Testing Private Methods with JUnit

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by Bill Venners
May 24, 2004



This article compares four different approaches to testing private methods in Java classes.

My very first use of JUnit was to build a conformance test kit for the ServiceUI API [1]. The purpose of a conformance test kit is to help ensure that alternate implementations of the same API are compatible with the API’s specification. Because an API specification defines only the public interface of the API, not the API’s implementation, a conformance test exercises only the public interface. In other words, a conformance test is a “black box” test. It treats the API under test as a black box, whose external interface can be seen, but whose internal implementation cannot. A conformance test of a Java API, therefore, need only access the public members of the packages and classes under test. There is no need to access package-level, protected, or private members.

When I later applied JUnit to the task of writing actual unit tests, as opposed to conformance tests, I found myself wanting to write white box tests—tests that employ knowledge of the internal implementation of the packages and classes under test. Whereas I only wanted to test public methods in my conformance tests, I wanted to write unit tests for package access and occasionally private methods as well as public methods.

Daniel Steinberg [2] showed me the common JUnit technique of using parallel source code trees, which allowed me to place test classes in the same package as the classes under test, but keep them in a different directory. This provided a clean separation of test and production code. By placing both source trees in the CLASSPATH, my test classes could access package-level methods and classes in the package under test. This still left me, however, with the problem of testing private methods.

When I asked Daniel about testing private methods, he gently suggested that I test the private methods indirectly by testing the package-access and public methods that call the private ones. This answer did not quite satisfy me, because on occasion I really did feel the urge to directly test a private method. My initial solution was to just make such private methods package access, which allowed me to test them directly with JUnit from the test classes in the same package in the parallel source tree. This worked fine, but made me feel a bit dirty somehow. Although in general I discovered that thinking about how to design interfaces so they could be easily unit tested helped me design better interfaces, in this case I felt I was making the design slightly worse to make it testable.

When I later ended up participating in the creation of what Frank Sommers, Matt Gerrans, and I eventually released as Artima SuiteRunner [3], I vowed that I would make the testing of private methods easier in SuiteRunner than it is in JUnit. But after investigating the various approaches to testing private methods, I decided not to do anything special in SuiteRunner to support testing private methods. So whether you are using JUnit or SuiteRunner, you have the same four basic approaches to testing private methods:

  • Don’t test private methods.
  • Give the methods package access.
  • Use a nested test class.
  • Use reflection.

In this article, I will discuss these four approaches to testing private methods in Java. I will look at the advantages and disadvantages of each and attempt to shed some light on when it makes sense to use each approach.

Approach 1: Don’t Test Private Methods

As I mentioned in the introduction, I first heard the advice to suppress my occasional urges to test private methods from Daniel Steinberg. But Daniel is not only source of this advice that I have encountered. It seems to be a common attitude in the Java community. For example, the JUnit FAQ [4] states:

Testing private methods may be an indication that those methods should be moved into another class to promote reusability.

Charles Miller expressed a similar point of view in his weblog [5]:

If you have a thorough suite of tests for a class’s exposed (non-private) interface, those tests should, by their nature, verify that any private method within the class also works. If this isn’t the case, or if you have a private method so complex that it needs to be tested out of the context of its public callers, I would consider that a code-smell.

And Dave Thomas and Andy Hunt, in their book Pragmatic Unit Testing [6], write:

In general, you don’t want to break any encapsulation for the sake of testing (or as Mom used to say, “don’t expose your privates!”). Most of the time, you should be able to test a class by exercising its public methods. If there is significant functionality that is hidden behind private or protected access, that might be a warning sign that there’s another class in there struggling to get out.

I believe all this advice. Most of the time, private methods can be most effectively tested via approach 1, indirectly by testing the package-level, protected, and public methods that call them. But inevitably, some people in some situations will feel that directly testing a private method is the right thing to do.

