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.

First, you need to tell maven where your test sources are 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>
        <includes><include>**/*.java</include></includes>
      </configuration>
    </plugin>

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 is an active member of the JSF 2 Expert Group and is happy to be a contributor to the Java Community. He has been writing software since the age of 8 and his other interests include languages, science, travel and surfing. You can follow Roger on Twitter and Google+.