I have been doing a lot of digging into Groovy and DSLs lately, getting ready for my OSCON talk next week.
The Groovy language has great built-in support for creating DSLs. Along with its dynamic nature, Groovy has the “use” keyword for working with categories, and coming in 1.1, the ExpandoMetaClass if you want to complete.
“Use”ing Categories
Categories help you do scoped “extension” to normal every day objects. An example is in order:
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“${i} tasty tacos”
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}
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}
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println 2.
tacos // outputs -> "2 tasty tacos"
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}
So, inside the use block, there is now (effectively) a new property on the Integer class, called tacos, that will return a string describing your delectable entree. This is a very powerful technique, and is very useful for developing your own expressive syntax.
One drawback to this technique is that you have to define where you are using it. Use can be extended into closures, but does not always extend into predefined methods.
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aClosure.
call() // outputs -> "3 tasty tacos"
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}
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aClosure.
call() // Exception No such property: tacos for class: java.lang.Integer
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// However, you can do this from Java (or Groovy):
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GroovyCategorySupport.
use(TacoCategory.
class, aClosure
) // outputs -> "3 tasty tacos"
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// calling methods on a class:
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4.tacos // has not been evaluated yet
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}
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}
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println new A
().
getTacos() // Exception No such property: tacos for class: java.lang.Integer
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// but you can do this:
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println new A
().
getTacos() // outputs -> "4 tasty tacos"
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}
Groovy + Annotations + Aspects = annotated “use” clause
I was talking about Groovy Categories with Blaine Buxton, and was telling him that I did not like having the “use” block inside my DSL code, and wanted a more transparent approach. He suggested that perhaps an Annotation and an Aspect would do what I was looking for.
With the 1.1-Beta1 Release, Groovy has built in support for Java Annotations, and since Groovy compiles down to byte code, there is implicit support for Aspects. Putting these two things together allows us to create an annotation for a more powerful use:
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@Uses
(category=
[MeasurementCategory.
class])
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(4.lbs.of(something)).toString()
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}
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}
With a simple annotation, @Uses(category=[list of category classes]), we now can define them for any method we want, without having to put them into the method body.
Here is the Annotation code:
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package com.nimblelogic.groovy.dsl;
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import java.lang.annotation.Inherited;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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@Inherited
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@Retention(RetentionPolicy.RUNTIME)
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public @interface Uses {
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public Class[] category();
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}
And here is the Aspect code:
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package com.nimblelogic.groovy.dsl;
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import groovy.lang.Closure;
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import java.util.ArrayList;
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import java.util.List;
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import org.aspectj.lang.JoinPoint;
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import org.aspectj.lang.annotation.SuppressAjWarnings;
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import org.aspectj.lang.reflect.MethodSignature;
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import org.codehaus.groovy.runtime.GroovyCategorySupport;
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public aspect UsesAspect {
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pointcut allMethods(): call(@Uses * *(..));
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@SuppressAjWarnings
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Object around
(): allMethods
() {
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List< class > classes = getCategories(thisJoinPoint);
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if ( classes.isEmpty() ) {
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result = proceed();
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} else {
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result =
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GroovyCategorySupport.use(classes,
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new Closure(thisJoinPoint.getTarget()) {
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@Override
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return proceed();
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}
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});
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}
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return result;
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}
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@SuppressWarnings(“unchecked”)
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private List< class > getCategories(JoinPoint thisJoinPoint) {
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List< class > classes = new ArrayList< class >();
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// get class level categories (if any)
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Class cl = thisJoinPoint.getTarget().getClass();
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addCategories((Uses) cl.getAnnotation(Uses.class), classes);
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// get method level categories (if any)
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if ( thisJoinPoint.getKind().equals(JoinPoint.METHOD_CALL)) {
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MethodSignature sig = (MethodSignature) thisJoinPoint.getSignature();
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addCategories((Uses) sig.getMethod().getAnnotation(Uses.class), classes);
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}
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return classes;
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}
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private void addCategories(Uses uses, List< class > classList) {
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if ( uses != null ) {
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Class[] classes = uses.category();
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for(Class cl : classes)
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classList.add(cl);
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}
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}
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}
One thing to note when working with Annotations / Aspects in Groovy: you have to keep track of the build order. I struggled for a while to figure out why I was not seeing the annotation in my Groovy classes. It was because it had not been built yet! I ended up having to split my compilation into 4 parts:
- Build the Annotation
- Compile the Groovy code
- Compile my other Java code (just test code)
- Weave the Aspects
