A DESCRIPTION OF THE PROBLEM :
HttpURLConnection sends invalid HTTP requests under certain circumstances. (it sends "User-Agent:" instead of "GET" for the HTTP method).

When HttpURLConnection initiates a POST on a reused connection but finds the socket closed, it will internally recover and open a new connection. If this POST succeeds and the server sends a redirect status (302) it will attempt to follow the redirect. However, internally it fails to correctly initialise the request headers: after creating a new set of headers (https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/net/www/protocol/http/HttpURLConnection.java#L2871), it will not set the HTTP method (https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/net/www/protocol/http/HttpURLConnection.java#L630-L634) because failedOnce is true at this point from the first POST on the closed socket.

Logging output from such a client server interaction:

Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection plainConnect0
FINEST: ProxySelector Request for http://localhost:8080/magnoliaAuthor/.magnolia/admincentral
Nov 03, 2021 8:37:01 PM sun.net.www.http.HttpClient logFinest
FINEST: KeepAlive stream retrieved from the cache, sun.net.www.http.HttpClient(http://localhost:8080/magnoliaAuthor/.magnolia/admincentral;jsessionid=44DEE0C8C535782A6B4932E9F0D455ED)
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection plainConnect0
FINEST: Proxy used: DIRECT
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection writeRequests
FINE: www.MessageHeader@15b170d89'>sun.net.www.MessageHeader@15b170d89 pairs: {POST /magnoliaAuthor/.magnolia/admincentral HTTP/1.1: null}{Content-Type: application/x-www-form-urlencoded}{Cookie: csrf=ZOxgq4P_WMgLjCm3J4mWPEFFNhmIUiHlq9of5JhtKys}{charset: utf-8}{User-Agent: Java/1.8.0_181}{Host: localhost:8080}{Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2}{Connection: keep-alive}{Content-Length: 92}
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection writeRequests
FINE: www.MessageHeader@15b170d89'>sun.net.www.MessageHeader@15b170d89 pairs: {POST /magnoliaAuthor/.magnolia/admincentral HTTP/1.1: null}{Content-Type: application/x-www-form-urlencoded}{Cookie: csrf=ZOxgq4P_WMgLjCm3J4mWPEFFNhmIUiHlq9of5JhtKys}{charset: utf-8}{User-Agent: Java/1.8.0_181}{Host: localhost:8080}{Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2}{Connection: keep-alive}{Content-Length: 92}
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection getInputStream0
FINE: www.MessageHeader@52ae5ad55'>sun.net.www.MessageHeader@52ae5ad55 pairs: {null: HTTP/1.1 302}{Set-Cookie: JSESSIONID=88DCCC46C9E70CBA8262CF8060A033C0; Path=/magnoliaAuthor; HttpOnly}{Location: /magnoliaAuthor/.magnolia/admincentral;jsessionid=88DCCC46C9E70CBA8262CF8060A033C0}{Content-Length: 0}{Date: Wed, 03 Nov 2021 19:37:01 GMT}
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection followRedirect0
FINE: Redirected from http://localhost:8080/magnoliaAuthor/.magnolia/admincentral to http://localhost:8080/magnoliaAuthor/.magnolia/admincentral;jsessionid=88DCCC46C9E70CBA8262CF8060A033C0
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection plainConnect0
FINEST: ProxySelector Request for http://localhost:8080/magnoliaAuthor/.magnolia/admincentral;jsessionid=88DCCC46C9E70CBA8262CF8060A033C0
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection plainConnect0
FINEST: Proxy used: DIRECT
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection writeRequests
FINE: www.MessageHeader@774fca3c4'>sun.net.www.MessageHeader@774fca3c4 pairs: {User-Agent: Java/1.8.0_181}{Host: localhost:8080}{Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2}{Connection: close}
Nov 03, 2021 8:37:01 PM sun.net.www.protocol.http.HttpURLConnection getInputStream0
FINE: www.MessageHeader@57256a0d6'>sun.net.www.MessageHeader@57256a0d6 pairs: {null: HTTP/1.1 400}{Content-Type: text/html;charset=utf-8}{Content-Language: en}{Content-Length: 2244}{Date: Wed, 03 Nov 2021 19:37:01 GMT}{Connection: close}



FREQUENCY : always