How do I test a class that has private methods, fields or inner classes?

Question

How do I use JUnit to test a class that has internal private methods, fields or nested classes?

It seems bad to change the access modifier for a method just to be able to run a test.

Adalai

Best way to test a private method is not testing it directly – Postman

Check the article Testing Private Methods with JUnit and SuiteRunner. – Amabel

why should test a private function ? It will automatically be tested anyway (and must be tested) when you test the feature/method which is using it the private function. – Ablaze

I've rolled this question back to its original state, prior to generifying the question. In its current state, the answers are highly Java-oriented, and the question was largely focused on the "how", meaning that the later versions erasing Java and generalizing for other languages, and at the end flipping the focus to C++ invalidates far too many of the answers. A generalized form asking "how to" in any arbitrary language and framework would also be considered far too broad, meaning the last few edits pushed this question towards being closed rather than being constructive. – Audreyaudri

@AkashVerma: For the same reason you want to do unit tests instead of just a system test? – Carper

See also: stackoverflow.com/questions/250692/…. The underlying philosophical question is language-agnostic. – Covenanter

See: github.com/ljr1981/stack_overflow_answers/blob/main/src/… and github.com/ljr1981/stack_overflow_answers/blob/main/testing/… – Adynamia

In the previous comment, I show you how easy this is in Eiffel. In this case, I have created a class called SO_34571 with a method feature called `my_private_method'. Notice that the "feature" group keyword is followed by a {TEST_SET_BRIDGE}. This means that any class that inherits from TEST_SET_BRIDGE (including descendants) has permission to access any feature of the feature-group. Clean. Simple. Elegant. Scalable. Controllable. Nice! – Adynamia

Answer 1

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

---

Notes:

1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Kenney

Useful if you don't know the API perhaps, but if you are having to test private methods in this manner there is something up with your design. As another poster says unit testing should test the class's contract: if the contract is too broad and instantiates too much of the system then the design should be addressed. – Advertisement

We develop methods that do a single specific task that allow the class to perform its specific task. These methods should not be public and still need to be tested independently. So yes reflection is a good way to do this and no your design does not need review if you are using reflection for this purpose. – Feuar

Answer 2

The best way to test a private method is via another public method. If this cannot be done, then one of the following conditions is true:

1. The private method is dead code
2. There is a design smell near the class that you are testing
3. The method that you are trying to test should not be private

Gal

Further, we never get close to 100% code coverage anyway, so why not focus your time doing quality testing on the methods that clients will actually be using directly. – Widget

@Widget Spot on. The only thing you would gain from testing private methods is debugging information, and that's what debuggers are for. If your tests of the class's contract have full coverage then you have all the information you need. Private methods are an implementation detail. If you test them you will have to change your tests every time your implementation changes, even if the contract doesn't. In the full life cycle of the software this is likely to cost a lot more than the benefit it gives. – Goolsby

I am impressed how often the "should not be private" claim is repeated across the answers here, as if "make it public so you can test it" could not easily lead to code-smell that invites others to abuse parts of a class. Exposing sensible internals just for easier testing of various code paths isn't always sensible, so don't follow this device here blindly! – Refugee

The "should not be private" here doesn't say "just make it public for test". If there is a need for testing the private method, there must be something like the three conditions he mentioned here. If you want 100% test coverage, that's still possible by calling the public methods of the class with various parameters, except when it is unused or contains dead code, eg. if (x == null) which is never reached because you nullchecked in the public method already. But then you should cleanup the private method, instead of inventing test cases that cannot happen. – Opsonize

Furthermore: Making it public could also mean: Refactor it out into a Util-Class and making it public there. If the method contains code for cases that never happen (but are implemented for future use cases), thats not the purpose of private methods to be generic and available for everyone for different use cases. – Opsonize

A counterexample (real) situation where none of these 3 points is valid (as far as I can think): A public method foo() will 1. use a private method bar() and 2. run in an infinite loop. As far as I know you can't test bar() by calling foo() otherwise the test case will run forever. – Mallee

I agree with this a 100% I see private methods as extensions of the public methods. I usually create private methods to make the code more readable. If I am doing TDD I write the tests for the public method and when all my tests are passing I may refactor the public method and create one or more private ones for readability but all my logic is already tested, I don't need to write new tests for the private methods I created. – Grindery

I second @Mallee comment. If your public method is expensive to call, and the only way to test a private implementation detail is by calling a public method that will lead to a problem. – Brie

I have a case where I can't cover the variable being passed to a switch statement being null, because in the code flow it's impossible to call the method with null parameters because an exception would be thrown. There is nothing wrong with the code, except that is impossible to test this case. The code would actually work fine in this scenario. – Valentia

Answer 3

When I have private methods in a class that are sufficiently complicated that I feel the need to test the private methods directly, that is a code smell: my class is too complicated.

My usual approach to addressing such issues is to tease out a new class that contains the interesting bits. Often, this method and the fields it interacts with, and maybe another method or two can be extracted in to a new class.

The new class exposes these methods as 'public', so they're accessible for unit testing. The new and old classes are now both simpler than the original class, which is great for me (I need to keep things simple, or I get lost!).

Note that I'm not suggesting that people create classes without using their brain! The point here is to use the forces of unit testing to help you find good new classes.

Ism

Why create a new class when you can make your method public anyway? – Octopod

If you have a mix of private and public methods on a class, that's a hint that multiple responsibilities are present. – Ism

Use reflection to access the private method in a test. For example, i have an implementation of an algorithm in a private method and this method is used in another public method. So i need to test this algorithm properly (even while i am writing this algorithm). So why do i need to do a hack in the productive code only to be able to test this method? Yes, using java reflection is also a hack, but only in the test. And if i decide to move the method (for some other reason) in to a utils class, i can just move the test too .. done. Easy peasy, readable, maintainable. – Faithfaithful

Answer 4

I have used reflection to do this for Java in the past, and in my opinion it was a big mistake.

Strictly speaking, you should not be writing unit tests that directly test private methods. What you should be testing is the public contract that the class has with other objects; you should never directly test an object's internals. If another developer wants to make a small internal change to the class, which doesn't affect the classes public contract, he/she then has to modify your reflection based test to ensure that it works. If you do this repeatedly throughout a project, unit tests then stop being a useful measurement of code health, and start to become a hindrance to development, and an annoyance to the development team.

What I recommend doing instead is using a code coverage tool, such as Cobertura, to ensure that the unit tests you write provide decent coverage of the code in private methods. That way, you indirectly test what the private methods are doing, and maintain a higher level of agility.

