Table of Contents

Annnotation driven development

AspectWerkz fully supports Java 5 annotations to define aspects or simply match on, and provides a Java 1.3/1.4 annotation API and custom compiler that allows to use the same feature with a very close user experience. Strongly typed Java 1.3/1.4 annotations facilities is thus provided.

1. Annotations
2. Matching on Annotations
3. Typed Annotations
4. Untyped Annotations
5. Compiling Annotations
6. Ant task for compiling Annotations
7. Runtime retrieval of Annotations
8. Introducing Annotations

Annotations

Java 5 Annotations are standardized thru the JSR-175. When you compile annotated source code the javac compiler will embed the annotations in the compiled class, and this information can then be read back using the reflection API.

Annotations in Java 5 are first class citizen thru the @interface keyword.

// A Java 5 Annotation
public @interface Asynchronous {
    int timeout() default 0;
    String label();
}

// A Java 5 annotated method
@Asynchronous(timeout=5, label="will run for a while")
public Object someMethod() {
    ...
}
                

AspectWerkz Java 1.3/1.4 Annotations provide the same user experience. Annotations are defined using a regular Java interface (hence providing strongly typed access to annotation values), and annotated source code appears in the form of doclet in the JavaDoc part.

The AspectWerkz AnnotationC compiler (available thru command line or as an Ant task) allows you to post-compile your classes to embed the annotation information. This extra compilation step is made useless when using Java 5 Annotations.

// A Java 1.3/1.4 Annotation with AspectWerkz
public interface Asynchronous {
    int timeout();
    String label();
}

// A Java 1.3/1.4 annotated method
// the resulting class will have to be post compiled 
// with AspectWerkz "AnnotationC -src *.java. -target compiled/"
/**
 * @Asynchronous(timeout=5, label="will run for a while")
 */
public Object someMethod() {
    ...
}
                

The AspectWerkz org.codehaus.aspectwerkz.Annotations API allows to retrieve reflectively both Java 5 annotations and Java 1.3/1.4 annotations in a consistent way, allowing you to adopt annotation driven development even without Java 5, while having a simple migration path.

Matching on Annotations

AspectWerkz supports matching on annotations. This means that you can define your pointcuts to pick out join points that are annotated with a certain annotation.

The matching will work equally good with JavaDoc-style annotations or a Java 1.5 annotations.

For example if you have annotated a method using the @Asynchronous annotation:

@Asynchronous(timeout=60)
public Object someMethod() {
    ...
}
                

You can then pick out this method (and all other methods that are annotated with the @Asynchronous annotation like this:

call(@Asynchronous * *..*.*(..))
                

execution(@Asynchronous * *..*.*(..))
                

Read more about this in the Join point selection pattern language section

Currently it is not possible to match on annotation element values.

Typed Annotations

Java 5 annotations are typed. To provide the same user experience in Java 1.3/1.4 and reduce the migration cost, AspectWerkz is using the concept of Annotation interfaces.

This concept makes it possible to achieve strong typing for JavaDoc-style annotations. Errors are reported already in the compilation phase. Which has many benefits compared to a the weakly typed, string based solution.

An annotation interface for Java 1.3/1.4 is a regular interface with one no-argument method per element in the annotation, whose returned value is of the wished type. The concept of defaults value that exists in Java 5 is not supported, and the default value will thus be null (or 0 / false for primitives).

For example if you have the JavaDoc annotation:

// Java 1.3/1.4 Annotations
/**
 * @Asynchronous(useThreadPool=true, timeout=60)
 * @Verbosity(level=2, prepend="LOG")
 * @VerbosityOther(level=2, prepend="LOG\"")
 */
 public Object someMethod() {
     ...
 }
                

This can be written like this using Java 1.5 annotations (mainly remove the JavaDoc comments and you are done)

@Asynchronous(useThreadPool=true, timeout=60)
@Verbosity(level=2, prepend="LOG")
@VerbosityOther(level=2 prepend="LOG\"")
public Object someMethod() {
    ...
}
                

When using Java 5, you have already written (or are using) the @Asynchronous annotation @interface component. For Java 1.3/1.4, you need to write (adapt this component). Here is an example with Java 5

// do not use a SOURCE retention policy if you plan to match on this annotation
// refer to Java 5 documentation about retention policy
public @interface Asynchronous {

    // use the exact annotation element name for the mehod name
    public boolean useThreadPool();

    public int timeout();
}
                

// retention policy will always be equivalent to Java 5 runtime retention policy
public interface Asynchronous {

    // use the exact annotation element name for the mehod name
    public boolean useThreadPool();

    public int timeout();
}                

The key points in this example are:

• The annotation interface for Java 1.3/1.4 is just an interface. For Java 5 it is a regular @interface.



• There is one method per annotation element whose name match exactly the element name as to be used in the annotated source code.



• Java 1.3/1.4 annotation will appear in the javadoc part of annotated elements. The retention will be runtime, accessible thru AspectWerkz Annotations API, and annotated sources will have to be post-compiled using AspectWerkz AnnotationC

    // A Java 5 Annotation with an anonymous element
    public @interface Asynchronous {
        int value() default 0;
        String label() default "";
    }

    // A Java 5 annotated method using the anonymous element
    // Note: we could write @Asynchronous(value=5) but this is useless
    // except when using label as in @Asynchronous(label="some text", value=5)
    @Asynchronous(5)
    public Object someMethod() {
        ...
    }
                    

    // A Java 1.3/1.4 Annotation with an anonymous element
    public interface Asynchronous {
        int value();
        String label();
    }

    // A Java 1.3/1.4 annotated method using the anonymous element
    // Note: we could write @Asynchronous(value=5) but this is useless
    // except when using label as in @Asynchronous(label="some text", value=5)
    /**
     * @Asynchronous(5)
     */
    public Object someMethod() {
        ...
    }
                    

    /**
     * @Asynchronous 5
     */
    public Object someMethod() {
        ...
    }
                    

Untyped Annotations

For those who wants it AspectWerkz also supports old style, untyped JavaDoc annotations. In such a case you don't have to write an annotation interface since a standard one is already provided.

