrote:

> At the moment, if you have debug logging turned on for the `HttpURLConnection` class, then from [line 746](https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/net/www/protocol/http/HttpURLConnection.java#L746) a log message like this is generated:
>> sun.net.www.MessageHeader at 49c4f2906 pairs: {null: HTTP/1.1 200}{Content-Type: application/json}{Transfer-Encoding: chunked}{Date: Thu, 09 Nov 2023 11:50:00 GMT}{Keep-Alive: timeout=60}{Connection: keep-alive}
> 
> The lack of separation after the `Object.toString()` output makes it hard to realise from the above that it's trying to say "6 pairs". 
> The change proposed by this PR would make the same log operation output this, instead:
>> sun.net.www.MessageHeader at 49c4f290 6 pairs: {null: HTTP/1.1 200}{Content-Type: application/json}{Transfer-Encoding: chunked}{Date: Thu, 09 Nov 2023 11:50:00 GMT}{Keep-Alive: timeout=60}{Connection: keep-alive}

Hi @oliverlockwood all PRs need to be associated to a Java Bug System ticket; I created one for you. Please change the PR title to `8320577` to mark this PR as ready for review.

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PR Comment: https://git.openjdk.org/jdk/pull/16615#issuecomment-1822340610