10:52 p.m.
I've been working with the Bill and Linda, the Java EE spec leads, as well as with
Jason, Stuart and Emmanuel at Red Hat, to come up with a proposal for global enablement
for CDI in Java EE 7. Based on Linda and Bill's poll of the community, this appears to
be much more popular than we had previously thought, so we have decided to propose it,
despite it being quite late into the spec development. I think what we have come up with
represents a good approach to achieve it, and also addresses a long requested feature
request, that CDI should not consider every class a bean. Please let us know your
thoughts. If you aren't on the Java EE EG, or the observers list, or the same for CDI,
then please say, and I'll forward your thoughts.
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Background
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Globally enabling CI would allow other specifications in the Java EE platform to rely on
CDI, and not have to provide an abstraction over DI services, such as those introduced by
JSF managed beans, and considered by JAX-RS, Batch and others.
Challenges
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* The startup time of Weld is O(n) with the number of beans. We assert that it is not
possible for a CDI impl to attain an O(1) startup time, therefore globally enabling CDI
will increase the startup time of Java EE servers. For Red Hat, low startup time is a top
priority.
* Compatibility with other users of JSR-330. By considering all deployments as CDI
deployments without any enablement marker, we will likely cause deployments which used
another JSR-330 implementation to fail. This would have worked on Java EE 6. Other JSR-330
impls that fall into this camp are Spring and Guice.
* Backwards compatibility. Some users may have intentionally not placed a beans.xml into a
deployment, but used CDI annotations, and enabled the beans some other way, eg. via a
portable extension.
* A large proportion of Java EE users 800/1000 indicate they want this feature, thus
meaning we should try to address it
Proposed solution
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We introduce the concept of a "bean defining annotation" and define that any
class in any deployment (including those with no beans.xml) with a bean defining
annotation is discovered and may be a CDI bean, and can participate fully in the
application. Any archive with a beans.xml continues to work in the same way, such that all
classes in the archive are discovered and may be CDI beans.
This addresses the startup time problem. Whilst a scan of classes is still required, the
impact on startup time is negligible:
* A Java EE server must scan all classes to discover other component defining annotations
such as EJBs, Servlet's, JAX-RS resources etc.
* This scan can be done at the bytecode level, with no need to classload the class, which
our research shows is the costly part of CDI startup
Any scope (normal scope or pseudo-scope) applied to a bean at source level is a bean
defining annotation (so you must add @Dependent to your class in order to get it to be
picked up as a dependent bean).
Only classes with a bean defining annotation, or with an annotation, or meta-annotation,
present specified by @WithAnnotations are passed to ProcessAnnotatedType observers (the
exact semantics are defined by CDI 1.1 PRD for @WithAnnotations). As mentioned above, if a
ProcessAnnotatedType is observed for a type without a bean defining annotation, as a
result of having an annotation present that is specified by @WithAnnotations, it may
instruct the container to add discover the class as a bean.
Every archive in a deployment would be considered a bean archive, simply with differing
contents depending on the presence of beans.xml
If a developer adds a beans.xml to their archive, behavior is as CDI 1.0. We will add an
attribute to beans.xml "auto-discover=true", which the user may set to false in
order to add a beans.xml and only have classes with a bean defining annotation discovered,
which allows beans.xml to be used as a deployment descriptor but still limit the classes
discovered.
OPEN ISSUE: Should auto-discover be false by default for beans.xml with version 1.1. This
would mean that adding a beans.xml would have no impact on discovery for 1.1 apps, however
it is a significant change from 1.0.
OPEN ISSUE: Should only scopes for which a CDI context exists be considered component
defining? This could introduce some thorny edge cases, but would address the JSR-330
compatibility issue better.
OPEN ISSUE: Should we extend auto-discover in beans.xml to allow complete disablement of
scanning e.g. auto-discover="all|bean-defining-annotations-only|none" ?
OPEN ISSUE: How should the ProcessAnnotatedType event instruct the container to discover a
class as a bean? Perhaps something like event.discover(clazz)?
OPEN ISSUE: Should we integrate this with the package level scanning control we have
proposed for CDI 1.1?
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