Using AOP to mock out dependecies for unit-test

[nizzy]

Hi Guys,

AOP newbie;

I hope someone can help me I'm developing a quick AOP proof of concept with the intention to roll it out company wide. The idea is to use AOP to mock-out dependencies in order to simplify unit testing. I'm sure I don't have to state the benefits here.

However I have created a basic J2EE project with a single SessionBean and a few entities.

I have dependencies created in the constructor of the SessionBean which I want to mock out.

private IPersistenceHelper helper;
public AOPUnitTestManagerBean() {
 helper = new PersistenceHelper(PERSISTENCE_CONTEXT_NAME);
}

I have written an simple Interceptor which intercepts the PersistenceHelper constructor and replaces it with a MockPersistenceHelper;

public class PersistenceHelperInterceptor implements Interceptor {

 /**
 * Log4j Logger for this class
 */
 private static final Logger log = Logger
 .getLogger(PersistenceHelperInterceptor.class);

 public String getName() {
 return "PersistenceHelperInterceptor";
 }

 public Object invoke(Invocation invocation) throws Throwable {
 if (log.isDebugEnabled()) {
 log.debug("inside invoke(arg0)");
 }
 Object helper = new MockPersistenceHelper(null);
 return helper;
 }
}

I can get this working if the MockPersistenceHelper extends the PersistenceHelper. This is not my preferred solution since it would be better if the MockPersistenceHelper implemented the same interface as the PersistenceHelper, this ensures that all methods have a mock implementations as oppsoed to simply over-riding the methods (open to abuse).

the jboss-aop.xml to get this to work;

<?xml version="1.0" encoding="UTF-8"?>
<aop>
 <bind pointcut="execution(public com.xxx.unittest.aop.PersistenceHelper->new(java.lang.String))">
 <interceptor class="com.xxx.unittest.aop.interceptor.PersistenceHelperInterceptor"/>
 </bind>
</aop>

So I have changed it to something like;

<?xml version="1.0" encoding="UTF-8"?>
<aop>

 <bind pointcut="execution(public $typedef{persistenceHelperTypeDef}->new(java.lang.String))">
 <interceptor class="com.xxx.unittest.aop.interceptor.PersistenceHelperInterceptor"/>
 </bind>

 <typedef name="persistenceHelperTypeDef"
 expr="class($instanceof{com.xxx.unittest.aop.IPersistenceHelper})
 AND !class($instanceof{com.xxx.unittest.aop.mock.*})" />
</aop>

However every option I have tries either gives me a ClassCastException, returned object not as expected, or a Stack overflow since the MockPersistenceHelper is itself intercepted since it implements the defined interface.

So I suppose the question is how can I define the typedef such that;

1. It intercepts the constructor of classes of IPersistenceHelper but does not intercept those classes in the xx.xx.xx.mock package.

Hope that wasn't too long winded to ask, hopefully, a straightforward question. This all occurs outside the JBossAS container!

[stalep]

hi, the expression you want should be possible, but from just looking at the code you provided i couldnt see any immediate errors. have you tried to group the typedef together with commas? - not sure if it would help but...
if you could provide a small example ill test it locally and probably help out more.

[nizzy]

Hi Stale,

Thanks for the response, I tried grouping the typedef with commas however I received a parser error. I had hoped this would be possible, all the documentation points toward this being possible.

My project is rather large, is it possible to zip up the eclipse project and attach it to a post, or to send it to you? Otherwise I will create a "small" test to reproduce, and post that!

[stalep]

the smaller test the better :)

[nizzy]

Hi Stale,

Couldn't get it any smaller, :)

Cut down Session Bean

public class AOPUnitTestManagerBean {

 private IPersistenceHelper helper;

 public AOPUnitTestManagerBean() {
 helper = new PersistenceHelper(null);
 }

 public void createObject(Object o) {
 helper.create(o);
 }
}

JUnit Test

import org.junit.Before;
import org.junit.Test;

public class AOPUnitTestManagerBeanTest {

 private AOPUnitTestManagerBean man;

 @Before
 public void setUp() throws Exception {
 man = new AOPUnitTestManagerBean();
 }

 @Test
 public void testCreateObject() {

 String s = new String("Hello");
 man.createObject(s);
 }
}

IPersistenceHelper interface

public interface IPersistenceHelper {

 public void create(Object o);
}

MockPersistenceHelper

public class MockPersistenceHelper implements IPersistenceHelper {

 public MockPersistenceHelper(String PERSISTENCE_CONTEXT_NAME) {
 }

 public void create(Object o) {
 System.out.println("inside MockPersistenceHelper.create()");
 }
}

PersistenceHelper

public class PersistenceHelper implements IPersistenceHelper {

 public PersistenceHelper(String persistenceContextName) {
 }

 public void create(Object o) {
 System.out.println("inside PersistenceHelper.create()");
 System.out.println("This should have been mocked-out --- did you set -javaagent in jvm args?");
 }
}

Interceptor

import org.jboss.aop.advice.Interceptor;
import org.jboss.aop.joinpoint.Invocation;

public class PersistenceHelperInterceptor implements Interceptor {

 public String getName() {
 return "PersistenceHelperInterceptor";
 }

 public Object invoke(Invocation invocation) throws Throwable {
 System.out.println("Inside PersistenceHelperInterceptor.invoke");

 Object helper = new MockPersistenceHelper(null);
 return helper;
 }
}

jboss-aop.xml

<?xml version="1.0" encoding="UTF-8"?>
<aop>

 <bind pointcut="execution(public $typedef{persistenceHelperTypeDef}->new(java.lang.String))">
 <interceptor class="PersistenceHelperInterceptor"/>
 </bind>

 <!-- This causes Stack Overflow Error since both PersistenceHelper
 and MockPersistenceHelper implement IPersistenceHelper, causing
 infinite loop (of sorts) -->
 <typedef name="persistenceHelperTypeDef" expr="class($instanceof{IPersistenceHelper})" />

 <!-- This causes ClassCastException -->
 <!-- typedef name="persistenceHelperTypeDef" expr="class($instanceof{IPersistenceHelper}) AND !class(MockPersistenceHelper)" /-->

</aop>

Hope this helps

[nizzy]

I see the same behaviour when I edit the typedef example bundled with jboss-aop-2.0.1.GA.

Tells me I've probably made the syntactic mistake in both :)

[stalep]

hi, ive tested the code you provided and i get a classcastexception too. ill take a closer look and let you know what find.

[stalep]

hi again. the problem is within the generated code and not in your pointcutdef. the PersistenceHelper constructor is "wrapped" in aop code when we instrument it, and that method will only accept an object of type PersistenceHelper. - not the interface. thats why it is throwing a classcastexception since it get a MockPersistenceHelper and not a PersistenceHelper.
atm there is no way around this, ill discuss with the team if this is something we should change, and ill let you know what we decide.

a solution for your problem would be to put the constructor in a method and just intercept that method. something like:

public class AOPUnitTestManagerBean {

 private IPersistenceHelper helper;

 public AOPUnitTestManagerBean() {
 helper = init();
 }

 private IPersistenceHelper init() {
 return new PersistenceHelper(null);
 }

 public void createObject(Object o) {
 helper.create(o);
 }
}

[nizzy]

Look forward to hearing from you.

Thanks for the help!

[nizzy]

Hi Stale,

Can I also point out that this behaviour is fundamental to being able to use AOP for unit-testing, certainly in our company, where we always have the "real" object and the "mock" object implementing the same interface.

I will either use you're suggested workaround for now, or make the "mock" object extend the "real" class, which also worked.

Thanks again for you're help.