Elka

+1 to this. In my opinion it's the best answer to the question. By testing private methods you are testing the implementation. This defeats the purpose of unit testing, which is to test the inputs/outputs of a class' contract. A test should only know enough about the implementation to mock the methods it calls on its dependencies. Nothing more. If you can not change your implementation without having to change a test - chances are that your test strategy is poor. – Chimera

@Colin M That's not really what he's asking though ;) Let him decide it, you don't know the project. – Minton

Not really true. It could take a lot of effort to test a small part of the private method through the public method using it. The public method could require significant setup before you reach the line that calls your private method – Halinahalite

I agree. You should test, if the contract is fulfilled. You should not test, how this is done. If the contact is fulfilled without reaching 100% code coverage (in private methods), that might be dead or useless code. – Reeder

non-trivial private methods are a code smell and indicate a need to refactor code into appropriate public methods/services. – Quinquepartite

The problem is, probably your test going to be complicated and complicated tests are hard to maintain. So there is a danger that someone who need to fix the test, fix the test in a wrong way and make them useless, or overlook the error in the code and fix the test instead. Using java reflection in tests can help to keep your tests small :) – Faithfaithful

Answer 5

From this article: Testing Private Methods with JUnit and SuiteRunner (Bill Venners), you basically have 4 options:

1. Don't test private methods.
2. Give the methods package access.
3. Use a nested test class.
4. Use reflection.

Dinorahdinosaur

5th option, like mentioned above is testing the public method that calls the private method? Correct? – Twinscrew

@LorenzoLerate I've been wrestling with this because I'm doing embedded development and I'd like my software to be as small as possible. Size constraints and performance is the only reason I can think of. – Crowfoot

@Twinscrew that's the same as #1 — don't test private methods – Bowleg

Answer 6

Generally a unit test is intended to exercise the public interface of a class or unit. Therefore, private methods are implementation detail that you would not expect to test explicitly.

Sublease

That's the best answer IMO, or as it is usually said, test behaviour, not methods. Unit testing is not a replacement for source code metrics, static code analysis tools and code reviews. If private methods are so complex that they need separates tests then it probably needs to be refactored, not more tests thrown at it. – Vindication

so don't write private methods, just create 10 other small classes ? – Raja

No, it basically means that you can test the functionality of the private methods using the public method that invokes them. – Mauricemauricio

@AkshatSharda correct - and morever, if you can't test some part of the private method through the public API then you probably don't need that functionality in the private method. I've found that a side effect of the "test through the API" methodology is that you write less dead code. – Arkose

I don't agree. If I want to be sure of my code, I would like to test every piece that the code flow goes through. Unit can be even private methods that implement self-contained logic. – Claman

Answer 7

Just two examples of where I would want to test a private method:

1. Decryption routines - I would not want to make them visible to anyone to see just for the sake of testing, else anyone can use them to decrypt. But they are intrinsic to the code, complicated, and need to always work (the obvious exception is reflection which can be used to view even private methods in most cases, when SecurityManager is not configured to prevent this).

2. Creating an SDK for community consumption. Here public takes on a wholly different meaning, since this is code that the whole world may see (not just internal to my application). I put code into private methods if I don't want the SDK users to see it - I don't see this as code smell, merely as how SDK programming works. But of course I still need to test my private methods, and they are where the functionality of my SDK actually lives.

I understand the idea of only testing the "contract". But I don't see one can advocate actually not testing code—your mileage may vary.

So my trade-off involves complicating the JUnit tests with reflection, rather than compromising my security and SDK.

Synchronize

While you should never make a method public just to test it, private is not the only alternative. No access modifier is package private and means that you can unit test it as long as your unit test lives in the same package. – Gluttonous

@Gluttonous that is all true, but is not what the question is about. OP asks about testing private methods. Default is not the same as private; as stated, you can easily see a default access method from a class just by declaring that class in the same package. With private access, you require reflection, which is a whole other ballgame. – Synchronize

I was commenting on your answer, point 1 in particular, not the OP. There is no need to make a method private just because you don't want it to be public. – Gluttonous

@Gluttonous true thx - I was lazy with the word "public". I have updated the answer to include public, protected, default (and to make mention of reflection). The point I was trying to make is that there is good reason for some code to be secret, however that shouldn't prevent us from testing it. – Synchronize

Thanks for giving real examples to everyone. Most answers are good, but in a theorical point of view. In real world, we do need to hide implementation from the contract, and we still need to test it. Reading all these, I think maybe this should be a whole new level of testing, with a different name – Bemean

One more example - Crawler HTML parsers. Having to parse a whole html page into domain objects requires a ton of nitty-gritty logic of validating little parts of the structure. It makes sense to break that up into methods and call it from one public method, yet it doesn't make sense to have all the smaller methods composing it be public, nor does it make much sense to create 10 classes per HTML page. – Pentheus

I'd argue that even in an SDK, private methods are part of the public API's functionality. So as long as the public API produces the same results, it's completely irrelevant which private method does which part. – Christianize

Answer 8

The private methods are called by a public method, so the inputs to your public methods should also test private methods that are called by those public methods. When a public method fails, then that could be a failure in the private method.

Xanthochroism

True fact. The problem is when the implementation is more complicated, like if one public method calls several private methods. Which one did fail? Or if it is a complicated private method, that can't be exposed nor transferred to a new class – Bemean

You already mentioned why the private method should be tested. You said "then that could be ", so you are not sure. IMO, whether a method should be tested is orthogonal to its access level. – Oria

Answer 9

In the Spring Framework you can test private methods using this method:

ReflectionTestUtils.invokeMethod()

For example:

ReflectionTestUtils.invokeMethod(TestClazz, "createTest", "input data");

Gainer

The cleanest and the most concise solution so far, but only if you are using Spring. – Bipinnate

What does this return? – Stockroom

Answer 10

Another approach I have used is to change a private method to package private or protected then complement it with the @VisibleForTesting annotation of the Google Guava library.

This will tell anybody using this method to take caution and not access it directly even in a package. Also a test class need not be in same package physically, but in the same package under the test folder.

For example, if a method to be tested is in src/main/java/mypackage/MyClass.java then your test call should be placed in src/test/java/mypackage/MyClassTest.java. That way, you got access to the test method in your test class.

Elka

I didn't know about this one, it's interresting, I still think that if you need that kind of annottaion you have design issues. – Semester

To me, this is like saying instead of giving a one-off key to your fire warden to test the fire drill, you leave your front door unlocked with a sign on the door saying - "unlocked for testing - if you're not from our company, please don't enter". – Arkose

@FrJeremyKrieg - what does that mean ? – Danialah

@MasterJoe2: the purpose of a fire test is to improve the safety of your building against the risk of fire. But you have to give the warden access to your building. If you permanently leave the front door unlocked, you increase the risk of unauthorised access and make it less safe. The same is true of the VisibleForTesting pattern - you're increasing the risk of unauthorised access in order to reduce the risk of "fire". Better to grant access as a one-off for testing only when you need to (eg, using reflection) rather than leave it permanently unlocked (non-private). – Arkose

@FrJeremyKrieg leaving your door unlocked would be equivalent to making your private method public just for testing. I'm assuming you know package-private means only classes in the same package have access. Making it package-private is more like giving your garage door code to your neighbors and the fire warden only. Is it less secure than a private method? Sure. Slightly. Question: How would you grant access as a one-off through reflection? Access to private methods is available to anyone through reflection. IMHO using reflection is not any better than package-private access. – Merodach

