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!

Example

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;

    @Before
    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);
        m.setAccessible(true);
        parameters = new Object[1];
    }

    @Test
    public void testPrivateMethod() throws Exception {
        parameters[0] = "someIdentifier";
        String result = (String) m.invoke(product, parameters); 

        //Do your assertions
        assertNotNull(result);
    }
}

Update

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.

I added validation, via annotations, to a Spring “Model” bean. I needed someway to Unit Test this validation, without running the container and without initialising the Spring context.

The bean (simplified)

package foo.bar;

import javax.validation.constraints.NotNull;
import javax.validation.constraints.Pattern;
import javax.validation.constraints.Size;

public final class ProductModel {

    @NotNull
    @Size(max=100)
    @Pattern(regexp="[^\n^\t^\r]+", message="Long Name must not contain New Lines, Carriage Returns or Tabs")
    private String longName;

    @Size(max=20)
    private String shortName;

    // rest snipped for brevity
}

The Unit Test

package foo.bar;

import java.util.Set;

import javax.validation.ConstraintViolation;

import junit.framework.Assert;

import org.hibernate.validator.HibernateValidator;
import org.junit.Before;
import org.junit.Test;
import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;

public class Temp {
    private LocalValidatorFactoryBean localValidatorFactory;

    @Before
    public void setup() {
        localValidatorFactory = new LocalValidatorFactoryBean();
        localValidatorFactory.setProviderClass(HibernateValidator.class);
        localValidatorFactory.afterPropertiesSet();
    }
    @Test
    public void testLongNameWithInvalidCharCausesValidationError() {
        final ProductModel productModel = new ProductModel();
        productModel.setLongName("A long name with\t a Tab character");
    	Set<ConstraintViolation<ProductModel>> constraintViolations = localValidatorFactory.validate(productModel);
    	Assert.assertTrue("Expected validation error not found", constraintViolations.size() == 1);
    }
}

More

It actually took me a few hours to work the above test out (simple as it is). If I hadn’t stumbled upon these Spring Unit Tests, I might never have got it.

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

public class SampleTest {
    private java.util.List emptyList; 

    /**
     * Sets up the test fixture.
     * (Called before every test case method.)
     */
    @Before
    public void setUp() {
        emptyList = new java.util.ArrayList();
    } 

    /**
     * Tears down the test fixture.
     * (Called after every test case method.)
     */
    @After
    public void tearDown() {
        emptyList = null;
    } 

    @Test
    public void testSomeBehavior() {
        assertEquals("Empty list should have 0 elements", 0, emptyList.size());
    } 

    @Test(expected=IndexOutOfBoundsException.class)
    public void testForException() {
        Object o = emptyList.get(0);
    }
}

Points of note

The static import of org.junit.Assert.*