Handling Annotations
====================

There are several different approaches to handling annotations in PHP. Doctrine Annotations
maps docblock annotations to PHP classes. Because not all docblock annotations are used
for metadata purposes a filter is applied to ignore or skip classes that are not Doctrine annotations.

Take a look at the following code snippet:

.. code-block:: php

    namespace MyProject\Entities;

    use Doctrine\ORM\Mapping AS ORM;
    use Symfony\Component\Validation\Constraints AS Assert;

    /**
     * @author Benjamin Eberlei
     * @ORM\Entity
     * @MyProject\Annotations\Foobarable
     */
    class User
    {
        /**
         * @ORM\Id @ORM\Column @ORM\GeneratedValue
         * @dummy
         * @var int
         */
        private $id;

        /**
         * @ORM\Column(type="string")
         * @Assert\NotEmpty
         * @Assert\Email
         * @var string
         */
        private $email;
    }

In this snippet you can see a variety of different docblock annotations:

- Documentation annotations such as ``@var`` and ``@author``. These annotations are on a blacklist and never considered for throwing an exception due to wrongly used annotations.
- Annotations imported through use statements. The statement ``use Doctrine\ORM\Mapping AS ORM`` makes all classes under that namespace available as ``@ORM\ClassName``. Same goes for the import of ``@Assert``.
- The ``@dummy`` annotation. It is not a documentation annotation and not blacklisted. For Doctrine Annotations it is not entirely clear how to handle this annotation. Depending on the configuration an exception (unknown annotation) will be thrown when parsing this annotation.
- The fully qualified annotation ``@MyProject\Annotations\Foobarable``. This is transformed directly into the given class name.

How are these annotations loaded? From looking at the code you could guess that the ORM Mapping, Assert Validation and the fully qualified annotation can just be loaded using
the defined PHP autoloaders. This is not the case however: For error handling reasons every check for class existence inside the AnnotationReader sets the second parameter $autoload
of ``class_exists($name, $autoload)`` to false. To work flawlessly the AnnotationReader requires silent autoloaders which many autoloaders are not. Silent autoloading is NOT
part of the `PSR-0 specification <https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md>`_ for autoloading.

This is why Doctrine Annotations uses its own autoloading mechanism through a global registry. If you are wondering about the annotation registry being global,
there is no other way to solve the architectural problems of autoloading annotation classes in a straightforward fashion. Additionally if you think about PHP
autoloading then you recognize it is a global as well.

To anticipate the configuration section, making the above PHP class work with Doctrine Annotations requires this setup:

.. code-block:: php

    use Doctrine\Common\Annotations\AnnotationReader;
    use Doctrine\Common\Annotations\AnnotationRegistry;

    AnnotationRegistry::registerFile("/path/to/doctrine/lib/Doctrine/ORM/Mapping/Driver/DoctrineAnnotations.php");
    AnnotationRegistry::registerAutoloadNamespace("Symfony\Component\Validator\Constraint", "/path/to/symfony/src");
    AnnotationRegistry::registerAutoloadNamespace("MyProject\Annotations", "/path/to/myproject/src");

    $reader = new AnnotationReader();
    AnnotationReader::addGlobalIgnoredName('dummy');

The second block with the annotation registry calls registers all the three different annotation namespaces that are used.
Doctrine saves all its annotations in a single file, that is why ``AnnotationRegistry#registerFile`` is used in contrast to
``AnnotationRegistry#registerAutoloadNamespace`` which creates a PSR-0 compatible loading mechanism for class to file names.

In the third block, we create the actual AnnotationReader instance. Note that we also add "dummy" to the global list of annotations
for which we do not throw exceptions. Setting this is necessary in our example case, otherwise ``@dummy`` would trigger an exception to
be thrown during the parsing of the docblock of ``MyProject\Entities\User#id``.

Setup and Configuration
-----------------------

To use the annotations library is simple, you just need to create a new ``AnnotationReader`` instance:

.. code-block:: php

    $reader = new \Doctrine\Common\Annotations\AnnotationReader();

This creates a simple annotation reader with no caching other than in memory (in php arrays).
Since parsing docblocks can be expensive you should cache this process by using
a caching reader.

You can use a file caching reader:

.. code-block:: php

    use Doctrine\Common\Annotations\FileCacheReader;
    use Doctrine\Common\Annotations\AnnotationReader;

    $reader = new FileCacheReader(
        new AnnotationReader(),
        "/path/to/cache",
        $debug = true
    );

If you set the debug flag to true the cache reader will check for changes in the original files, which
is very important during development. If you don't set it to true you have to delete the directory to clear the cache.
This gives faster performance, however should only be used in production, because of its inconvenience
during development.

