Thursday, November 15, 2012

Meta-Annotations


Meta-Annotations

Meta-Annotations are an experiment in annotation inheritance, abstraction and encapsulation with a Java SE mindset
A meta-annotation is any annotation class annotated with @Metatype. The other annotations used on the meta-annotation become part of its definition. If any of those annotations happen to also be meta-annotations, they are unrolled as well and their annotations become part of the definition.

@Metatype

The recursion that is the meta-annotation concept only happens when an annotation is marked as a @javax.annotation.Metatype.
When @Metatype is seen the basic contract is "carry the surrounding annotations forward". When a class, method or other target uses an annotation annotated with @Metatype the annotations on that annotation are "unrolled" or carried forward and effectively placed on that class, method or other target as if they were explicitly declared.
If any of the annotations that are carried forward also are annotated with @Metatype the recursion continues. The result is a simple algorithm or design pattern that provides inheritance or reuse in a way that is not specific to any domain, API, or specification.
APIs and specifications can choose to formally adopt annotation reuse in this fashion, but the core concept and implementations of@Metatype do not need to be expanded to support these APIs or specifications.
The simple elegance of this not being domain specific is that it could be used to combine several annotations from different specifications into one reusable annotation. Say JAX-RS @PathParam("id") with Bean Validation @NotNull to create a new annotation called @ValidId.

Creating Meta-Annotations

If the annotation in question can be applied to ElementType.ANNOTATION_TYPE or ElementType.TYPE, creating a meta-annotation version of it is quite easy.
@TransactionManagement(TransactionManagementType.CONTAINER)
@Metatype
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface ContainerManagedTransactions {
}
When the annotation in question cannot be applied to ElementType.ANNOTATION_TYPE or ElementType.TYPE, things get interesting. This is where meta-annotations depart from things like @Stereotype. The goal of meta-annotations is to be completely generic and not specific to any one domain or API. A such, you cannot really require all existing APIs change to allow for meta-annotations. The goal is that meta-annotations can be used generically and do not need to be "designed" into an API.
To allow annotations that apply to FIELDMETHODPARAMETERCONSTRUCTORLOCAL_VARIABLE, or PACKAGE, as well as any other location where annotations may be applied in the future a compromise is made.
import javax.ejb.Schedule;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Metatype
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)

public @interface Daily {
    public static class $ {

        @Daily
        @Schedule(second = "0", minute = "0", hour = "0", month = "*", dayOfWeek = "*", year = "*")
        public void method() {
        }
    }
}
An inner class named $. This is enough to bind together the @Daily and @Schedule in the context to which they both apply.
Ugly but effective. Alternate proposals welcome.
The above is considered the public API portion of the meta-annotation concept.
The concept itself is born out of standards based systems like EJB and CDI where annotation processing is invisible to the application itself. In those settings the above is enough and no additional APIs would be needed to support meta-annotations in standard APIs.

Under the covers

The "guts" of this particular implementation is designed to look and feel as much like the reflection API as possible. Obviously, with VM level control, you could do much better. A clean Java SE API might be just what is needed and its very possible that meta-annotations should really be a Java SE concept.
Here's a glimpse as to how things can look under the covers:
final java.lang.reflect.AnnotatedElement annotated = new org.metatype.MetaAnnotatedClass(Triangle.class);
assertNotNull(annotated);

assertTrue(annotated.isAnnotationPresent(Color.class));
assertTrue(annotated.getAnnotation(Color.class) != null);
assertTrue(!contains(Color.class, annotated.getDeclaredAnnotations()));
assertTrue(contains(Color.class, annotated.getAnnotations()));
assertEquals("red", annotated.getAnnotation(Color.class).value());

assertTrue(annotated.isAnnotationPresent(Red.class));
assertTrue(annotated.getAnnotation(Red.class) != null);
assertTrue(!contains(Red.class, annotated.getDeclaredAnnotations()));
assertTrue(contains(Red.class, annotated.getAnnotations()));

assertTrue(annotated.isAnnotationPresent(Crimson.class));
assertTrue(annotated.getAnnotation(Crimson.class) != null);
assertTrue(contains(Crimson.class, annotated.getDeclaredAnnotations()));
assertTrue(contains(Crimson.class, annotated.getAnnotations()));
The application classes would look like so:
@Crimson
// -> @Red -> @Color
public static class Triangle {

}

@Metatype
@Color("red")
// one level deep
@Target(value = {TYPE})
@Retention(value = RUNTIME)
public static @interface Red {
}

@Metatype
@Red
// two levels deep
@Target(value = {TYPE})
@Retention(value = RUNTIME)
public static @interface Crimson {
}

Best Practices
It is recommended to have an api package or some other package where "approved' annotations are defined and to prohibit usage of the non-meta versions of those annotations. All the real configuration will then be centralized in the api package and changes to the values of those annotations will be localized to that package and automatically be reflected throughout the application.
An interesting side-effect of this approach is that if the api package where the meta-annotation definitions exist is kept in a separate jar as well, then one can effectively change the configuration of an entire application by simply replacing the api jar.

Future concepts

XML Overriding

The unrolling of meta-annotations happens under the covers. In that same vein, so could the concept of overriding.
The above @Red annotation might theoretically be overridden via xml as follows:

<org.superbiz.api.Red>
  <org.superbiz.api.Color value="dark red"/>
</org.superbiz.api.Red>
Or take more complex meta-annotation definition like the following:
package org.superbiz.corn.meta.api;

import javax.ejb.Schedule;
import javax.ejb.Schedules;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Metatype
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)

public @interface PlantingTime {
    public static interface $ {

        @PlantingTime
        @Schedules({
                @Schedule(month = "5", dayOfMonth = "20-Last", minute = "0", hour = "8"),
                @Schedule(month = "6", dayOfMonth = "1-10", minute = "0", hour = "8")
        })
        public void method();
    }
}
This might theoretically be overridden as:

<org.superbiz.corn.meta.api.PlantingTime>
  <javax.ejb.Schedules>
    <value>
      <javax.ejb.Schedule month="5" dayOfMonth="15-Last" minute="30" hour="5"/>
      <javax.ejb.Schedule month="6" dayOfMonth="1-15" minute="30" hour="5"/>
    </value>
  </javax.ejb.Schedules>
</org.superbiz.corn.meta.api.PlantingTime>

Merging or Aggregating definitions
Certain annotations take lists and are designed to be multiples. In the current definition of meta-annotations, the following is illegal.
@RolesAllowed({"Administrator", "SuperUser"})
@Metatype
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Admins {
}

@RolesAllowed({"Employee", "User"})
@Metatype
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Users {
}


public static class MyBean {

    @Admin
    @User
    public void doSomething() {
        // ...
    }
}
Here the @Admin and @User annotation each resolve to @RolesAllowed. Since only one @RolesAllowed annotation is allowed on the method per the Java language specification, this results in an error.
The intention is clear however and aggregating metadata together in this way is natural.
A theoretical way to support something like this is with an annotation to describe that this aggregation is intended and desired. Note the addition of the theoretical @Merge annotation.
@RolesAllowed({"Administrator", "SuperUser"})
@Metatype
@Merge
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Admins {
}

@RolesAllowed({"Employee", "User"})
@Metatype
@Merge
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Users {
}


public static class MyBean {

    @Admin
    @User
    public void doSomething() {
        // ...
    }
}
A new @RolesAllowed annotation would be created containing the list {"Administrator", "SuperUser", "Employee", "User"} and that would represent the final @RolesAllowed usage for the doSomething() method.

209 comments:

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