Session handling changes

Maven Resolver 2.x introduced “onSessionEnd” hooks, that became required for some of the new features (like HTTP/2 transports are). While existing “Resolver 1.x” way of handling session will still work, it may produce resource leaks. Client code managing Resolver (like Maven) is strongly advised to upgrade session handling. Client code using Resolver (like Maven Mojos) do not have to change anything, they should be able to continue to function in very same way as before (as with Resolver 1.x).

What changed on surface:

• introduction of RepositorySystemSession nested interfaces CloseableRepositorySystemSession and SessionBuilder.
• introduction of RepositorySystem new method createSessionBuilder that creates SessionBuilder instances.
• deprecation of DefaultRepositorySystemSession default constructor, this constructor is actually the “Resolver 1.x way” of using sessions.

Required changes in client code managing Resolver 2.x:

• do not use DefaultRepositorySystemSession default constructor anymore.
• instead, use RepositorySystem#createSessionBuilder to create SessionBuilder and out of it CloseableRepositorySystemSession instances.
• handle sessions as resources: each created instance should be closed once finished their use.
• session instances created by given RepositorySystem should be used only with that same instance.
• to shallow-copy session instances (for alteration purposes) using existing DefaultRepositorySystemSession copy constructor is acceptable (this is what Mojos do).
• to shallow-copy session instances (for alteration purposes) there is CloseableRepositorySystemSession#copy method as well, if closeable session is needed.
• to shallow-copy session instances but have new lifecycle as well, use SessionBuilder#withRepositorySystemSession on newly created builder instances.