Mocking and Stubbing in Unit Tests

Introduction to Testing in Java

Maria Milusheva

Senior Software Engineer

Motivation: Dependencies

Recall the dependency flow in the previous lesson: The currency exchange app depends on the bank server: A Mermaid graph of the dependency flow in the banking demo

Impossible to verify calculations because currency exchange rates are inherently unpredictable.

Dependency management in testing

Mocking - creating simulated objects or behaviors for real objects that are difficult to use in tests, allowing us to test without relying on external dependencies.

Mockito - powerful Java library for creating mocks.

import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

Mocking example

Mocking

Mock = like a stand-in actor in a play, pretending to be the real object only as far as the test is concerned

Consider the previous lesson:

public double convertEuroTo(String currency, double amount) {
    double rate = this.bank.getRateEuroTo(currency);
    return amount * rate;
}

➡ Make predicable object for EuropeanCentralBankServer for testing

Mocking: Currency exchange

This is how we test with mocks:

@Test
void convert_returnsExpectedValue() {
    EuropeanCentralBankServer bank = mock(EuropeanCentralBankServer.class);
    ExchangeApp exchangeApp = new ExchangeApp(bank);

    // Need to program the mock so it works
    when(bank.getRateEuroTo("USD")).thenReturn(1.1);


    // Calls getRateEuroTo from the mock object, obtaining our pre-programmed values
    double result = exchangeApp.convertEuroTo("USD", 1000); 

    assertEquals(1100, result); // We can now assert on exact return values
}

Common mistake

We should never forget to program our mocks:

@Test
void convert_returnsExpectedValue() {
    EuropeanCentralBankServer bank = mock(EuropeanCentralBankServer.class);
    ExchangeApp exchangeApp = new ExchangeApp(bank);

    // No mock programming - mock bank object doesn't know how to respond to getRateEuroTo
    double result = exchangeApp.convertEuroTo("USD", 1000);

    assertEquals(1100, result); // Will fail due to lack of mock setup
}

Stubbing

The test fails with:

org.opentest4j.AssertionFailedError: 
Expected :1100.0
Actual   :0.0

Programming a mock is referred to as stubbing.

Stubbing - providing minimal, pre-defined answers to calls made during the test, usually not responding at all to anything outside what’s programmed for the test.

Exceptions with Mockito

Mockito can also simulate error scenarios:

EuropeanCentralBankServer bank = mock(EuropeanCentralBankServer.class);
ExchangeApp exchangeApp = new ExchangeApp(bank);
// Instead of thenReturn() use thenThrow()
when(bank.getRateEuroTo("USD"))
        .thenThrow(new RuntimeException("Bank server unavailable."));

exchangeApp.convertEuroTo("USD", 1000); // getRateEuroTo will throw exception

Let's practice!

Introduction to Testing in Java