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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 to read or file server read to test functionality. Mock objects do the mocking of the real service. A mock object returns 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 a 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.

package com.adglob.mock;

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);

      stocks.add(googleStock);

      stocks.add(microsoftStock);       

      //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

      when(stockServiceMock.getPrice(googleStock)).thenReturn(50.00);

      when(stockServiceMock.getPrice(microsoftStock)).thenReturn(1000.00);

      //add stocks to the portfolio

      portfolio.setStocks(stocks);

      //set the stockService to the portfolio

      portfolio.setStockService(stockServiceMock);

      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.