Testing Private Methods in Java

Posted: August 19th, 2011 | Author: simeshev | Filed under: General

Java 5 provides vargargs API that allows to test private methods in a very concise way. Here is how.

Yesterday, during a meetup at Loopt, a question of an API for testing private methods was brought up. I used to use JUnit add-on PrivateAccessor [1] when I was testing private methods. The problem was that applying PrivateAccessor produced prohibitively verbose code, so you had to think really hard to understand what the test was doing.

invoke() with varargs

Luckily, Java 5 provides vargargs API that allows to write really concise code for invoking private methods and return results. Here is the invoke() method that makes use of vargargs. Feel free to use it in your code:

   /**
    * Invokes a private method on an object.
    *
    * @param obj        the object on what to invoke the method.
    * @param methodName the name of the method to invoke.
    * @param parameters a list of parameter
    * @return the result of dispatching the method represented by the object.
    */
   public static <T> T invoke(Object obj, String methodName, Object... parameters) {

      try {

         // Create a list of parameter classes
         final Class[] parameterTypes = new Class[parameters.length];
         for (int i = 0; i < parameters.length; i++) {
            parameterTypes[i] = parameters[i].getClass();
         }

         // Get method
         Class clazz = obj.getClass();
         Method method = clazz.getDeclaredMethod(methodName, parameterTypes);
         method.setAccessible(true);

         // Call method
         return (T)method.invoke(obj, parameters);
      } catch (RuntimeException e) {

         throw e;
      } catch (Exception e) {

         throw new IllegalArgumentException(e);
      }
   }

Complete Example

Below is a complete JUnit tests that tests StringBuilder’s private method append(). Notice how neatly the invocation of append() fits into a short single line:

import java.lang.reflect.Method;
import junit.framework.TestCase;

public final class StringBuilderTest extends TestCase {


   /**
    * Object under test
    */
   private StringBuilder object;


   /**
    * Tests String's private method 'append(StringBuilder sb)'.
    */
   public void testAppend() {
      
      StringBuilder toAppend = new StringBuilder("Test");
      StringBuilder result = invoke(object, "append", toAppend);
      assertEquals("Test", result.toString());
   }


   /**
    * Invokes a private method on an object.
    *
    * @param obj        the object on what to invoke the method.
    * @param methodName the name of the method to invoke.
    * @param parameters a list of parameter
    * @return the result of dispatching the method represented by the object.
    */
   public static <T> T invoke(Object obj, String methodName, Object... parameters) {

      try {

         // Create a list of parameter classes
         final Class[] parameterTypes = new Class[parameters.length];
         for (int i = 0; i < parameters.length; i++) {
            parameterTypes[i] = parameters[i].getClass();
         }

         // Get method
         Class clazz = obj.getClass();
         Method method = clazz.getDeclaredMethod(methodName, parameterTypes);
         method.setAccessible(true);

         // Call method
         return (T)method.invoke(obj, parameters);
      } catch (RuntimeException e) {

         throw e;
      } catch (Exception e) {

         throw new IllegalArgumentException(e);
      }
   }


   public void setUp() throws Exception {

      super.setUp();

      object = new StringBuilder(10);
   }
}

Making invoke() reusable

To make invoke() reusable, I suggest to move it to a separate utility class or at least to create a super class for all your tests that extends junit.TestCase and move invoke() there.

Regards,

Slava Imeshev
Cacheonix: Reliable Distributed Java Cache

References

1. JUnit Add-On PrivateAccessor

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