JUnit Testing private variables? [duplicate]

I have been assigned the task of unit testing a class that I never worked directly on with JUnit, and am strictly forbidden to change the code in the package. This is usually no issue, since most of our unit testing is just for functionality and input/output consistency, which can be done simply by running routines and checking their return values.

However, occasionally there is a need to check a private variable within the class, or directly edit a private variable to check some internal behavior. Is there a way to gain access to these, whether through JUnit or any other way, for the purpose of unit testing without actually changing any of the code in the original source package? And if not, how do programmers handle this issue in the real world where a unit tester may not be the same person as the coder?

Solution 1:

First of all, you are in a bad position now - having the task of writing tests for the code you did not originally create and without any changes - nightmare! Talk to your boss and explain, it is not possible to test the code without making it "testable". To make code testable you usually do some important changes;

Regarding private variables. You actually never should do that. Aiming to test private variables is the first sign that something wrong with the current design. Private variables are part of the implementation, tests should focus on behavior rather of implementation details.

Sometimes, private field are exposed to public access with some getter. I do that, but try to avoid as much as possible (mark in comments, like 'used for testing').

Since you have no possibility to change the code, I don't see possibility (I mean real possibility, not like Reflection hacks etc.) to check private variable.

Solution 2:

Yeah you can use reflections to access private variables. Altough not a good idea.

Check this out:

http://en.wikibooks.org/wiki/Java_Programming/Reflection/Accessing_Private_Features_with_Reflection

Solution 3:

Reflection e.g.:

public class PrivateObject {

  private String privateString = null;

  public PrivateObject(String privateString) {
    this.privateString = privateString;
  }
}
PrivateObject privateObject = new PrivateObject("The Private Value");

Field privateStringField = PrivateObject.class.
            getDeclaredField("privateString");

privateStringField.setAccessible(true);

String fieldValue = (String) privateStringField.get(privateObject);
System.out.println("fieldValue = " + fieldValue);