How to invoke an object’s private method in Java

The private methods are designed not accessible from outside. But sometimes we want to call it directly, for example in unit testing. Java actually provides ways to call private method with the help of reflection, java.lang.reflect.Method

PowerMock, a widely used mock library also provide helper class to make it easy  in JUnit test cases. (PowerMock indeed wraps the reflection for you)

In this demo, there is a HellowWorld class under test.

package com.shengwang.demo;

public class HelloWorld {

  private String sayHello(String name) {
    return = "Dear " + name;
  }
}

The private method sayHello will be tested in JUnit test case.

1. Method-1 Using reflection directly

  1. 1 What you need

• JDK  (no third party library is required )

  1.2 Invoke private method in testcase

package com.shengwang.demo;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.fail;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import org.junit.Test;


public class HelloWorldByReflectionTest {

  @Test
  public void testPrivateMethod() {
    try {
      HelloWorld helloWorld = new HelloWorld();
      Method method;
      // get reflected method
      method = HelloWorld.class.getDeclaredMethod("sayHello",new Class[] { String.class });
      // change accessiblity
      method.setAccessible(true);
      // invoke the method
      assertEquals("Dear Tom", method.invoke(helloWorld, "Tom"));
    } catch (NoSuchMethodException | SecurityException | 
             IllegalAccessException | IllegalArgumentException | 
             InvocationTargetException e) {
      e.printStackTrace();
      fail();
    }
  }
}

2. Method-2 Using PowerMock

  2. 1 What you need

• Mockito
• PowerMock

PowerMock usually need to work with either Mockito or EasyMock, you can choose anyone. In this demo Mockito is used.

2.2 Add Maven dependencies

Add dependencies to the maven pom.xml

<dependency>
  <groupId>org.mockito</groupId>
  <artifactId>mockito-all</artifactId>
  <version>1.9.5</version>
  <scope>test</scope>
</dependency>

<dependency>
  <groupId>org.powermock</groupId>
  <artifactId>powermock-module-junit4</artifactId>
  <version>1.5.6</version>
  <scope>test</scope>
</dependency>

<dependency>
  <groupId>org.powermock</groupId>
  <artifactId>powermock-api-mockito</artifactId>
  <version>1.5.6</version>
  <scope>test</scope>
</dependency>

  2.3 Invoke private method in testcase

package com.shengwang.demo;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.fail;
import static org.powermock.reflect.Whitebox.invokeMethod;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ HelloWorld.class })
public class HelloWorldByPowerMockTest {
  @Test
  public void testPrivateMethod() {
    String result;
    try {
      HelloWorld helloWorld = new HelloWorld();
      result = invokeMethod(helloWorld, "sayHello", "Tom");
      assertEquals("Dear Tom", result);
    } catch (Exception e) {
      e.printStackTrace();
      fail();
    }
  }
}

The org.powermock.reflect.Whitebox.invokeMethod from PowerMock is imported at the beginning and used to invoke private method.