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Using Reflection for Testing

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Using Reflection for Testing

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I am working on a presentation about the ‘Single Responsibility Principle’, based on my previous post. It takes most of my time.

In the meantime, I want to share a sample code of how I  test inner fields in my classes.
I am doing it for a special case of testing, which is more of an integration test.
In the standard unit testing of the dependent class, I am using mocks of the dependencies.

The Facts

  1. All of the fields (and dependencies in our classes are private
  2. The class do not have getters for its dependencies
  3. We wire things up using Spring (XML context)
  4. I wan to verify that dependency interface A is wired correctly to dependent class B

One approach would be to wire everything and then run some kind of integration test of the logic.
I don’t want to do this. It will make the test hard to maintain.

The other approach is to check wiring directly.
And for that I am using reflection.

Below is a sample code of the testing method, and the usage.
Notice how I am catching the exception and throws a RuntimeException in case there is a problem.
This way, I have cleaner tested code.

// Somewhere in a different utility class for testing
	public static <T> T realObjectFromField(Class<?> clazz, String fieldName, Object object) {
		Field declaredField = accessibleField(clazz, fieldName);
		try {
			return (T) declaredField.get(object);
		} catch (IllegalArgumentException | IllegalAccessException e) {
			throw new RuntimeException(e);
	private static Field accessibleField(Class<?> clazz, String fieldName) {
		try {
			Field declaredField = clazz.getDeclaredField(fieldName);
			return declaredField;
		} catch (NoSuchFieldException | SecurityException e) {
			throw new RuntimeException(e);
	// This is how we use it in a test method
	import static mypackage.ReflectionUtils.realObjectFromField;
	ItemFiltersMapperByFlag mapper = realObjectFromField(ItemsFilterExecutor.class, "filtersMapper", filterExecutor);
	assertNotNull("mapper is null. Check wiring", mapper);

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