The "if you're not from our company" thing doesn't apply for most dev shops. I've never worked at a company that left their code repos open to the outside. Unless your code repo is public (ie open-source), it's safe to assume no one with malicious intent has access. The threat really comes from internal devs who could through their code unknowingly access your sensitive methods/members, not realizing the danger. Thus the heads-up with @VisibleForTesting. Is it less secure? Sure. Slightly. But tested code is worth it. Private methods safeguard against naivete, not maliciousness. – Merodach

@SamuraiSoul thank you for your response! To be sure, the analogy is imperfect and could be tweaked (eg, reflection is more like a lockpick than a one-off code). But as you yourself said, making methods private protects against naivete - making them package-private reduces this protection. And I never argued that tested code isn't worth it - I only argued that I don't think "@VisibleForTesting" is the best way to test it. In the rare case I need to directly test private members, I always prefer using reflection than reducing my protection. – Arkose

Answer 11

To test legacy code with large and quirky classes, it is often very helpful to be able to test the one private (or public) method I'm writing right now.

I use the junitx.util.PrivateAccessor-package for Java. It has lots of helpful one-liners for accessing private methods and private fields.

import junitx.util.PrivateAccessor;

PrivateAccessor.setField(myObjectReference, "myCrucialButHardToReachPrivateField", myNewValue);
PrivateAccessor.invoke(myObjectReference, "privateMethodName", java.lang.Class[] parameterTypes, java.lang.Object[] args);

Knee

Be sure to download the entire JUnit-addons (sourceforge.net/projects/junit-addons) package, not the Source Forge-Recommended Project PrivateAccessor. – Loraineloralee

And be sure that, when you're using a Class as your first parameter in these methods, that you're accessing only static members. – Loraineloralee

Answer 12

Having tried Cem Catikkas' solution using reflection for Java, I'd have to say his was a more elegant solution than I have described here. However, if you're looking for an alternative to using reflection, and have access to the source you're testing, this will still be an option.

There is possible merit in testing private methods of a class, particularly with test-driven development, where you would like to design small tests before you write any code.

Creating a test with access to private members and methods can test areas of code which are difficult to target specifically with access only to public methods. If a public method has several steps involved, it can consist of several private methods, which can then be tested individually.

Advantages:

• Can test to a finer granularity

Disadvantages:

• Test code must reside in the same file as source code, which can be more difficult to maintain
• Similarly with .class output files, they must remain within the same package as declared in source code

However, if continuous testing requires this method, it may be a signal that the private methods should be extracted, which could be tested in the traditional, public way.

Here is a convoluted example of how this would work:

// Import statements and package declarations

public class ClassToTest
{
    private int decrement(int toDecrement) {
        toDecrement--;
        return toDecrement;
    }

    // Constructor and the rest of the class

    public static class StaticInnerTest extends TestCase
    {
        public StaticInnerTest(){
            super();
        }

        public void testDecrement(){
            int number = 10;
            ClassToTest toTest= new ClassToTest();
            int decremented = toTest.decrement(number);
            assertEquals(9, decremented);
        }

        public static void main(String[] args) {
            junit.textui.TestRunner.run(StaticInnerTest.class);
        }
    }
}

The inner class would be compiled to ClassToTest$StaticInnerTest.

See also: Java Tip 106: Static inner classes for fun and profit

Tude

Answer 13

If using Spring, ReflectionTestUtils provides some handy tools that help out here with minimal effort. For example, to set up a mock on a private member without being forced to add an undesirable public setter:

ReflectionTestUtils.setField(theClass, "theUnsettableField", theMockObject);

Flowing

Answer 14

As others have said... don't test private methods directly. Here are a few thoughts:

1. Keep all methods small and focused (easy to test, easy to find what is wrong)
2. Use code coverage tools. I like Cobertura (oh happy day, it looks like a new version is out!)

Run the code coverage on the unit tests. If you see that methods are not fully tested add to the tests to get the coverage up. Aim for 100% code coverage, but realize that you probably won't get it.

Keenankeene

Up for code coverage. No matter what kind of logic is in the private method, you are still invoking those logic through a public method. A code coverage tool can show you which parts are covered by the test, therefore you can see if your private method is tested. – Whitehurst

I have seen classes where the only public method is main[], and they pop up GUI plus connect the database and a couple of web servers worldwide. Easy to say "do not test indirectly"... – Lizbeth

Answer 15

Private methods are consumed by public ones. Otherwise, they're dead code. That's why you test the public method, asserting the expected results of the public method and thereby, the private methods it consumes.

Testing private methods should be tested by debugging before running your unit tests on public methods.

They may also be debugged using test-driven development, debugging your unit tests until all your assertions are met.

I personally believe it is better to create classes using TDD; creating the public method stubs, then generating unit tests with all the assertions defined in advance, so the expected outcome of the method is determined before you code it. This way, you don't go down the wrong path of making the unit test assertions fit the results. Your class is then robust and meets requirements when all your unit tests pass.

Contemporaneous

Although this is true, it can lead to some very complicated tests--it's a better pattern to test a single method at a time (Unit testing) rather than a whole group of them. Not a terrible suggestion but I think there are better answers here--this should be a last resort. – Dabbs

Answer 16

If you're trying to test existing code that you're reluctant or unable to change, reflection is a good choice.

If the class's design is still flexible, and you've got a complicated private method that you'd like to test separately, I suggest you pull it out into a separate class and test that class separately. This doesn't have to change the public interface of the original class; it can internally create an instance of the helper class and call the helper method.

If you want to test difficult error conditions coming from the helper method, you can go a step further. Extract an interface from the helper class, add a public getter and setter to the original class to inject the helper class (used through its interface), and then inject a mock version of the helper class into the original class to test how the original class responds to exceptions from the helper. This approach is also helpful if you want to test the original class without also testing the helper class.

Dichromaticism

Answer 17

Testing private methods breaks the encapsulation of your class because every time you change the internal implementation you break client code (in this case, the tests).

So don't test private methods.

Elka

unit test and src code are a pair. If you change the src, maybe you have to change the unit test. That is the sense of junit test. They shall garuantee that all works as before. and it is fine if they break, if you change the code. – Perrone

Do not agree that unit test should change if code changes. If you are ordered to add functionality to a class without changing the existing functionality of the class, then the exist unit tests ought to pass even after you've changed your code. As Peter mentions, unit tests ought to test the interface, not how it is done internally. In Test Driven Development unit tests are created before the code is written, to focus on the interface of the class, not on how it is solved internally. – Hedrick

Answer 18

The answer from JUnit.org FAQ page:

But if you must...

If you are using JDK 1.3 or higher, you can use reflection to subvert the access control mechanism with the aid of the PrivilegedAccessor. For details on how to use it, read this article.

