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Make Our Code More Testable with Proxy Design Pattern


If you heard about the term separation of concerns, you could agree this concept is very important for making a system "clean". One of the most important characteristics of a clean system is testable.

Let me tell you my story about how I've just come acrosss the design pattern Proxy.

Note: to get started, you can find a very simple example of the pattern Proxy here. 

Let's start!

I have an interface as below:
public interface DocumentGenerator {
    File generate(Document document) throws BusinessException;
}

The following is my first implementation for a concrete class of DocumentGenerator.
public class DocumentGeneratorImpl implements DocumentGenerator {
 private Dossier dossier;
 private Locale locale;
 
 public DocumentGeneratorImpl(Dossier dossier, Locale locale) {
  validateNotNullParams(dossier, locale);
  this.dossier = dossier;
  this.locale = locale;
 }
 
 private void validateNotNullParams(LibertyDossier dossier, Locale locale) {
  if(Objects.isNull(dossier)) {
   throw new IllegalArgumentException("Dossier must not be null!");
  }
  
  if(Objects.isNull(dossier.getDossierType())){
   throw new IllegalArgumentException("Dossier type must not be null!");
  }
  
  if(Objects.isNull(locale)) {
   throw new IllegalArgumentException("Locale must be defined!");
  }
 }
 
 @Override
 public File generate(Document document, boolean temporary) 
  throws BusinessException {
  setCobIdForDossierIfNotExist();
  switch (dossier.getDossierType()) {
  case TYPE_A:
   // Do some logic here in case TYPE_A
  case TYPE_BA:
   // Do some logic here in case TYPE_B
  default:
   throw new BusinessException("Not supported dossier type");
  }
 }
 
 private void setCobIdForDossierIfNotExist() {
  if(StringUtils.isEmpty(dossier.getCobId())) {
   String generatedCobId = DossierService.getInstance().generateCobId();
   dossier.setCobId(generatedCobId);
  }
 }
}

The client code constructs the concrete class DocumentGeneratorImpl.
DocumentGenerator  generator = new DocumentGeneratorImpl(dossier, locale);

I saw that some private methods "validateNotNullParams(dossier, locale)" and "setCobIdForDossierIfNotExist()" are just a second responsibility of class DocumentGeneratorImpl.

Follow "S" of SOLID  - Single Responsibility

Firstly, I was thinking about how to separate these methods into another class. Then, I create a class called DocumentGeneratorHelper which contains to these methods. It was just improved a bit.

public class DocumentGeneratorImpl implements DocumentGenerator {
 private Dossier dossier;
 private Locale locale;
 
 public DocumentGeneratorImpl(Dossier dossier, Locale locale) {
  DocumentGeneratorHelper.validateNotNullParams(dossier, locale);
  this.dossier = dossier;
  this.locale = locale;
 }
 
 @Override
 public File generate(Document document, boolean temporary) 
   throws BusinessException {
  DocumentGeneratorHelper.setCobIdForDossierIfNotExist();
  switch (dossier.getDossierType()) {
  case TYPE_A:
   // Do some logic here in case TYPE_A
  case TYPE_BA:
   // Do some logic here in case TYPE_B
  default:
   throw new BusinessException("Not supported dossier type");
  }
 }
 
}

However, if I create tests for DocumentGeneratorImpl, I need to mock DocumentGeneratorHelper. Huh....! Any better approach?

I was thinking about that it is somehow I need to apply an Aspect-Oriented Programming (AOP) approach. As my google searching result, there are some approaches but they are quite complicated and heavy. They use Java Proxy or AOP Frameworks such as AspectJ.

A Simple AOP Approach

Fortunately, the keyword "proxy" leads me to the pattern Proxy. The idea of Proxy is really simple and cool!



Okay, now I create a Proxy instead of a Helper as before.

public class DocumentGeneratorProxy implements DocumentGenerator{
 private DocumentGenerator generator;
 private Dossier dossier;
 
 public DocumentGeneratorProxy(Dossier dossier, Locale locale) {
  validateNotNullParams(dossier, locale);
  this.dossier = dossier;
  this.generator = new DocumentGenerator(this.dossier, locale);
 }

 private void validateNotNullParams(Dossier dossier, Locale locale) {
  if(Objects.isNull(dossier)) {
   throw new IllegalArgumentException("Dossier must not be null!");
  }
  
  if(Objects.isNull(dossier.getDossierType())){
   throw new IllegalArgumentException("Dossier type must not be null!");
  }
  
  if(Objects.isNull(locale)) {
   throw new IllegalArgumentException("Locale must be defined!");
  }
 }

 @Override
 public File generate(Document document, boolean temporary)
   throws BusinessException {
  setCobIdForDossierIfNotExist();
  return generator.generate(document, temporary);
 }

 private void setCobIdForDossierIfNotExist() {
  if(StringUtils.isEmpty(dossier.getCobId())) {
   String generatedCobId = DossierService.getInstance().generateCobId();
   dossier.setCobId(generatedCobId);
  }
 }

}

Done! DocumentGeneratorImpl  doesn't contain its second responsibilities anymore. These methods are tested by the Proxy and we don't need to mock in DocumentGeneratorImpl.
the public class DocumentGeneratorImpl implements DocumentGenerator {
 private Dossier dossier;
 private Locale locale;
 
 public LibertyDocumentGenerator(Dossier dossier, Locale locale) {
  this.dossier = dossier;
  this.locale = locale;
 }
 
 @Override
 public File generate(Document document, boolean temporary) 
   throws BusinessException {
  switch (dossier.getDossierType()) {
  case TYPE_A:
   // Do some logic here in case TYPE_A
  case TYPE_BA:
   // Do some logic here in case TYPE_B
  default:
   throw new BusinessException("Not supported dossier type");
  }
 }
 
}

So, instead of calling directly DocumentGeneratorImpl, we call DocumentGeneratorProxy in the client code as below:

DocumentGenerator  generator = new DocumentGeneratorProxy(dossier, locale);

Happy codings!
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