Summary
-------

Change the Java programming language to allow multiple application
of annotations with the same type to a single program element.


Goals
-----

Improve the readability of source code which logically applies
multiple instances of the same annotation type to a given program
element.


Motivation
----------

Frequently-used idioms of programming with annotations in EE and
elsewhere awkwardly use a container annotation just to simulate the
ability to apply multiple annotations.  Building in support for
repeated annotations will improve the readability of source code.


Description
-----------

The basic approach to implement the language feature is to desugar
repeated annotations of a base type into a single container
annotation; the container annotation has a `values` method which
returns an array of the base annotation type.  For repeated
annotations to be enabled for a particular annotation type, the
declaration of the base annotation type will need to include a new
meta-annotation, say `@ContainerAnnotation`, to declare which other
annotation type should be used as a container.  Warnings and errors
should be issued if the container is not suitability compatible with
the base annotation, including problematic differences in retention
policy or target.

Open design issues include whether or not multiple levels of
compiler-generated containers will be supported.  For example, should

    @A(1)
    @A(2)
    @AContainer
    @AContainerContainer
    foo();

be treated as logically equivalent to

    @AContainerContainer(@AContainer({@A(1), @A2}), @AContainer)
    foo();

or a compilation error after one level of nesting to

    @AContainer({@A(1), @A(2)})
    @AContainer
    @AContainerContainer
    foo();

At a libraries level, the implementations of the reflective APIs in
the platform, including core reflection and `javax.lang.model`, will
need to be updated to handle the repeated annotation information.  For
example, the `AnnotatedElement.getAnnotation(BaseAnnotation.class)`
method will be redefined to look for in a container annotation if the
base annotation is not directly present.  One or more methods to query
for the presence of absence of annotations may be added to the
`AnnotatedElement` interface.  If multiple levels of compiler-generated
nesting is supported, the library changes will be more extensive.


Testing
-------

As with all language changes, the corresponding compiler JCK tests
will need to be updated.


Risks and Assumptions
---------------------

One risk is the possibility of currently unforeseen interactions
between this language feature and existing library semantics or
between this language feature other language features present in Java
SE 8.  In particular, the interaction, if any, between repeated
annotations and annotations on types will need to be defined.

An assumption is that existing EE annotation types which use the
manual container annotation pattern will migrate to use the repeating
annotations pattern and thereby validate the feature through usage.

If the various Java IDEs do not support this language change during
development, experimentation with the feature and validation of its
design will be slowed.


Dependences
-----------

The interaction between repeating annotations and
[JEP 104: Annotations on Java Types](JDK-8046094), if any, needs to be defined.


Impact
------

  - Other JDK components: Direct use of repeated annotations in the
    JDK code base is expected to be minimal.

  - Compatibility: Interfaces not usually expected to have
    implementations outside of the JDK may have methods added to them.
    Defender methods from Project Lambda can help limit the source
    compatibility impact.

  - Performance/scalability: There should be no degradation in
    compilation speed if the feature is not used.