How To Unit Test Private Methods

I've discovered that one reason I don't write as many unit tests as I should is because they live in a separate source tree, out of sight and out of mind. So I've moved my unit tests to be inner classes on the classes they test. A side effect of this is you can easily test the private methods of a class since the inner class has direct access to these methods.

Here is the configuration you will need to do this. Note, I am using TestNG, but it should be pretty similar in JUnit.

By default surefire will not execute tests in inner classes so first you need to fix this and also tell maven where your test code is by adding this to your <plugins> section:

    <!-- unit tests live inside the java source files themselves -->
    <plugin>
      <artifactId>maven-surefire-plugin</artifactId>
      <version>2.12</version>
      <configuration>
        <testSourceDirectory>src/main</testSourceDirectory>
        <testClassesDirectory>target/classes</testClassesDirectory>
        <excludes><exclude>[none]</exclude></excludes>
      </configuration>
    </plugin>

Note that [none] is not a special keyword, it only exists to match no class files. You can put whatever rubbish in there (but a blank entry has no effect).

Your unit tests must be public static inner classes annotated with @Test, like this:

    @Test
    public static class UserEditorTest {
        public void test_private_method() {
           assertEquals(new UserEditor().somePrivateMethod(), "success");
        }
    }

Since the test class is an inner class, it can call the private method.

To run the test from maven, I use a wildcard on the test name so I don't have to specify OuterClass$InnerClass.

    $ mvn test -Dtest=*UserEditorTest

To get Netbeans to run the tests, you can use a wildcard so it finds the inner class. Go to the project properties and look for the Actions settings. You will need to set the parameters like this:

    -Dtest=*{className}

You can also run from a normal testng.xml test suite like this:

<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">
<suite name="Suite1" verbose="1">
  <test name="MyTest" >
    <classes>
       <class name="com.example.Example$ExampleTest"/>
    </classes>
  </test>
</suite>

This setup has improved my productivity and (more importantly) the coverage of my unit tests. Do you think using inner classes for unit testing is a bad idea? Let me know why in the comments below.

About Roger Keays

Roger Keays is an artist, an engineer, and a student of life. He has no fixed address and has left footprints on 40-something different countries around the world. Roger is addicted to surfing. His other interests are music, psychology, languages, the proper use of semicolons, and finding good food.