If you are using JDK 1.6 or higher and you annotate your tests with @Test, you can use Dp4j to inject reflection in your test methods. For details on how to use it, see this test script.

P.S. I'm the main contributor to Dp4j. Ask me if you need help. :)

Contreras

Answer 19

PowerMockito is made for this.

Use a Maven dependency:

    <dependency>
        <groupId>org.powermock</groupId>
        <artifactId>powermock-core</artifactId>
        <version>2.0.7</version>
        <scope>test</scope>
    </dependency>

Then you can do

import org.powermock.reflect.Whitebox;
...
MyClass sut = new MyClass();
SomeType rval = Whitebox.invokeMethod(sut, "myPrivateMethod", params, moreParams);

Wheeling

Answer 20

If you want to test private methods of a legacy application where you can't change the code, one option for Java is jMockit, which will allow you to create mocks to an object even when they're private to the class.

Sclerotomy

As part of the jmockit library, you have access to the Deencapsulation class which makes testing private methods easy: Deencapsulation.invoke(instance, "privateMethod", param1, param2); – Mineralogy

I use this approach all the time. Very helpful – Predominance

Answer 21

If you're using JUnit, have a look at junit-addons. It has the ability to ignore the Java security model and access private methods and attributes.

Fulfillment

Answer 22

I tend not to test private methods. There lies madness. Personally, I believe you should only test your publicly exposed interfaces (and that includes protected and internal methods).

Ulane

Answer 23

I would suggest you refactoring your code a little bit. When you have to start thinking about using reflection or other kind of stuff, for just testing your code, something is going wrong with your code.

You mentioned different types of problems. Let's start with private fields. In case of private fields I would have added a new constructor and injected fields into that. Instead of this:

public class ClassToTest {

    private final String first = "first";
    private final List<String> second = new ArrayList<>();
    ...
}

I'd have used this:

public class ClassToTest {

    private final String first;
    private final List<String> second;

    public ClassToTest() {
        this("first", new ArrayList<>());
    }

    public ClassToTest(final String first, final List<String> second) {
        this.first = first;
        this.second = second;
    }
    ...
}

This won't be a problem even with some legacy code. Old code will be using an empty constructor, and if you ask me, refactored code will look cleaner, and you'll be able to inject necessary values in test without reflection.

Now about private methods. In my personal experience when you have to stub a private method for testing, then that method has nothing to do in that class. A common pattern, in that case, would be to wrap it within an interface, like Callable and then you pass in that interface also in the constructor (with that multiple constructor trick):

public ClassToTest() {
    this(...);
}

public ClassToTest(final Callable<T> privateMethodLogic) {
    this.privateMethodLogic = privateMethodLogic;
}

Mostly all that I wrote looks like it's a dependency injection pattern. In my personal experience it's really useful while testing, and I think that this kind of code is cleaner and will be easier to maintain. I'd say the same about nested classes. If a nested class contains heavy logic it would be better if you'd moved it as a package private class and have injected it into a class needing it.

There are also several other design patterns which I have used while refactoring and maintaining legacy code, but it all depends on cases of your code to test. Using reflection mostly is not a problem, but when you have an enterprise application which is heavily tested and tests are run before every deployment everything gets really slow (it's just annoying and I don't like that kind of stuff).

There is also setter injection, but I wouldn't recommended using it. I'd better stick with a constructor and initialize everything when it's really necessary, leaving the possibility for injecting necessary dependencies.

Nonunionism

Disagree with the ClassToTest(Callable) injection. That makes ClassToTest more complicated. Keep it simple. Also, that then requires someone else to tell ClassToTest something that ClassToTest should be the boss of. There is a place for injecting logic, but this is not it. You have just made the class harder to maintain, not easier. – Ptyalism

Also, it is preferable if your method of testing X does not increase X complexity to the point where it might cause more problems... and therefore require additional testing... which if you've implemented in a way that can cause more problems... (this is not an infinite loops; each iteration is likely smaller than the one before it, but still annoying) – Ptyalism

Could you explain why would it make class ClassToTest harder to maintain? Actually it makes your application easier to maintain, what do you suggest making new class for every different value you'll need in first and 'second' variables? – Nonunionism

Method of testing doesn't increase complexity. It's just your class which is poorly written, so poorly that it can't be tested. – Nonunionism

Harder to maintain because the class is more complicated. class A{ A(){} f(){} } is simpler than class A { Logic f; A(logic_f) } class B { g() { new A( logic_f) } }. If that were not true, and if it were true that supplying a class' logic as constructor arguments were easier to maintain, then we would pass all logic as constructor arguments. I just don't see how you can claim class B{ g() { new A( you_dont_know_your_own_logic_but_I_do ) } } ever makes A easier to maintain. There are cases where injecting like this makes sense, but I don't see your example as "easier to maintain" – Ptyalism

As for "what do you suggest making new class for every different value you'll need"... I don't understand how that follows. No. I'm saying keep it simple. class A { M m; N n; f() { } } super simple. Don't break from that unless you have a reason. If you had to in order to test private members, sure, fine; I just don't see how you can claim it is "easier to maintain." Dependency injection only makes things easier to maintain when it reduces complexity (decouples classes, makes graphs/charts easier to follow, something), which it did not in your example. – Ptyalism

Note that I am not suggesting that your actual answer is invalid. Supplying the logic from the outside can in fact answer the original question. I am only commenting on a specific statement within the answer, a statement which the answer does not hinge on. – Ptyalism

Actually, on third (or fourth?) read over, I see I may have missed a point. In your no-arg public ClassToTest() { this(...); }, is that implying that the class no-arg constructor is generally used, that it can supply its own logic known internally to the class, and the other constructor used only for testing (or if B wants to change its behavior for some reason; a handy side effect)? I did not catch that the first couple reads over. – Ptyalism

Yes, I gotcha now. The fact that you do not have a default implementation known internally to ClassToTest threw me off, but you do have it implied by the ellipse. For some reason, I was originally thinking you intended that the logic must be supplied only from the outside. For suckers like me, could you just edit the "..." to be something like "defaultLogic"; assuming I understand you correctly now. – Ptyalism

Answer 24

Please see below for an example;

The following import statement should be added:

import org.powermock.reflect.Whitebox;

Now you can directly pass the object which has the private method, method name to be called, and additional parameters as below.

Whitebox.invokeMethod(obj, "privateMethod", "param1");

Rossner

Answer 25

Here is my generic function to test private fields:

protected <F> F getPrivateField(String fieldName, Object obj)
    throws NoSuchFieldException, IllegalAccessException {
    Field field =
        obj.getClass().getDeclaredField(fieldName);

    field.setAccessible(true);
    return (F)field.get(obj);
}

Haga

Answer 26

A private method is only to be accessed within the same class. So there is no way to test a “private” method of a target class from any test class. A way out is that you can perform unit testing manually or can change your method from “private” to “protected”.

And then a protected method can only be accessed within the same package where the class is defined. So, testing a protected method of a target class means we need to define your test class in the same package as the target class.