You can also use one of the ``Doctrine\Common\Cache\Cache`` cache implementations to cache the annotations:

.. code-block:: php

    use Doctrine\Common\Annotations\AnnotationReader;
    use Doctrine\Common\Annotations\CachedReader;
    use Doctrine\Common\Cache\ApcCache;

    $reader = new CachedReader(
        new AnnotationReader(),
        new ApcCache(),
        $debug = true
    );

The debug flag is used here as well to invalidate the cache files when the PHP class with annotations changed
and should be used during development.

.. warning ::

    The AnnotationReader works and caches under the
    assumption that all annotations of a doc-block are processed at
    once. That means that annotation classes that do not exist and
    aren't loaded and cannot be autoloaded (using the AnnotationRegistry) would never be visible and not
    accessible if a cache is used unless the cache is cleared and the
    annotations requested again, this time with all annotations
    defined.

By default the annotation reader returns a list of annotations with numeric indexes. If you want your annotations
to be indexed by their class name you can wrap the reader in an IndexedReader:

.. code-block:: php

    use Doctrine\Common\Annotations\AnnotationReader;
    use Doctrine\Common\Annotations\IndexedReader;

    $reader = new IndexedReader(new AnnotationReader());

.. warning::

    You should never wrap the indexed reader inside a cached reader only the other way around. This way you can re-use
    the cache with indexed or numeric keys, otherwise your code may experience failures due to caching in an numerical
    or indexed format.

Registering Annotations
~~~~~~~~~~~~~~~~~~~~~~~

As explained in the Introduction Doctrine Annotations uses its own autoloading mechanism to determine if a
given annotation has a corresponding PHP class that can be autoloaded. For Annotation Autoloading you have
to configure the ``Doctrine\Common\Annotations\AnnotationRegistry``. There are three different mechanisms
to configure annotation autoloading:

-   Calling ``AnnotationRegistry#registerFile($file)`` to register a file that contains one or more Annotation classes.
-   Calling ``AnnotationRegistry#registerNamespace($namespace, $dirs = null)`` to register that the given namespace
    contains annotations and that their base directory is located at the given $dirs or in the include path if NULL is passed.
    The given directories should *NOT* be the directory where classes of the namespace are in, but the base directory
    of the root namespace. The AnnotationRegistry uses a namespace to directory separator approach to resolve the correct path.
-   Calling ``AnnotationRegistry#registerLoader($callable)`` to register an autoloader callback. The callback accepts the
    class as first and only parameter and has to return true if the corresponding file was found and included.

.. note::

    Loaders have to fail silently, if a class is not found even if it matches for example the namespace prefix of that loader.
    Never is a loader to throw a warning or exception if the loading failed otherwise parsing doc block annotations will become
    a huge pain.

A sample loader callback could look like:

.. code-block:: php

    use Doctrine\Common\Annotations\AnnotationRegistry;
    use Symfony\Component\ClassLoader\UniversalClassLoader;

    AnnotationRegistry::registerLoader(function($class) {
        $file = str_replace("\\", DIRECTORY_SEPARATOR, $class) . ".php";

        if (file_exists("/my/base/path/" . $file)) {
            // file exists makes sure that the loader fails silently
            require "/my/base/path/" . $file;
        }
    });

    $loader = new UniversalClassLoader();
    AnnotationRegistry::registerLoader(array($loader, "loadClass"));


Ignoring missing exceptions
~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default an exception is thrown from the AnnotationReader if an annotation was found that:

- Is not part of the blacklist of ignored "documentation annotations".
- Was not imported through a use statement
- Is not a fully qualified class that exists

You can disable this behavior for specific names if your docblocks do not follow strict requirements:

.. code-block:: php

    $reader = new \Doctrine\Common\Annotations\AnnotationReader();
    AnnotationReader::addGlobalIgnoredName('foo');

PHP Imports
~~~~~~~~~~~

By default the annotation reader parses the use-statement of a php file to gain access to the import rules
and register them for the annotation processing. Only if you are using PHP Imports you can validate the correct
usage of annotations and throw exceptions if you misspelled an annotation. This mechanism is enabled by default.

To ease the upgrade path, we still allow you to disable this mechanism. Note however that we will remove this
in future versions:

.. code-block:: php

    $reader = new \Doctrine\Common\Annotations\AnnotationReader();
    $reader->setEnabledPhpImports(false);