The log reveals that the server has exited with an assertion error:

                    assert method.equalsIgnoreCase("POST")
                            || method.equalsIgnoreCase("GET");

Most probably this indicates that something else than our test is connecting to our server. The server should be made reliable and just close the accepted connection and go back to accept in that case.

----------System.err:(16/961)----------
java.lang.AssertionError
at ServerCloseTest$DummyServer.run(ServerCloseTest.java:305)
java.lang.Exception: failures: 1
at com.sun.javatest.regtest.agent.TestNGRunner.main(TestNGRunner.java:96)
at com.sun.javatest.regtest.agent.TestNGRunner.main(TestNGRunner.java:54)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:833)