If all the above does not suits your requirement, use the reflection way to access the private method.

Ollie

You're mixing "protected" with "friendly." A protected method can only be accessed by a class whose objects are assignable to the target class (i.e subclasses). – Photograph

Actually there is no "Friendly" in Java, the term is "Package" and it's indicated by the lack of a private/public/protected modifier. I would have just corrected this answer but there is a really good one that already says this--so I'd just recommend deleting it. – Dabbs

I almost downvoted, thinking all along "This answer is just plain wrong." until I got to the very last sentence, which contradicts the rest of the answer. The last sentence should have been the first. – Ptyalism

Answer 27

Today, I pushed a Java library to help testing private methods and fields. It has been designed with Android in mind, but it can really be used for any Java project.

If you got some code with private methods or fields or constructors, you can use BoundBox. It does exactly what you are looking for. Here below is an example of a test that accesses two private fields of an Android activity to test it:

@UiThreadTest
public void testCompute() {

    // Given
    boundBoxOfMainActivity = new BoundBoxOfMainActivity(getActivity());

    // When
    boundBoxOfMainActivity.boundBox_getButtonMain().performClick();

    // Then
    assertEquals("42", boundBoxOfMainActivity.boundBox_getTextViewMain().getText());
}

BoundBox makes it easy to test private/protected fields, methods and constructors. You can even access stuff that is hidden by inheritance. Indeed, BoundBox breaks encapsulation. It will give you access to all that through reflection, but everything is checked at compile time.

It is ideal for testing some legacy code. Use it carefully. ;)

Seabolt

Answer 28

As many above have suggested, a good way is to test them via your public interfaces.

If you do this, it's a good idea to use a code coverage tool (like EMMA) to see if your private methods are in fact being executed from your tests.

Elka

You should not indirectly test! Not only touching via coverage ; test that the expected result is delivered! – Perrone

Answer 29

First, I'll throw this question out: Why do your private members need isolated testing? Are they that complex, providing such complicated behaviors as to require testing apart from the public surface? It's unit testing, not 'line-of-code' testing. Don't sweat the small stuff.

If they are that big, big enough that these private members are each a 'unit' large in complexity—consider refactoring such private members out of this class.

If refactoring is inappropriate or infeasible, can you use the strategy pattern to replace access to these private member functions / member classes when under unit test? Under unit test, the strategy would provide added validation, but in release builds it would be simple passthrough.

Pinson

Because often a particular piece of code from a public method is refactored into an internal private method and is really the critical piece of logic which you might have got wrong. You want to test this independently from the public method – Postoperative

Even the shortest code sometimes without unit test is not correct. Just try to caluclate the difference between 2 geograhpical angles. 4 lines of code, and most will not do it correct at first try. Such methods needs unit test, because the form the base of a trustfull code. (Ans such usefull code can be public, too; less usefull protected – Perrone

Answer 30

I want to share a rule I have about testing which particularly is related to this topic:

I think that you should never adapt production code in order to indulge easer writing of tests.

There are a few suggestions in other posts saying you should adapt the original class in order to test a private method - please red this warning first.

If we change the accessibility of a method/field to package private or protected, just in order to have it accessible to tests, then we defeat the purpose of existence of private access directive.

Why should we have private fields/methods/classes at all when we want to have test-driven development? Should we declare everything as package private, or even public then, so we can test without any effort?—I don't think so.

From another point of view: Tests should not burden performance and execution of the production application.

If we change production code just for the sake of easier testing, that may burden performance and the execution of the application in some way.

If someone starts to change private access to package private, then a developer may eventually come up to other "ingenious ideas" about adding even more code to the original class. This would make additional noise to readability and can burden the performance of the application.

With changing of a private access to some less restrictive, we are opening the possibility to a developer for misusing the new situation in the future development of the application. Instead of enforcing him/her to develop in the proper way, we are tempting him/her with new possibilities and giving him ability to make wrong choices in the future.

Of course there might be a few exceptions to this rule, but with clear understanding, what is the rule and what is the exception? We need to be absolutely sure we know why that kind of exception is introduced.

Km

While this is a good principle, another is that the code should be testable. If the transformation the code should do is simple to understand and break down into small pieces (something like parsing JSON), testing the public API should be sufficient. If the code does several complicated calculations to produce its final result (e.g., a Kalman Filter) then being able to test the individual steps in the algorithm can make it easier to test. In particular, testing the private methods can indicate where the problem is. – Galaxy

Answer 31

I recently had this problem and wrote a little tool, called Picklock, that avoids the problems of explicitly using the Java reflection API, two examples:

Calling methods, e.g. private void method(String s) - by Java reflection

Method method = targetClass.getDeclaredMethod("method", String.class);
method.setAccessible(true);
return method.invoke(targetObject, "mystring");

Calling methods, e.g. private void method(String s) - by Picklock

interface Accessible {
  void method(String s);
}

...
Accessible a = ObjectAccess.unlock(targetObject).features(Accessible.class);
a.method("mystring");

Setting fields, e.g. private BigInteger amount; - by Java reflection

Field field = targetClass.getDeclaredField("amount");
field.setAccessible(true);
field.set(object, BigInteger.valueOf(42));

Setting fields, e.g. private BigInteger amount; - by Picklock

interface Accessible {
  void setAmount(BigInteger amount);
}

...
Accessible a = ObjectAccess.unlock(targetObject).features(Accessible.class);
a.setAmount(BigInteger.valueOf(42));

Andromede

Answer 32

In C++: before including the class header that has a private function that you want to test.

Use this code:

#define private public
#define protected public

Elka

Ouch. I guess it works fine, but I'd prefer using friend declarations instead of messing around with #defines on such a basic level. – Among

Yeah that'll work...but holy **** is that (likely) awful for your design – Ostiary

FYI this no longer works in VS2019, it is not allowed to use macros that way any longer. – Gca

Answer 33

For Java I'd use reflection, since I don't like the idea of changing the access to a package on the declared method just for the sake of testing. However, I usually just test the public methods which should also ensure the private methods are working correctly.

you can't use reflection to get private methods from outside the owner class, the private modifier affects reflection also

This is not true. You most certainly can, as mentioned in Cem Catikkas's answer.

Gillian

you can't use reflection to get private methods from outside the owner class, the private modifier affects reflection also Here's a good article on the question's subject – Rociorock

@jmfsg The article you link to link to specifically says "Testing private methods is a little more involved; but we can still do it using System.Reflection." (Apparently you need ReflectionPermission, but that's not normally a problem.) – Kinchinjunga

Answer 34

I think most answers are too crude. It depends on your code if you should test private methods or not.

I used private method testing in the past, and I did this by reflection. It works for me. I realize the problems with it, but for me it was the best solution.

I have a heavy application that simulates human behaviour in a population of over a million persons. Every person is represented by an object. The main purpose of the application is to follow how something (disease, information, ideas) spreads through the population.

