[JAVA] How to test private scope with JUnit

Test what is private scope

In Java, call private scope fields, methods, inner classes from the outside, Let's test it with JUnit.

First, let's try testing with JUnit.


There is a compilation error. It is a specification. Cannot be executed. Since it is private, it cannot be called in the same way as public.

So we use Reflection to call something private scope.

What is Reflection?

Reflection is a type of Java standard library. It is dealing with information about the class itself. As a concrete example With a string of class name, method name, field name Generate that class, execute methods, access fields I can do it.

The following is an external site, but it looks like this, for example. http://java.keicode.com/lang/reflection.php

"If it's so convenient, use it positively." You may think that. However, Reflection is a double-edged sword. Be careful when handling. ** But I will introduce it. ** **

Problems with Reflection

Reflection depends on how you use it

--Class design collapses --Code is hard to write and hard to read --Performance is worse than calling a method normally

There are also problems such as. ** But I will introduce it. ** **

"Test private scope with JUnit" This is because it is used for a limited purpose.

Return to the main subject

Let's fix the JUnit test class, which is full of errors.

package jp.co.illmatics.sample;
 *Class to be tested
public class PrivateSample {

    private String field = "Hello field!";

    private final String finalField = "Hello final field!";

    private String doIt()    {
        return "Hello method!";

    private class InnerClass {
        public InnerClass() {

        public String getInnerMethod() {
            return "Hello inner method!";
package jp.co.illmatics.sample;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import org.junit.Test;

import junit.framework.TestCase;

 *Test class
public class PrivateSampleTest extends TestCase {

    PrivateSample privateSample = new PrivateSample();
    String expected = "";
    String actual = "";

    public void testField() throws Exception {
        final String fieldName = "field";
        expected = "Hello field!";
        actual = (String) getFieldValue(privateSample, fieldName);
        assertEquals(expected, actual);

    public void testFinalField() throws Exception {
        final String finalFieldName = "finalField";
        expected = "Hello updated final Field!";
        actual = (String) getUpdatedFinalFieldValue(privateSample, expected, finalFieldName);
        assertEquals(expected, actual);

    public void testMethod() throws Exception {
        final String methodName = "doIt";
        expected = "Hello method!";
        actual = (String) getMethodValue(privateSample, methodName);
        assertEquals(expected, actual);

    public void testInnerClassMethod() throws Exception {
        final String innerClassName = "jp.co.illmatics.sample.PrivateSample$InnerClass";
        final String innerMethodName = "getInnerMethod";
        expected = "Hello inner method!";
        actual = (String) getInnerClassMethod(privateSample, innerClassName, innerMethodName);
        assertEquals(expected, actual);

    private Object getFieldValue(PrivateSample privateSample, String fieldName)
            throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException,
            SecurityException {
        return getPrivateField(privateSample, fieldName).get(privateSample);

    private Field getPrivateField(PrivateSample privateSample, String fieldName)
            throws NoSuchFieldException, SecurityException, IllegalArgumentException,
            IllegalAccessException {
        Class<?> clazz = privateSample.getClass();
        Field field = clazz.getDeclaredField(fieldName);
        return field;

    private Object getUpdatedFinalFieldValue(PrivateSample privateSample, String newValue,
            String fieldName) throws NoSuchFieldException, SecurityException,
                    IllegalArgumentException, IllegalAccessException {
        Field finalField = getPrivateField(privateSample, fieldName);
        finalField.set(privateSample, newValue);
        return finalField.get(privateSample);

    private Object getMethodValue(PrivateSample privateSample, String name)
            throws NoSuchMethodException, SecurityException, IllegalAccessException,
            IllegalArgumentException, InvocationTargetException {
        Method doIt = PrivateSample.class.getDeclaredMethod(name);
        return doIt.invoke(privateSample);

    private Object getInnerClassMethod(PrivateSample parent, String classFullName,
            String methodName) throws ClassNotFoundException, NoSuchMethodException,
                    SecurityException, InstantiationException, IllegalAccessException,
                    IllegalArgumentException, InvocationTargetException {
        ClassLoader loader = ClassLoader.getSystemClassLoader();
        Class<?> innerClazz = loader.loadClass(classFullName);
        Constructor<?> constructor = innerClazz.getDeclaredConstructor(parent.getClass());
        Object innerObj = constructor.newInstance(parent);
        return innerClazz.getDeclaredMethod(methodName).invoke(innerObj);

resolved.png I was able to fix it.

testStart.png Test

testSucceeded.png It's done.

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