It treats everything after the annotation declaration as one single value of type String. Which means that if you write an annotation like this:

/**
 * @SampleUntypedAnnotation this (is
 *                    one single
 *                    value
 */
                

All untyped annotations will be wrapped in an instance of org.codehaus.aspectwerkz.annotation.UntypedAnnotation which has to be used when retrieving the annotations at runtime. For example:

UntypedAnnotation annotation = ... // see next sections
String value = annotation.value(); //"this (is one single value" - note that cariage returns are lost
String name = annotation.name(); // "SampleUntypedAnnotation"
                 

The untyped annotations still needs to be compiled, since they need to be put into the bytecode of the annotated classes.

Such an annotation can also be written this way (as an anonymous one whose value() element is of type String)

/**
 * @SampleUntypedAnnotation("this (is
 *                    one single
 *                    value
 * ")
 */
                

Compiling Annotations

If you are using custom JavaDoc-style annotations then you have to compile in into bytecode of the classes. This is done with the AnnotationC compiler.

Please note that this is not needed for Java 1.5 annotations.

You can run AnnotationC from the command line. (It might be useful to run the ASPECTWERKZ_HOME/bin/setEnv.{bat|sh} script first.)

You invoke the compiler like this:

java [options...] org.codehaus.aspectwerkz.annotation.AnnotationC
    [-verbose]
    -src <path to src dir>
    -classes <path to classes dir>
    [-dest <path to destination dir>]
    [-custom <property file(s) for custom annotations>]
                

The last option -custom property_file(s)_for_custom_annotations points to the (or several files separated by classpath separator - ; or : depending on you OS) property file which defines the annotations by mapping the names to the fully qualified names of the annotation interface.

Note that if you are using the -dest option, the anonymous inner classes will not be copied to the destination directory, since the anonymous classes are not taken into account by the Annotation compiler. In such a case it is recommended to add the following (if using Ant) just after the call to AnnotationC when the -dest option is used: (adapt according to the directories you are using)

<copy todir="classes/annotated" overwrite="false"> <fileset dir="classes/regular"/> </copy>

# Typed annotations
Requires      = test.expression.RequiresAnnotation
Serializable  = test.expression.SerializableAnnotation

# Untyped annotations
loggable
readonly
                    

Ant task for compiling Annotations

An Ant task is provided to compile the annotations.

<!-- we assume we defined a classpath with the id="aw.class.path" for AspectWerkz jars -->
<path id="aw.class.path">
    ...
    <pathelement path="pathToAspectWerkz.jar"/>
    ...
</path>

<!-- define the custom task (annotationc can be changed to what you prefer)
<taskdef name="annotationc" 
         classname="org.codehaus.aspectwerkz.annotation.AnnotationCTask" 
         classpathref="aw.class.path"/>
<!-- Note: the <taskdef> element can be nested within a <target> element at your convenience -->

<!-- invoke the annotationc defined task -->
<target name="samples:task:annotationc" depends="init, compile:all">
    <annotationc
        verbose="true"
        destdir="${basedir}/target/samples-classes"
        properties="${basedir}/src/samples/annotation.properties"
        copytodest="**/*.dtd">

        <src path="${basedir}/src/samples"/>
        <src path="${basedir}/src/test"/>
        <classpath path="${basedir}/target/samples-classes"/>
        <classpath path="${basedir}/target/test-classes"/>
        <classpath path="${basedir}/target/classes"/>
        <fileset dir="other">
            <include name="**/BAZ.java"/>
        </fileset>
    </annotationc>
</target>
                 

Runtime retrieval of Annotations

You can retrieve the annotations at runtime using the Annotations class.

Here are some examples. The name in these examples is the annotation name for JavaDoc-style annotations and the fully qualified name of the annotation interface for Java 1.5 annotations.

All these methods return an instance of the type org.codehaus.aspectwerkz.annotation.Annotation. The instance needs to be casted to the correct annotation interface. If there are more than one it returns the first one found. This method is useful when working with Java 1.5 annotations in which there can be only one instance per member or class.

Annotation annotation = Annotations.getAnnotation("Session", klass);
Annotation annotation = Annotations.getAnnotation("Transaction", method);
Annotation annotation = Annotations.getAnnotation("ReadOnly", field);
                

All these methods return a list with all Annotation instances with the specific name. For Java 1.5 annotations this list will always be of size 0-1 while JavaDoc-style annotations can be declared multiple times per member/class.

List annotations = Annotations.getAnnotations("Session", klass);
List annotations = Annotations.getAnnotations("Transaction", method);
List annotations = Annotations.getAnnotations("ReadOnly", field);
                

These methods return a list with org.codehaus.aspectwerkz.annotation.AnnotationInfo instances which contains the:

• name of the annotation
• annotation instance (implements the annotation interface)

List annotationInfos = Annotations.getAnnotationInfos(klass);
List annotationInfos = Annotations.getAnnotationInfos(method);
List annotationInfos = Annotations.getAnnotationInfos(field);
                

Introducing Annotations

TODO - not yet implemented (targetted for 2.0 final)