For this I have methods that pass on the disease or information from one object to another. There is absolutely no reason to make these methods public, for the end user is not interested in one pass from person to person. The end user is only interested in the grand picture of how it spreads through the population. So those methods are private.

But I want to know for sure if the single act of passing one bit of information from one person to another is doing what it should. Testing it via public user interfaces is also not possible, because it's just not public, and I think it's awkward to make it public just for the purpose of testing. The end output of the application is defined by hundreds of millions of such single steps being performed all the time. I cannot test the end output either, because that has complicated stochastability involved, which makes it too unpredictable to test.

So the way for me to test the application is by testing a single step of passing information from one person to another person. These are all private methods. And so I use reflection to test that.

I'm telling this story to show that it's not that plain black and white story. It depends on your application what is best. Under some circumstances, testing private methods via reflection might be your best option.

If some people here know better solutions in my use case, I'd happily stand corrected of course....

Claudclauddetta

Answer 35

Android has the @VisibleForTesting annotation from android.support.annotation package.

The @VisibleForTesting annotation indicates that an annotated method is more visible than normally necessary to make the method testable. This annotation has an optional otherwise argument that lets you designate what the visibility of the method should have been if not for the need to make it visible for testing. Lint uses the otherwise argument to enforce the intended visibility.

In practice, it means that you should make a method open for testing and the @VisibleForTesting annotation will show a warning.

For example

package com.mypackage;

public class ClassA {

    @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
    static void myMethod() {

    }
}

And when you call ClassA.myMethod() within the same package(com.mypackage) you will see the warning.

Spotter

Answer 36

You can turn off Java access restrictions for reflection so that private means nothing.

The setAccessible(true) call does that.

The only restriction is that a ClassLoader may disallow you from doing that.

See Subverting Java Access Protection for Unit Testing (Ross Burton) for a way to do this in Java.

Keenankeene

Answer 37

Use this utility class if you are on Spring:

ReflectionTestUtils.invokeMethod(new ClassName(), "privateMethodName");

Glassware

Answer 38

Here is my Lombok sample:

public static void main(String[] args) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
    Student student = new Student();

    Field privateFieldName = Student.class.getDeclaredField("name");
    privateFieldName.setAccessible(true);
    privateFieldName.set(student, "Naruto");

    Field privateFieldAge = Student.class.getDeclaredField("age");
    privateFieldAge.setAccessible(true);
    privateFieldAge.set(student, "28");

    System.out.println(student.toString());

    Method privateMethodGetInfo = Student.class.getDeclaredMethod("getInfo", String.class, String.class);
    privateMethodGetInfo.setAccessible(true);
    System.out.println(privateMethodGetInfo.invoke(student, "Sasuke", "29"));
}


@Setter
@Getter
@ToString
class Student {
    private String name;
    private String age;
    
    private String getInfo(String name, String age) {
        return name + "-" + age;
    }
}

Handle

The text is incomprehensible (use of machine translation?). Can you fix it? (But ************** without ************** "Edit:", "Update:", or similar - the answer should appear as if it was written today.) – Elka

Answer 39

The best and proper legal way to test a Java private method from a test framework is the @VisibleForTesting annotation over the method, so the same method will be visible for the test framework like a public method.

Elka

Really like this option and need to evaluate if this works for me / us :) – Faithfaithful

Answer 40

If you are worried by not testing private methods as many of the posts suggest, consider that a code coverage tool will determine exactly how much of your code is tested and where it leaks, so it is quantifiably acceptable to do this.

Answers that direct the author of the question towards a 'workaround' are doing a massive disservice to the community. Testing is a major part of all engineering disciplines. You would not want to buy a car that is not properly tested, and tested in a meaningful way, so why would anyone want to buy or use software that is tested poorly? The reason people do this anyway is probably because the effects of badly tested software are felt way after the fact, and we don't usually associate them with bodily harm.

This is a very dangerous perception which will be difficult to change, but it is our responsibility to deliver safe products regardless of what even management is bullying us to do. Think the Equifax hack...

We must strive to build an environment that encourages good software engineering practices. This does not mean ostracizing the weak/lazy among us who do not take their craft seriously, but rather, to create a status quo of accountability and self reflection that would encourage everyone to pursue growth, both mentally and skillfully.

I am still learning, and may have wrong perceptions/opinions myself, but I do firmly believe that we need to hold ourselves accountable to good practices and shun irresponsible hacks or workarounds to problems.

Elka

You make good points (some/most of which have already been made by others), but please try to avoid antagonizing by using words such as "pseudo developers/engineers". As you said (and I agree), we should not ostracize, but instead create a better status quo and help everyone to learn and improve. – Timehonored

@Timehonored Thanks for pointing that out. I recall when I was writing this response that I was frustrated at the lack of resources (or perception thereof) to learn good practices. Having barely graduated college at that time, I was seeking a criterion that could guide me on how to write good code. I agree that the language I used was not ok and I have edited that out. Thank you. – Simultaneous

This is presumptive and arrogant. You cannot possibly know the reasons why someone might want to test a private method or introspect a private field. Even if it is best practice to not directly test private methods you don't need elitist drivel to make your point. – Calore

Answer 41

A quick addition to Cem Catikka's answer, when using ExpectedException:

Keep in mind that your expected exception will be wrapped in an InvocationTargetException, so in order to get to your exception you will have to throw the cause of the InvocationTargetException you received. Something like (testing private method validateRequest() on BizService):

@Rule
public ExpectedException thrown = ExpectedException.none();

@Autowired(required = true)
private BizService svc;


@Test
public void testValidateRequest() throws Exception {

    thrown.expect(BizException.class);
    thrown.expectMessage(expectMessage);

    BizRequest request = /* Mock it, read from source - file, etc. */;
    validateRequest(request);
}

private void validateRequest(BizRequest request) throws Exception {
    Method method = svc.getClass().getDeclaredMethod("validateRequest", BizRequest.class);
    method.setAccessible(true);
    try {
        method.invoke(svc, request);
    }
    catch (InvocationTargetException e) {
        throw ((BizException)e.getCause());
    }
 }

Indeterminate

Answer 42

I am not sure whether this is a good technique, but I developed the following pattern to unit test private methods:

I don't modify the visibility of the method that I want to test and add an additional method. Instead I am adding an additional public method for every private method I want to test. I call this additional method Test-Port and denote them with the prefix t_. This Test-Port method then simply accesses the according private method.

Additionally, I add a Boolean flag to the Test-Port method to decide whether I grant access to the private method through the Test-Port method from outside. This flag is then set globally in a static class where I place e.g. other global settings for the application. So I can switch the access to the private methods on and off in one place, e.g., in the corresponding unit test.

Colley

Answer 43

I feel exactly the same ... change the access modifier for a method just to be able to run a test looks for me as a bad idea. Also in our company we had a lot of discussions about it and in my opinion, really nice way to test a private method is to using Java reflection or another framework which making the method testable. I did so multiple times for complex private methods and this helped me to keep the test small, readable and maintainable.

