Mockito - Overview

What is Mocking?

Mocking is a way to test the functionality of a class in isolation. 

Mocking does not require a database connection or properties file read or file server read to test a functionality. 

Mock objects do the mocking of the real service. A mock object returns a dummy data corresponding to some dummy input passed to it.

Mockito :-

Mockito facilitates creating mock objects seamlessly.

It uses Java Reflection in order to create mock objects for a given interface. Mock objects are nothing but proxy for actual implementations.

Consider a case of Stock Service which returns the price details of a stock. 

During development, the actual stock service cannot be used to get real-time data. So we need a dummy implementation of the stock service. Mockito can do the same very easily, as its name suggests.

Benefits of Mockito :-

  • No Handwriting − No need to write mock objects on your own.
  • Refactoring Safe − Renaming interface method names or reordering parameters will not break the test code as Mocks are created at runtime.
  • Return value support − Supports return values.
  • Exception support − Supports exceptions.
  • Order check support − Supports check on order of method calls.
  • Annotation support − Supports creating mocks using annotation.

Consider the following code snippet.

import java.util.ArrayList;
import java.util.List;
import static org.mockito.Mockito.*;

public class PortfolioTester {

   public static void main(String[] args){

      //Create a portfolio object which is to be tested
      Portfolio portfolio = new Portfolio();

      //Creates a list of stocks to be added to the portfolio
      List<Stock> stocks = new ArrayList<Stock>();
      Stock googleStock = new Stock("1","Google", 10);
      Stock microsoftStock = new Stock("2","Microsoft",100);

      //Create the mock object of stock service
      StockService stockServiceMock = mock(StockService.class);

      // mock the behavior of stock service to return the value of various stocks

      //add stocks to the portfolio

      //set the stockService to the portfolio
      double marketValue = portfolio.getMarketValue();

      //verify the market value to be 
      //10*50.00 + 100* 1000.00 = 500.00 + 100000.00 = 100500
      System.out.println("Market value of the portfolio: "+ marketValue);

Let's understand the important concepts of the above program. The complete code is available in the chapter First Application.

  • Portfolio − An object to carry a list of stocks and to get the market value computed using stock prices and stock quantity.
  • Stock − An object to carry the details of a stock such as its id, name, quantity, etc.
  • StockService − A stock service returns the current price of a stock.
  • mock(...) − Mockito created a mock of stock service.
  • when(...).thenReturn(...) − Mock implementation of getPrice method of stockService interface. For googleStock, return 50.00 as price.
  • portfolio.setStocks(...) − The portfolio now contains a list of two stocks.
  • portfolio.setStockService(...) − Assigns the stockService Mock object to the portfolio.
  • portfolio.getMarketValue() − The portfolio returns the market value based on its stocks using the mock stock service.