In my case, I tend to create many private utility methods. These utility methods often do nothing with instance data, they just operate on the passed parameters and return a result. I create such methods to make the calling method easier to understand. It is a way to manage the complexity of the implementation of the class. Now, if I extract the private method out of a method that already works and has good unit test coverage, then those existing unit tests will likely suffice. I needn’t write more unit tests just for the private method. But if I want to write the private method before its calling method, and I want to write the unit tests before writing the private method, I’m back to wanting to directly test the private method. In the case of private utility methods, I don’t feel my urge to directly test the methods is, as the JUnit FAQ put it, “an indication that those methods should be moved into another class to promote reusability.” These methods are really only needed in the class in which they reside, and in fact are often only called by one other method.

Another reason I sometimes feel the urge to test private methods directly is that I tend to think of unit testing as helping me achieve a robust system by building that system out of robust parts. Each part is a “unit” for which I can write “unit tests.” The unit tests help me ensure each unit is functioning correctly, which in turn helps me build a system that functions correctly as a whole. The primary unit I think in terms of when programming in Java is the class. I build systems out of classes, and unit tests give me confidence that my classes are robust. But to some extent I also feel the same way about the private methods out of which I compose package-access, protected, and public methods. These private methods are units that can be tested individually. Such unit tests give me confidence that the private methods are working correctly, which helps me build package-access, protected, and public methods that are robust.

Approach 2: Give the Methods Package Access

As I mentioned in the introduction, giving methods package access was my first approach to testing private methods with JUnit. This approach actually works just fine, but it does come with a slight cost. When I see a private access specifier on a method, it tells me something I like to know—that this is part of the implementation of the class. I know I can ignore the method if I am just trying to use the class from another class in the package. I could figure this out about a package-access method by looking more closely at the name, documentation, and code of the method, but the word private communicates this far more efficiently. Moreover, the main problem I have with this approach is philosophical. Although I don’t mind “breaking encapsulation for the sake of testing,” as Dave and Andy would put it, I just don’t feel good about breaking encapsulation in a way that changes the package-level API. In other words, although I am quite enthusiastic to test non-public methods of classes, i.e., to create “white-box” unit tests, I’d rather the API of the classes under test, including the package-level API, not be changed to facilitate those tests.

Approach 3: Use a Nested Test Class

A third approach to testing private methods is to nest a static test class inside the production class being tested. Given that a nested class has access to the private members of its enclosing class, it would be able to invoke the private methods directly. The static class itself could be package access, allowing it to be loaded as part of the white box test.

The downside to this approach is that if you don’t want the nested test class being accessible in your deployment JAR file, you have to do a bit of extra work to extract it. Also, some people may not like having test code mixed in the same file as production code, though others may prefer that approach.

Approach 4: Use Reflection

The fourth approach to testing private methods was suggested to me by Vladimir R. Bossicard, who wrote JUnit Addons [7]. One day over lunch, Vladimir enlightened me that the java.lang.reflect API included methods that allowed client code to circumvent access protection mechanism of the Java virtual machine. He also told me that his JUnit Addons project included a class, junitx.util.PrivateAccessor [8], to assist in using the reflection API for just this purpose: to write unit tests that manipulate private members of the classes under test. The JUnit FAQ points to a similar class, called PrivilegedAccessor [9], written by Charlie Hubbard and Prashant Dhotke.

One advantage of using the reflection approach to testing private methods is that it provides a clean separation of test code and production code. The tests need not be nested inside the class under test, as in approach 3. Rather, they can be placed alongside the other tests that exercise the package-level and public methods of the class. In addition, you need not alter the API of the class under test. Unlike approach 2, private methods can remain private. Unlike approach 3, you need not add any extra nested class at package access level. The main disadvantage of this approach is that the test code is far more verbose because it uses the reflection API. In addition, refactoring IDEs such as Eclipse and IntelliJ usually aren’t as adept at changing the names of methods where they are referred to as Strings passed to the methods of the reflection API. So if you change the name of the private method with your refactoring IDE, you may still have to make some changes by hand in the test code.

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