After I have read all the answers here, I just disagree with persons who say "if you need to test a private method, then there is a code smell" or even "don´t test private methods" ... so I have a little example for you:

Imagine I have a class with one public method and a couple of private methods:

public class ConwaysGameOfLife {

    private boolean[][] generationData = new boolean[128][128];

    /**
     * Compute the next generation and return the new state
     * Also saving the new state in generationData
     */
    public boolean[][] computeNextGeneration() {
        boolean[][] tempData = new boolean[128][128];

        for (int yPos=0; yPos<=generationData.length; yPos++) {
            for (int xPos=0; xPos<=generationData[yPos].length; xPos++) {
                int neighbors = countNeighbors(yPos, xPos);
                tempData[yPos][xPos] = determineCellState(neighbors, yPos, xPos);
            }
        }

        generationData = tempData;
        return generationData;
    }

    /**
     * Counting the neighbors for a cell on given position considering all the edge cases
     *
     * @return the amount of found neighbors for a cell
     */
    private int countNeighbors(int yPos, int xPos) {}

    /**
     * Determine the cell state depending on the amount of neighbors of a cell and on a current state of the cell
     *
     * @return the new cell state
     */
    private boolean determineCellState(int neighborsAmount, int yPos, int xPos) {}
}

So at least for the method "countNeighbors" I need to test eight edge cases and also some general cases (cells directly in the corners, cells directly on the edges of the matrix and cells somewhere in the middle). So if I am only trying to cover all the cases through the "computeNextGeneration" method and after refactoring, some tests are red, it is probably time consuming to identify the place where the bug is.

If I am testing "determineCellState" and "countNeighbors" separately and after refactoring and optimization, tests of "computeNextGeneration" and "determineCellState" are red, I am pretty sure that the error will be in the "determineCellState" method.

Also, if you write unit tests for those methods from the beginning, this tests will help you to develop the methods / the algorithms without a need to considering and wrapping other method calls and cases inside the public method. You can just write fast small tests covering your cases in the method ... for example if the test with the name "countNeighbors_should_return_right_amount_of_neighbors_for_the_right_top_corner_cell()" fails, then it is pretty clear where to look for the bug.

Faithfaithful

I completely agree with you. One of the typical programming patterns is to have small public methods that deals with high level logic and offloads heavier logic(implementation details) to private methods like in your example. Such private logic would be in most cases nightmare to test through public parent method, and for those situations reflection is obvious choice. Changing visibility modifiers to make code testable is crazy when you have the adequate tooling to test code without changeing contract. Reflection in this case is completely justified in my opinion! – Suspect

Answer 44

PowerMock.Whitebox is the best option I have seen, but when I read its source code, it reads private fields with reflection, so I think I have my answer:

• test private internal states (fields) with PowerMock, or just reflection without the overhead of introducing another independency

• for private methods: actually, the upvote for this question itself, and the huge number of comments and answers, shows that it is a very concurrent and controversial topic where no definite answer could be given to suit every circumstance. I understand that only the contract should be tested, but we also have coverage to consider. Actually, I doubt that only testing contracts will 100% make a class immune to errors. Private methods are those who process data in the class where it is defined and thus does not interest other classes, so we cannot simply expose to make it testable. I will try not to test them, but when you have to, just go for it and forget all the answers here. You know better your situation and restrictions than any other one on the Internet. When you have control over your code, use that. With consideration, but without overthinking.

---

After some time, when I reconsider it, I still believe this is true, but I saw better approaches.

First of all, Powermock.Whitebox is still usable.

And, Mockito Whitebox has been hidden after v2 (the latest version I can find with Whitebox is testImplementation 'org.mockito:mockito-core:1.10.19') and it has always been part of org.mockito.internal package, which is prone of breaking changes in the future (see this post). So now I tend not to use it.

In Gradle/Maven projects, if you define private methods or fields, there aren't any other ways than reflection to get access to them, so the first part stays true. But, if you change the visibility to "package private", the tests following the same structure in test package will have access to them. That is also another important reason why we are encouraged to create the same hierarchy in main and test package. So, when you have control over production code as well as tests, delete that private access modifier may be the best option for you because relatively it does not cause huge impact. And, that makes testing as well as private method spying possible.

@Autowired
private SomeService service; // With a package private method "doSomething()"

@Test
void shouldReturnTrueDoSomething() {
    assertThat(doSomething(input), is(true)); // Package private method testing
}

@Test
void shouldReturnTrueWhenServiceThrowsException() {
    SomeService spy = Mockito.spy(service); // Spying real object
    doThrow(new AppException()).when(spy).doSomething(input); // Spy package private method
    ...

}

When it comes to internal fields, in Spring you have ReflectionUtils.setField().

At last, sometimes we can bypass the problem itself: if there is a coverage requirement to meet, maybe you can move these private methods into an inner static class and ignore this class in Jacoco. I just found some way to ignore the inner class in Jacoco Gradle tasks. Another question.

Lookin

Answer 45

You can create a special public method to proxy the private method to test. The @TestOnly annotation is out of the box available when using IntelliJ. The downside is is that if somebody want to to use the private method in a public context, he can do it. But he will be warned by the annotation and the method name. On IntelliJ a warning will appear when doing it.

import org.jetbrains.annotations.TestOnly

class MyClass {

    private void aPrivateMethod() {}

    @TestOnly
    public void aPrivateMethodForTest() {
        aPrivateMethod()
    }
}

Bluepoint

Also i like this option and need to evaluate how it feels in the code itself. Unfortunately not all colleges use intelliJ but some also uses Eclipse or VSCode. So probably this Annotation is not an option ./ – Faithfaithful

Answer 46

If you have a case where you really need to test a private method/class etc.. directly, you can use reflection as already mentioned in other answers. However if it comes to that, instead of dealing directly with reflection I'd rather use utility classes provided by your framework. For instance, for Java we have:

• ReflectionUtils in Spring Framework

• ReflectionUtils in JUnit

As per how to use them, you can find plenty of articles online. Here one that I particularly liked:

• Java Code Examples for org.springframework.util.ReflectionUtils

Elka

Answer 47

JML has a spec_public comment annotation syntax that allows you to specify a method as public during tests:

private /*@ spec_public @*/ int methodName(){
...
}

This syntax is discussed at 2.4 Privacy Modifiers and Visibility. There also exists a program that translates JML specifications into JUnit tests. I'm not sure how well that works or what its capabilities are, but it doesn't appear to be necessary since JML is a viable testing framework on its own.

Noontide

Answer 48

What if your test classes are in the same package as the class that should be tested?

But in a different directory of course, src & classes for your source code, test/src and test/classes for your test classes. And let classes and test/classes be in your classpath.

Riesling

tests can be in different folders (like src/test/java and app code in src/main/java) but the package needs to be the same. – Raja

Answer 49

In C# you could have used System.Reflection, though in Java I don't know. If you "feel you need to unit test private methods", my guess is that there is something else which is wrong...

