# Upgrading Resolver (major versions)
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This page will collect quick guides for developers upgrading from one to
another major version of Resolver.

# Upgrading from 1.x to 2.x

Maven Resolver upcoming major version 2.x should be "smooth sailing", as long you
do not depend (directly or indirectly) on **deprecated** classes from Resolver
1.x line. Always use latest 1.x release to check for deprecated classes.

## 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 `CloseableSession` 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 `CloseableSession` 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 but have new lifecycle as well, use `SessionBuilder#withRepositorySystemSession` on newly created builder instances.
