It looks like on macOS the test manages to pick up the default system proxy configuration configured on the test host - and that returns a proxy for the IPv6 loopback address.

un 04, 2025 4:49:52 PM sun.net.www.protocol.http.HttpURLConnection plainConnect0
FINEST: ProxySelector Request for http://[0:0:0:0:0:0:0:1]:64804/
Jun 04, 2025 4:49:52 PM sun.net.www.protocol.http.HttpURLConnection plainConnect0
FINEST: Proxy used: HTTP @ xxxx-xxxx-xxx.xxx.xxx:80

(proxy address redacted)
 
The test should probably set a proxy selector that always returns NO_PROXY to avoid these kinds of intermittent failures.

This does not appear to be a problem for recent versions of the JDK - but I have seen this failure with JDK 11 and JDK 17.


test HttpURLConnectionExpectContinueTest.testChunkedRequestAndNoExpect100ContinueResponse(): failure
java.io.IOException: Server returned HTTP response code: 403 for URL: http://[0:0:0:0:0:0:0:1]:64804
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
at java.base/sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:2019)
at java.base/sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:2014)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:551)
at java.base/sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:2013)
at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1575)
at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1559)
at HttpURLConnectionExpectContinueTest.testChunkedRequestAndNoExpect100ContinueResponse(HttpURLConnectionExpectContinueTest.java:277)
       ...
Caused by: java.io.IOException: Server returned HTTP response code: 403 for URL: http://[0:0:0:0:0:0:0:1]:64804
at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1963)
at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1559)
at java.base/java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:527)
at HttpURLConnectionExpectContinueTest.testChunkedRequestAndNoExpect100ContinueResponse(HttpURLConnectionExpectContinueTest.java:276)
... 33 more