Annotation For Decorator Design Pattern

This project contains the validation and implementation of decorator pattern using annotation.

It is multi-module project where :

• annotation module defines the decorator annotation, which can be used to annotate classes to be used as decorators.

• processor module implements the annotation processor for the decorator. The processor

  • Validates the annotated classes/entities
  • Raises errors/warnings (based on the given config) upon violating the decorators usage
  • Generates classes corresponding to the annotated classes as decorators.

• usage module implements the decorator pattern for a simple use case of decorating a christmas tree with and without annotations. Demonstrating the equivalence of pattern usage with annotations.

The following is the definition of the annotation to decorate a christmas tree.

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.CLASS)

public @interface Decorator {

    /**
     * Name of the Decorator
     */
    String name();

    /**
     * Specifies the name of the decorator class to be applied
     */
    Class type();

    /**
     * The decorator to decorate with
     */
    String decorate();

    /**
     * The cost for decorating
     */

    float costOfDecorating();

    Class implementingType();


}

The following is sample usage:

@Decorator(
        name = ApplicationConstants.ELECTRIC_LIGHT_DECORATOR,
        type = IChristmasTree.class,
        decorate = " with ~~Electric Lights~~ ",
        costOfDecorating = 100,
        implementingType = TreeDecorator.class
)

Steps to run

• Clone this repo :

git clone https://Iam_MohammedSiddiq@bitbucket.org/Iam_MohammedSiddiq/mohammed_siddiq_474_hw2.git

mvn clean compile test

Will validate the usage of annotation and run the tests over annotated and non-annotated implementation

---

Validations

The decorator processor validates the usage of the annotation and raises errors on violating the use of the annotation

• have a non zero length name

• have an implementing type

• have the decorator description

• must be applied to a class

• cannot be applied to static classes

• usage class should have public modifier

• should not be implementing or extending any other class

Violated errors : uncomment the invalidUsage package under the usage module to have a invalid run.

[WARNING] Class DecoratorPattern.invalidUsage.AbstractClassDecoratoris Abstract, Class must be concrete
[WARNING] Class DecoratorPattern.invalidUsage.AbstractClassDecorator.tesUsageis not public, Class must be public

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project usage: Compilation failure
[ERROR] /Users/mohammedsiddiq/Downloads/CS474_HW2/decorator/usage/src/main/java/DecoratorPattern/invalidUsage/AbstractClassDecorator.java:[15,17] decorate() in @Decorator for class DecoratorPattern.invalidUsage.AbstractClassDecorator is empty! Invalid use of Decorator pattern
[ERROR] 

[WARNING] Class DecoratorPattern.invalidUsage.AbstractClassDecorator is extending DecoratorPattern.TreeDecorator

On successful validations the processor then generates corresponding java files.

sample constructed .java file for the above decorator usage

public class ElectricLight extends DecoratorPattern.TreeDecorator {
  public ElectricLight(DecoratorPattern.IChristmasTree ichristmastree) {
    super(ichristmastree);
  }

  public java.lang.String decorate() {
    return super.decorate() + decorateWithElectricLight();
  }

  public java.lang.String decorateWithElectricLight() {
    return " with ~~Electric Lights~~ ";
  }
}

The patterns annotated usage is verified with patterns non-annotated usage:

2019-11-03 15:36:03 INFO  TestAnnotatedPatternWithActual:53 - Non annotated decoration : Simple Christmas Tree! with ~~Electric Lights~~  with {Garlands}  with ~~Electric Lights~~  with {Garlands}  
2019-11-03 15:36:03 INFO  TestAnnotatedPatternWithActual:58 - Annotated decoration : Simple Christmas Tree! with ~~Electric Lights~~  with {Garlands}  with ~~Electric Lights~~  with {Garlands}