Unit Test Private Java Methods using Reflection

Thursday, May 5th, 2011


I realise that the thought of Unit Testing private methods in Java classes can be like a red rag to a bull for some people and I understand the arguments.

Therefore, I present the following as a “how to”, not a moral argument for or against!


The class under test

public class Product() {
    private String privateMethod(String id) {
    //Do something private
    return "product_" + id;

The (Reflection Based) Unit Test

import java.lang.reflect.Method;

import static org.junit.Assert.*;
import org.junit.Test;

public class ProductTest {
    private Product product; // the class under test
    private Method m;
    private static String METHOD_NAME = "privateMethod";
    private Class[] parameterTypes;
    private Object[] parameters;

    public void setUp() throws Exception {
        product = new Product();
        parameterTypes = new Class[1];
        parameterTypes[0] = java.lang.String.class;
        m = product.getClass().getDeclaredMethod(METHOD_NAME, parameterTypes);
        parameters = new Object[1];

    public void testPrivateMethod() throws Exception {
        parameters[0] = "someIdentifier";
        String result = (String) m.invoke(product, parameters); 
        //Do your assertions


I’ve since been told that if dp4j.jar is in the classpath at compile-time, it will inject the necessary reflection to make this work. I haven’t had time to try this yet so YMMV.

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