Mark Lorenz on Technology

Thursday, February 23, 2006

Generate meaningful unit tests quickly & easily

Last time, I introduced the topics of TDD and DBC. These techniques lead you to:
  • define what it is you are trying to build,
  • create test cases to see if you are building what is required (and no more), and
  • iteratively and incrementally verify that the code you are writing is correct.
Never fear - I wouldn't expect you to do all this without any help. Awhile back, I checked out multiple tools to help with this effort. The one I chose as the best for this situation was Instantiation's CodePro.

Note: CodePro can create test cases for existing code too! It does a great job of testing based on your current design. It parses the code as well as actually running it to examine the behavior of your system. So, these techniques can be used with new or existing efforts.

CodePro works with Eclipse and uses JUnit. Both of these are free. For more information about JUnit, I recommend JUnit Recipes by Rainsberger.

UML Model

We are going to go through a relatively simple example of how to use CodePro to generate meaningful test cases for your designs. The package we are going to target is the event package, as shown in this Unified Modeling Language class diagram:

Note: if you don't know UML, I recommend Applying UML and Patterns by Larman and UML Distilled by Fowler. This diagram was created using MagicDraw, which is an excellent product.

I have noted the classes and methods that I will show assertions for:
  • Loggable
    The ability to take part in a SystemEvent.
    @invariant (getEventString() != null && getEventString().length() > 0)
    @invariant (getEvents() != null)
  • Loggable.add()
    Add anEvent to my events. Throw an Exception if anEvent is not one of my
    @throws Exception
    @post (getEvents().contains(anEvent))
  • UpdateEvent()
    @throws Exception
    @pre target instanceof Loggable
    @pre EventType.contains( type )
    @pre attributeName != null && attributeName.length() > 0
    @pre oldValue != newValue
  • SystemEvent.getType()
    @return a String to uniquely indicate my type
    @post ($ret == type)
Note that Loggable is an interface. CodePro is smart enough to use the concrete implementors to test the assertions for Loggable!

In some complex portions of the design, CodePro cannot figure out how to create valid instances for the test fixtures. In this case, you can provide a <className>Factory class in the <projectName>Test project with static methods that return valid instances. You find these situations when you encounter a NullPointerException when running a test case and the test fixture has null(s) in its logic.


The process we will follow looks like this:

Assertions are entered into our modeling tool, such as MagicDraw, as comments on the classes (for invariants) and methods (for pre and post conditions). These are carried over to our IDE (Eclipse in this case) as JavaDocs. The assertions can be edited or entered in the IDE as needed, with roundtrip engineering updating our model.

The generated test cases
We then use CodePro to generate test cases from our Java code. Here's some of what we'd get for LoggableTest:
* Return an instance of the class being tested.
* @return an instance of the class being tested
* @see Loggable
* @generatedBy CodePro at 1/18/06 1:33 PM
public Loggable getFixture7()
throws Exception
if (fixture7 == null) {
fixture7 = new TestOrder(...);
return fixture7;
* Return an instance of the class being tested.
* @return an instance of the class being tested
* @see Loggable
* @generatedBy CodePro at 1/18/06 1:33 PM
public Loggable getFixture8()
throws Exception
if (fixture8 == null) {
fixture8 = new Specimen( SpecimenNumberFactory.sampleSpecimenNumber(), new PatientRole(PersonFactory.heathcliffPeterman()...);
return fixture8;

* Run the void add(SystemEvent) method test.
* @targetAssertion @post (getEvents().contains(anEvent))
* @targetAssertion @invariant (getEvents() != null)
* @targetAssertion @invariant (getEventString() != null && getEventString().length() > 0)
* @generatedBy CodePro at 2/24/06 10:57 AM
public void testAdd_fixture28_5() throws Exception {
Loggable fixture = getFixture28();
SystemEvent anEvent = new UpdateEvent("Anäßt-1.0.txt", (Loggable) null, "Anäßt-1.0.txt", "Anäßt-1.0.txt", "Anäßt-1.0.txt");
// add test code here
assertTrue(fixture.getEvents() != null);
assertTrue(fixture.getEventString() != null && fixture.getEventString().length() > 0);
Note that both concrete implementors of Loggable (TestOrder and Specimen) are used as fixtures! Also note that, when available, our factory static methods are used.

