Summary

Remove obsolete usage of the undocumented "impl.prefix" JDK system property from InetAddress.

Problem

The InetAddress class uses "impl.prefix" JDK internal system property to locate and install classes which implement "java.net.InetAddressImpl" package private interface.

This mechanism allows replacing built-in implementations used by InetAddress. The following built-in implementations are used by default:
java.net.Inet4AddressImpl and java.net.Inet6AddressImpl.

To replace the built-in implementations the custom class should be located in 'java.net.' package and its full name is constructed from "impl.prefix" value by following this rule:
"java.net." + value + implName
Where implName is either "Inet6AddressImpl" or "Inet4AddressImpl".

With strong encapsulation this logic is obsolete and can be removed.

Solution

• Remove "impl.prefix" property usage in InetAddress.
• Seal internal java.net.InetAddressImpl interface to permit only java.net.Inet4AddressImpl and java.net.Inet6AddressImpl implementations.
• A release note is planned for this change.

Specification

The webrev with changes is attached: JDK-8274227_webrev.zip
There is no specification changes associated with the proposed change, since all classes are package private in 'java.net' package.