[java] annotation to make a private method public only for test classes

Who has a solution for that common need.

I have a class in my application.

some methods are public, as they are part of the api, and some are private, as they for internal use of making the internal flow more readable

now, say I want to write a unit test, or more like an integration test, which will be located in a different package, which will be allowed to call this method, BUT, I want that normal calling to this method will not be allowed if you try to call it from classes of the application itself

so, I was thinking about something like that

public class MyClass {

   public void somePublicMethod() {
    ....
   }

   @PublicForTests
   private void somePrivateMethod() {
    ....
   }
}

The annotation above will mark the private method as "public for tests" which means, that compilation and runtime will be allowed for any class which is under the test... package , while compilation and\or runtime will fail for any class which is not under the test package.

any thoughts? is there an annotation like this? is there a better way to do this?

it seems that the more unit tests you write, to more your inforced to break your encapsulation...

I am not aware of any such annotation, however the following may be of value: unit testing private methods
or the following: JMockit

Okay, so here we have two things that are being mixed. First thing, is when you need to mark something to be used only on test, which I agree with @JB Nizet, using the guava annotation would be good.

A different thing, is to test private methods. Why should you test private methods from the outside? I mean.. You should be able to test the object by their public methods, and at the end that its behavior. At least, that we are doing and trying to teach to junior developers, that always try to test private methods (as a good practice).

dp4j has what you need. Essentially all you have to do is add dp4j to your classpath and whenever a method annotated with @Test (JUnit's annotation) calls a method that's private it will work (dp4j will inject the required reflection at compile-time). You may also use dp4j's @TestPrivates annotation to be more explicit.

If you insist on also annotating your private methods you may use Google's @VisibleForTesting annotation.

An article on Testing Private Methods lays out some approaches to testing private code. using reflection puts extra burden on the programmer to remember if refactoring is done, the strings aren't automatically changed, but I think it's the cleanest approach.

Or you can extract this method to some strategy object. In this case you can easily test extracted class and don't make method public or some magic with reflection/bytecode.

I just put the test in the class itself by making it an inner class: https://rogerkeays.com/how-to-unit-test-private-methods

Consider using interfaces to expose the API methods, using factories or DI to publish the objects so the consumers know them only by the interface. The interface describes the published API. That way you can make whatever you want public on the implementation objects and the consumers of them see only those methods exposed through the interface.

You can't do this, since then how could you even compile your tests? The compiler won't take the annotation into account.

There are two general approaches to this

The first is to use reflection to access the methods anyway

The second is to use package-private instead of private, then have your tests in the same package (but in a different module). They will essentially be private to other code, but your tests will still be able to access them.

Of course, if you do black-box testing, you shouldn't be accessing the private members anyway.

As much as I know there is no annotation like this. The best way is to use reflection as some of the others suggested. Look at this post:
How do I test a class that has private methods, fields or inner classes?

You should only watch out on testing the exception outcome of the method. For example: if u expect an IllegalArgumentException, but instead you'll get "null" (Class:java.lang.reflect.InvocationTargetException).
A colegue of mine proposed using the powermock framework for these situations, but I haven't tested it yet, so no idea what exactly it can do. Although I have used the Mockito framework that it is based upon and thats a good framework too (but I think doesn't solve the private method exception issue).

It's a great idea though having the @PublicForTests annotation.

Cheers!

We recently released a library that helps a lot to access private fields, methods and inner classes through reflection : BoundBox

For a class like

public class Outer {
    private static class Inner {
        private int foo() {return 2;}
    }
}

It provides a syntax like :

Outer outer = new Outer();
Object inner = BoundBoxOfOuter.boundBox_new_Inner();
new BoundBoxOfOuter.BoundBoxOfInner(inner).foo();

The only thing you have to do to create the BoundBox class is to write @BoundBox(boundClass=Outer.class) and the BoundBoxOfOuter class will be instantly generated.

If your test coverage is good on all the public method inside the tested class, the privates methods called by the public one will be automatically tested since you will assert all the possible case.

The JUnit Doc says:

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

The common way is to make the private method protected or package-private and to put the unit test for this method in the same package as the class under test.

Guava has a @VisibleForTesting annotation, but it's only for documentation purposes.