All our assertions are being tested. If we change them, we can just regenerate the affected objects (method, class, package, project).

CodePro and JUnit provide tools to run the test cases, see the results, fix any problems, rerun the tests, ... This supports TDD.

But that's not all: CodePro also will parse, run, and create test cases for existing code too:

(the concrete subclass chosen for this test case)
* Method getParameters.
* @return my parameters to my BIRT design
protected HashMap getParameters() {
HashMap params = new HashMap();
params.put("department", department);
params.put("dateRange", dateRange);
return params;

* Return an instance of the class being tested.
* @return an instance of the class being tested
* @see Report
* @generatedBy CodePro at 2/24/06 11:36 AM
public Report getFixture5() throws Exception

if (fixture5 == null) {
fixture5 = new EmployeeEfficiencyReport(new Department("", ""),
new DateRange(new GregorianCalendar(1999, 11, 31, 23, 59, 59),
new GregorianCalendar(1999, 11, 31, 23, 59, 59)));
return fixture5;
* Run the java.util.HashMap getParameters() method test.
* @generatedBy CodePro at 2/24/06 11:36 AM
public void testGetParameters_fixture5_1()
throws Exception
Report fixture = getFixture5();
java.util.HashMap result = fixture.getParameters();
// add test code here
assertEquals(2, result.size());
You can see that I didn't have any assertions - CodePro just parsed and ran the code and figured out what to test! How easy is that! (Hope you don't want to hold onto your old excuses for not unit testing adequately.)

This has been a short (well, not that short) look at a process and tools to use TDD on your project in a way that makes you more productive, rather than a drag on productivity.

CodePro Guidelines

As with any product, there are some lessons you learn along the way. These are mine, in no particular order:
  • Turn off the verification preference
Window / Preferences / CodePro / JUnit, Test Methods tab, Test Verification group - uncheck "Mark new test methods as unverified"

If verification is active, every test case will have a "fail(unverified)" line at the end. EPT does have a menu action to verify test cases so make it easy to eliminate these lines. Their thinking is that you should check out every test case to make sure it is doing the right thing. Considering that one class will have many test cases, I choose to take my chances with their generated code. I tend to look at a few of them, but nowhere near all of them.

  • Keep the default preferences (other than ones specifically listed here).
This will result in a project named Test with packages named exactly as those being tested. The classes in the packages are named Test. Note: you must leave the "Design by Contract" options selected or else your test cases will not test your assertions!

  • If you edit a test case, e.g. to add logic not generated for you, then you should delete the comment line that includes "@generatedBy CodePro".
This will keep CodePro from replacing that test case if another generate action is taken.

  • If you have not edited test cases, delete the test class(es) before regenerating test cases.
This will make the generate action run much faster, since it doesn't have to check for edited logic that must be preserved.

  • Before deploying, the code should be "uninstrumented" to remove unnecessary logic during production.
CodePro includes code coverage collection and reporting. Instantiations is able to collect coverage information because they "instrument" the code (add metric collection logic to the .class files).

  • Create Factory classes in the Test
  • project for cases where CodePro doesn't handle complex situations as well as you'd like.
If you have complicated business objects with nested compound collaborating objects, CodePro may use a meaningless String or even null for portions. If you create a factory class in the test project, CodePro will use it in the generated test cases. The factory class should be in the package with the same name as the real class' package and should have static methods (e.g. public static Person aCustomer() { ...} ) to return a valid instance. The Instantiations folks are working on enhancements that will flag when they had to use null.

  • Generate test cases even if an Exception is thrown.
Preferences/CodePro/JUnit, Execution tab, "When an exception is thrown, generate a test method:", select "always". If you don't do this, you will have situations where you are scratching you head as to why there are no test cases generated for a class. When you select "always", you can run the tests, see the Exception and resolve the problem without having to guess what's wrong.

