When you are dealing with legacy code base that doesn’t have enough test coverage it would be sometime very necessary to test private methods/behaviors without going through the public methods/behavior. If your public method/behavior that invoke private method/behavior is too long and too complicated to cover with the limited time frame and you are only interested in newly written private method/behavior then you can use JMockit Deencapsulation class to test those private method/behavior.

The complete source code can be cloned from github.

JMockit API to test private method in java

Class : mockit.Deencapsulation
Method : public static T invoke(Object objectWithMethod, String methodName, Object… nonNullArgs)

package com.ourownjava.tdd.mockit;
import java.util.List;
/**
*
* @author Sanju Thomas
*
*/
public class ClassWithPrivateMethod {
private void privateMethod(){
throw new RuntimeException("Private Method is invoked!");
}
private void populateSomeData(final List listOfString){
for(char c : "Sanju Thomas".toCharArray()){
listOfString.add(String.valueOf(c));
}
}
private String [] splitUsingDelimiter(final String token, final String delimiter){
return token.split(delimiter);
}
}

package com.ourownjava.tdd.jmockit;
import static org.junit.Assert.assertEquals;
import java.util.ArrayList;
import java.util.List;
import mockit.Deencapsulation;
import org.junit.Before;
import org.junit.Test;
import com.ourownjava.tdd.mockit.ClassWithPrivateMethod;
/**
*
* @author Sanju Thomas
*
*/
public class TestClassWithPrivateMethod {
private ClassWithPrivateMethod classWithPrivateMethod;
@Before
public void setUp(){
classWithPrivateMethod = new ClassWithPrivateMethod();
}
@Test(expected = RuntimeException.class)
public void shouldInvokePrivateMethod(){
Deencapsulation.invoke(classWithPrivateMethod, "privateMethod", null);
}
@Test
public void shouldInvokePrivateMethodWithParameter(){
final List someDate = new ArrayList();
Deencapsulation.invoke(classWithPrivateMethod, "populateSomeData", someDate);
assertEquals(12, someDate.size());
}
@Test
public void shouldInvokePrivateMethodWithParametersThatReturnsSomething(){
final String [] tokens = Deencapsulation.invoke(classWithPrivateMethod, "splitUsingDelimiter", new Object[] {"Sanju Thomas", " "});
assertEquals(2, tokens.length);
assertEquals("Sanju", tokens[0]);
assertEquals("Thomas", tokens[1]);
}
}