I would seriously consider looking at my architecture again with fresh eyes...

Elka

Answer 50

If you are only using Mockito:

You can consider the private method as a part of public method being tested. You can make sure you cover all the cases in private method when testing public method.

Suppose you are a Mockito-only user (you are not allowed or don't want to use PowerMock or reflection or any such tools) and you don’t want to change the existing code or libraries being tested, this might be the best way.

The only thing you need to handle if you choose this way is the variables (user-defined objects) declared locally within private methods. If the private method depends on locally declared variable objects and their methods, make sure you declare those user-defined objects globally as private object instead of locally declared objects. You can instantiate these objects locally.

This allows you to mock these objects and inject them back to testing object. You can also mock (using when/then) their methods.

This will allow you test private method without errors when testing the public method.

Advantages

1. Code coverage
2. Able to test the complete private method.

Disadvantages

1. Scope of the object—if you don't want the object to be exposed to other methods within same class, this might not be your way.
2. You might end up testing the private method multiple times when invoked at different public methods and/or in the same method multiple times.

Reprehend

Answer 51

For C++ (since C++11) adding the test class as a friend works perfectly and does not break production encapsulation.

Let's suppose that we have some class Foo with some private functions which really require testing, and some class FooTest that should have access to Foo's private members. Then we should write the following:

// prod.h: some production code header

// forward declaration is enough
// we should not include testing headers into production code
class FooTest;

class Foo
{
  // that does not affect Foo's functionality
  // but now we have access to Foo's members from FooTest
  friend FooTest;
public:
  Foo();
private:
  bool veryComplicatedPrivateFuncThatReallyRequiresTesting();
}

// test.cpp: some test
#include <prod.h>

class FooTest
{
public:
  void complicatedFisture() {
    Foo foo;
    ASSERT_TRUE(foo.veryComplicatedPrivateFuncThatReallyRequiresTesting());
  }
}

int main(int /*argc*/, char* argv[])
{
  FooTest test;
  test.complicatedFixture();  // and it really works!
}

Windbroken

There is also a template trick which allows to get to private members (through some pointer indirection IIRC) or just -fno-access-control (which just ignores private and protected restrictions). – Hotze

Answer 52

There is another approach to test your private methods.

If you "enable assertion" in run configurations then you can unit test your method inside the method itself. For example;

assert ("Ercan".equals(person1.name));
assert (Person.count == 2);

Sauternes

Answer 53

My team and I are using Typemock, which has an API that allows you to fake non-public methods.

Recently they added the ability to fake non-visible types and to use xUnit.

Vanburen

Answer 54

You can use PowerMockito to set return values for private fields and private methods that are called/used in the private method you want to test:

For example, setting a return value for a private method:

MyClient classUnderTest = PowerMockito.spy(new MyClient());

// Set the expected return value
PowerMockito.doReturn(20).when(classUnderTest, "myPrivateMethod", anyString(), anyInt());
// This is very important. Otherwise, it will not work
classUnderTest.myPrivateMethod();

// Setting the private field value as someValue:
Whitebox.setInternalState(classUnderTest, "privateField", someValue);

Then finally you can validate your private method under test is returning the correct value based on set values above by:

String msg = Whitebox.invokeMethod(obj, "privateMethodToBeTested", "param1");
Assert.assertEquals(privateMsg, msg);

Or

If the classUnderTest private method does not return a value, but it sets another private field then you can get that private field value to see if it was set correctly:

// To get the value of a private field
MyClass obj = Whitebox.getInternalState(classUnderTest, "foo");
assertThat(obj, is(notNull(MyClass.class))); // Or test value

Wahlstrom

Answer 55

Define implementation class where the reason of public methods to be public is that they implement the clearly defined functionality, rather than if they are accessed from the outside of the class or not. If the functionality you want to test conveniently fits into a single method, make it into public function.

If you write the algorithm for exactly sum, there is nothing great in design the has a private sum() method returning sum minus one, to accomodate "one off" somewhere else.

The "production" class may have third party classes that require license dongle, call network services, produce non deterministic output, use specific hardware and the like. It may be totally no way how this could be unit tested.

Cataclysmic

Answer 56

The following Reflection TestUtil could be used generically to test the private methods for their atomicity.

import com.google.common.base.Preconditions;

import org.springframework.test.util.ReflectionTestUtils;

/**
 * <p>
 * Invoker
 * </p>
 *
 * @author
 * @created Oct-10-2019
 */
public class Invoker {
    private Object target;
    private String methodName;
    private Object[] arguments;

    public <T> T invoke() {
        try {
            Preconditions.checkNotNull(target, "Target cannot be empty");
            Preconditions.checkNotNull(methodName, "MethodName cannot be empty");
            if (null == arguments) {
                return ReflectionTestUtils.invokeMethod(target, methodName);
            } else {
                return ReflectionTestUtils.invokeMethod(target, methodName, arguments);
            }
        } catch (Exception e) {
           throw e;
        }
    }

    public Invoker withTarget(Object target) {
        this.target = target;
        return this;
    }

    public Invoker withMethod(String methodName) {
        this.methodName = methodName;
        return this;
    }

    public Invoker withArguments(Object... args) {
        this.arguments = args;
        return this;
    }

}

Object privateMethodResponse = new Invoker()
  .withTarget(targetObject)
  .withMethod(PRIVATE_METHOD_NAME_TO_INVOKE)
  .withArguments(arg1, arg2, arg3)
  .invoke();
Assert.assertNotNutll(privateMethodResponse)

Pettifogging

Answer 57

Groovy has a bug/feature, through which you can invoke private methods as if they were public. So if you're able to use Groovy in your project, it's an option you can use in lieu of reflection. Check out this page for an example.

Harriettharrietta

That is a hack not a real solution. – Domela

@Domela I disagree...this is one of the reasons testing Java code using Groovy is so powerful. – Intinction

Answer 58

I only test the public interface, but I have been known to make specific private methods protected so I can either mock them out entirely, or add in additional steps specific for unit testing purposes. A general case is to hook in flags I can set from the unit test to make certain methods intentionally cause an exception to be able to test fault paths; the exception triggering code is only in the test path in an overridden implementation of the protected method.

I minimize the need for this though and I always document the precise reasons to avoid confusion.

Tildatilde

Answer 59

In your class:

namespace my_namespace {
    #ifdef UNIT_TEST
        class test_class;
    #endif

    class my_class {
        public:
            #ifdef UNIT_TEST
                friend class test_class;
            #endif
        private:
            void fun() { cout << "I am private" << endl; }
    }
}

In your unit test class:

#ifndef UNIT_TEST
    #define UNIT_TEST
#endif

#include "my_class.h"

class my_namespace::test_class {
    public:
        void fun() { my_obj.fun(); }
    private:
        my_class my_obj;
}

void my_unit_test() {
    test_class test_obj;
    test_obj.fun(); // here you accessed the private function ;)
}

Taxiway

question is about java-this answer is for C++ – Anglaangle