I've been playing with TestNG for a few days now. I really like it. It makes me want to write more tests.

I was wondering if there was a way to create a test for a private method, so I went to the source. I asked Cédric about it. He mentioned that I should use the Reflection API.

The goal was to write a test for the validate method in the following class:

package net.thauvin.erik;

public class Example {
  // …
  
  private static int validate(String username, String password, int days) {
    // …
    
    return days;
  }
}

In order to invoke the validate method in my test, I simply needed to:

• Retrieve the method using Class.getDeclaredMethod()
• Make it accessible using Method.setAccessible()

as follows:

package net.thauvin.erik;

import org.testng.annotations.*;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class ExampleTest {
  // …

  @Test
  public void validateTest() throws NoSuchMethodException,
      IllegalAccessException, InvocationTargetException {
    final Method validate = Example.class.getDeclaredMethod("validate",
        String.class, String.class, int.class);
    validate.setAccessible(true);

    assert (new Integer(10).equals(validate.invoke(validate, "test",
        "passw0rd", new Integer(10))));
  }
}

Pretty nifty trick.

[@514]

Bob emailed me the following:

I would just make the method package-private (default access) and put the test in the same package.

Bob

As I told him, friendly access doesn't always work for me because of the way I obfuscate private methods. I could make exceptions in my obfuscator settings, but that's a lot more work.

You might want to read Bob's post on Package Scoping and Unit Testing.

[@395]
Eugene rightfully pointed out that Commons Lang provides helpers to reflectively invoke methods.