This test creates a server socket and binds it to an IPv4 address.
Then it connects a Socket to that address & port and expects it to succeed.
Then it tries to connect a Socket using the machine's IPv6 address and expects it to fail, since the server is bound to the IPv4 only.
But nothing prevents something else to be listening by chance on that address, so connecting to IPv6 is not *guaranteed* to fail... We would need to fail only if the server we connected to was *our* server.

As a result this test sometimes fails with the following exception:

java.lang.RuntimeException: connect to IPv6 address should be refused
at TcpTest.test2(TcpTest.java:136)
at TcpTest.main(TcpTest.java:76)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:565)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:830)