PHP class autoloading changed to PSR-4 instead of PSR-0: files move from lib/Drupal/amodule/ to just src/ within modules

The class loader in Drupal 8 implemented support for the PSR-0 standard previously.

The class loader has been changed to use Package-Oriented Autoloading, the PSR-4 standard, by default. All core modules have been migrated to PSR-4. See the Documentation page for PSR-4 in Drupal 8

This change does not affect your namespaces and use statements in code; it affects file locations only.

Before (PSR-0)

/modules/my_module/my_module.info.yml
/modules/my_module/lib/Drupal/my_module/MyModuleClass.php
/modules/my_module/lib/Drupal/my_module/Entity/MyEntity.php              # Plugin
/modules/my_module/lib/Drupal/my_module/Tests/MyEntityTest.php           # Simpletest
/modules/my_module/tests/Drupal/my_module/Tests/Entity/MyEntityTest.php  # PHPUnit

After (PSR-4)

/modules/my_module/my_module.info.yml
/modules/my_module/src/MyModuleClass.php
/modules/my_module/src/Entity/MyEntity.php            # Plugin
/modules/my_module/src/Tests/MyEntityTest.php         # Simpletest
/modules/my_module/tests/src/Entity/MyEntityTest.php  # PHPUnit

The effective changes:

1. Autoloaded classes are moved directly into the ./src subdirectory:

   - ./lib/Drupal/my_module/MyModuleClass.php
   + ./src/MyModuleClass.php
   

   This applies to all classes in your namespace. Classes in sub-namespaces are retained in subdirectories as-is. In short:

   $ mkdir src
   $ git mv lib/Drupal/my_module/* src/
   

2. PHPUnit tests are moved directly into the ./tests/src subdirectory:

   - ./tests/Drupal/Tests/my_module/MyModuleTest.php
   + ./tests/src/MyModuleTest.php
   

   This applies to all classes in the Tests namespace. Classes in sub-namespaces are retained in subdirectories as-is. In short:

   $ mkdir -p tests/src
   $ git mv tests/Drupal/my_module/Tests/* tests/src/
   

\Drupal\Core and \Drupal\Component

Files located in core/lib - i.e. classes in the \Drupal\Core and \Drupal\Component namespace - are not moved. They remain in the same place after this change.

Temporary transition period

For a certain period, both PSR-4 and PSR-0 autoloading will be supported in parallel, so as to facilitate conversions in Drupal core and contributed modules. The length of this period is being discussed at #2247287: Drop automatic PSR-0 support for modules. To ensure forward compatibility, it is recommend that modules are converted to the PSR-4 file layout at the earliest opportunity.

Note that PSR-0 theoretically supported underscores (_) as namespace separators, such that Foo_Bar_Baz.php could contain a Baz class in the Foo\Bar namespace. With the switch to PSR-4 this is no longer supported. Because module classes in lib directories - i.e. those that are still in the PSR-0 location - are internally registered using the PSR-4 mechanism as well this applies to those classes as well. Because such file names violate Drupal coding standards this should generally not affect anyone.