  • Add or edit Design by Contract (DBC) tags if you have additional business rules you want tested or the DBC tags are erroneous. Instantiations is working on an enhancement to indicate when an assertion has invalid syntax (Eclipse ignores the assertions).
Tags should be in the same place you would put JavaDoc tags. The tags currently supported by CodePro are:
  • @pre
This tag is used to indicate a precondition, i.e. something that should be true (or false) upon entry to a method. This tag can only be used for methods. For example:
@pre (aName != null && aName.length() > 0)
  • @post
This tag is used to indicate a postcondition, i.e. something that should be true (or false) after a method executes. This tag can only be used for methods. For example:
@post specimens.contains( newSpecimen )

  • @inv(ariant)
This tag is used to indicate something that should always be true (or false). This tag can only be used for classes. For example:
@inv (numEmployees >= 0)
The variables used within these tags' expressions currently supported by CodePro are:
  • $ret
This tag is used to test a return value. This tag can only be used for methods. For example:
@post ($ret == type)
  • $pre
This tag makes use of a value upon entry to a method. This tag can only be used for methods. For example:
@post count == $pre(int, count) + 1
  • $result
This tag is used to check a non-void return from a method. This tag can only be used for non-void methods. For example:
@post ($result != null)
Note: Be careful how you specify assertions or EPT will generate compile errors in the test cases! Syntax checking of assertions in JavaDocs will be added in a future release. Until then, watch your parentheses, method name spelling, …


Symptom: You are getting a NullPointerException where you shouldn't.
Possible reason: You may need a Factory class. Look for null in your fixture code to determine where.

Symptom: You don't get test cases where you should.
Possible reason: You might have a problem with a (super)class at load or construction time. Check static blocks, constructors, and variable initialization.

Symptom: There are errors in the test cases.
Possible reason: Your assertions may have a syntax error. Look at related pre, post, or invariant lines.

Symptom: All your test cases are failing, when some should pass.
Possible reason: Make sure Preferences/CodePro/JUnit, Test Methods tab, Test Verification is unchecked. Alternatively, verify the test cases from the Test Case Outline view.

Symptom: Test results seem to be based on old code.
Possible reason: A class' source is out of sync with the binary file. If you have not edited the test case logic, delete the ClassNameTest class and regenerate it. If you have edited the test case logic, try forcing a rebuild of the Test project.

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TDD using DBC

Test Driven Development using Design by Contract


Test-Driven Development Cycle in general involves these steps:
  1. Write the test
  2. Write the code
  3. Run the automated test
  4. Refactor
  5. Repeat
This is a laudable process to aspire to. The problem is that unit testing is time-consuming and arduous without help. For Java developers using Eclipse as their IDE, there is a solution (read on).
Note: A good book on TDD is Agile Java by Langr.


Design by Contract has a history that goes back to the 1980s with work by Bertrand Meyer. The primary pragmatic idea we will use today is the concept of an assertion. There are three types of assertions:
  1. Precondition
  2. Postcondition
  3. Invariant
The goal is to specify business rules that must be enforced for the services to take place.
A precondition is a condition that must hold true before a method can be executed. The format is:
@pre (boolean expression)
  • @pre !isCancelled()
    This might be used to make sure an Order can be modified.
  • @pre quantityOrdered > 0
    This might be used to make sure an amount being Ordered is a positive integer.
A postcondition is a condition that must hold true after a method is executed. The format is:
@post (boolean expression)
  • @post ($ret == (inventory.contains(aProduct))
    This can be read as "answer whether my inventory contains a product". It may be an assertion on the Business.sells( aProduct ) method.
  • @post savingsAcct.getBalance() == $pre(savingsAcct.getBalance()) - anAmount
    This business rule checks to make sure that after the SavingsAccount.withdraw( anAmount ) method that the balance is debited correctly.
An invariant is a condition that must always hold true for a class. The format is:
@inv (boolean expression)
  • @inv balance >= 0.0
    This would certainly be a rule for Accounts in a banking application.
  • @inv number.length() == 9
    This would be a rule for a SocialSecurityNumber class.

Development Process using TDD and DBC

To use the assertions we've been discussing, put them in as comments into your modeling tool or in JavaDocs in your IDE.

Alright - that's a lot of information to digest before actually doing anything productive. In my next post, I'll introduce a tool to automate much of the heavy lifting, making TDD a real possibility on your project.

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