RFR [9] 8075139: Restore java.protocol.handler.pkgs to work as a fallback for migration

[Chris Hegarty]

The work done as part of JDK-8064924 [1] to introduce a new service type, java.net.spi.URLStreamHandlerProvider, provides a clean provider mechanism that will work well with modules. That part of the change should remain, but the removal of support to find handlers through the java.protocol.handler.pkgs system property is problematic, and could be an impediment to moving to JDK 9. Applications making use of custom protocol handlers and not packaged as modules should continue to be able to use this property.

The proposal is to reinstate the JDK 8 specification about java.protocol.handler.pkgs in java.net.URL.<init>, that documents the search order for handlers. Additionally reinstate the JDK 8 implementation code to search the system property after the service loader lookup.

http://cr.openjdk.java.net/~chegar/8075139/webrev.00/

The spec and code changes in the above webrev are almost identical to the original code pre 8064924.

-Chris.

[1] https://bugs.openjdk.java.net/browse/JDK-8064924