戻り値のスタブ化
Mockオブジェクトのメソッドが呼び出されたときに、特定の値を返したり例外をスローしたりするよう設定します。
「戻り値のスタブ化」はCoddyKit上の無料Testing Mastery: JUnit, Mockito & Integration Testsレッスンです。 これはレッスン1/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはTesting Mastery: JUnit, Mockito & Integration Tests学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Testing Mastery: JUnit, Mockito & Integration Testsコースには全4レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
Intro to Stubbing Mocks
When you create a mock object, it doesn't have any real behavior. Its methods do nothing by default, often returning null, 0, or empty collections.
Stubbing is the process of teaching a mock object how to respond to specific method calls. It's like giving your mock a script to follow!
This allows you to control the exact conditions for your tests, isolating the code you're actually testing.
Making Mocks Return Values
The most common way to stub a mock is using when().thenReturn(). This tells Mockito: "When this specific method is called on the mock, return this predefined value."
when(mock.someMethod()).thenReturn(value);
The value can be any object or primitive type that matches the method's return type.
thenReturn() Code Example
Try running this simple example. Notice how the mock's add method returns our stubbed value only when called with (1, 1).
import static org.mockito.Mockito.*;
interface CalculatorService {
int add(int a, int b);
}
public class Main {
public static void main(String[] args) {
// Create a mock of CalculatorService
CalculatorService mockCalc = mock(CalculatorService.class);
// Stub the add(1, 1) method to return 5
when(mockCalc.add(1, 1)).thenReturn(5);
// Call the stubbed method
System.out.println("1 + 1 = " + mockCalc.add(1, 1));
// Call a method that was NOT stubbed
System.out.println("2 + 2 = " + mockCalc.add(2, 2));
}
}Mocks Throwing Errors
Sometimes you need to test how your code handles errors. You can configure a mock to throw an exception when a specific method is called, using when().thenThrow().
when(mock.failingMethod()).thenThrow(new SomeException("Error!"));
This is crucial for testing error paths and ensuring your application gracefully handles unexpected situations.
thenThrow() Code Demo
In this example, our DataService mock is configured to throw an IllegalArgumentException if we try to fetch data with an "invalid" ID.
import static org.mockito.Mockito.*;
interface DataService {
String fetchData(String id);
}
public class Main {
public static void main(String[] args) {
DataService mockData = mock(DataService.class);
// Stub fetchData("invalid") to throw an exception
when(mockData.fetchData("invalid"))
.thenThrow(new IllegalArgumentException("ID not found!"));
// Call a method that was NOT stubbed (returns null by default)
System.out.println("Fetching 'valid': " + mockData.fetchData("valid"));
// Call the stubbed method that throws an exception
try {
System.out.println("Fetching 'invalid': " + mockData.fetchData("invalid"));
} catch (IllegalArgumentException e) {
System.out.println("Caught error: " + e.getMessage());
}
}
}Dynamic Responses: thenAnswer()
What if you need the mock's response to depend on the arguments passed to it, or to perform some custom logic?
when().thenAnswer() is your solution. It takes an Answer object (often a lambda or anonymous class) which provides more control:
- You get access to the actual method arguments.
- You can perform calculations or complex logic.
- You can return a value or throw an exception dynamically.
thenAnswer() Code Demo
Here, the multiply method on our mock actually performs the multiplication based on the arguments it receives. This is more dynamic than a fixed return value.
import static org.mockito.Mockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
interface MathService {
int multiply(int a, int b);
}
public class Main {
public static void main(String[] args) {
MathService mockMath = mock(MathService.class);
// Stub multiply for any two integers to perform actual multiplication
when(mockMath.multiply(anyInt(), anyInt())).thenAnswer(
new Answer<Integer>() {
public Integer answer(InvocationOnMock invocation) {
Object[] args = invocation.getArguments();
Integer arg1 = (Integer) args[0];
Integer arg2 = (Integer) args[1];
return arg1 * arg2; // Custom logic based on arguments
}
}
);
System.out.println("3 * 4 = " + mockMath.multiply(3, 4));
System.out.println("5 * 2 = " + mockMath.multiply(5, 2));
System.out.println("7 * 0 = " + mockMath.multiply(7, 0));
}
}Multiple Returns: Chaining
What if you want a mock method to return different values on successive calls?
Mockito allows you to chain stubbing methods. You can provide multiple return values to thenReturn(), or chain multiple thenReturn(), thenThrow(), or thenAnswer() calls.
when(mock.method()).thenReturn(val1, val2, val3);when(mock.method()).thenReturn(val1).thenThrow(ex).thenReturn(val3);
After all specified values or actions are exhausted, the last one will be repeated for subsequent calls.
Chained Stubbing Demo
Observe how our mock Queue returns "Item A", then "Item B", and then throws an exception on subsequent poll() calls.
import static org.mockito.Mockito.*;
import java.util.Queue;
import java.util.LinkedList;
public class Main {
public static void main(String[] args) {
Queue<String> mockQueue = mock(Queue.class);
// Stub poll() to return different values/actions on successive calls
when(mockQueue.poll())
.thenReturn("Item A")
.thenReturn("Item B")
.thenThrow(new IllegalStateException("Queue is empty!"));
System.out.println("Poll 1: " + mockQueue.poll()); // Returns "Item A"
System.out.println("Poll 2: " + mockQueue.poll()); // Returns "Item B"
try {
System.out.println("Poll 3: " + mockQueue.poll()); // Throws exception
} catch (IllegalStateException e) {
System.out.println("Caught error on Poll 3: " + e.getMessage());
}
// Subsequent calls will repeat the last action (throw exception)
try {
System.out.println("Poll 4: " + mockQueue.poll());
} catch (IllegalStateException e) {
System.out.println("Caught error on Poll 4: " + e.getMessage());
}
}
}Check Your Understanding
You are testing a method that processes items from a queue. You need the queue's poll() method to return "Task 1", then "Task 2", and finally null (indicating an empty queue) for subsequent calls. Which Mockito stubbing approach would you use?
Stubbing Return Values: Recap
In this lesson, you learned how to control the behavior of your mock objects by stubbing their method calls. This is fundamental for isolating the unit under test and creating predictable test environments.
when().thenReturn(): To make a mock method return a specific value.when().thenThrow(): To make a mock method throw an exception.when().thenAnswer(): For dynamic responses based on method arguments or custom logic.- Chaining: To define a sequence of different behaviors for successive calls to the same method.
Next, we'll explore Mockito's argument matchers for even more flexible stubbing and verification!
よくある質問
「戻り値のスタブ化」レッスンは無料ですか？
はい。「戻り値のスタブ化」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Testing Mastery: JUnit, Mockito & Integration Testsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Testing Mastery: JUnit, Mockito & Integration Testsコースには全4レッスンが含まれています。
「戻り値のスタブ化」で何を学びますか？
Mockオブジェクトのメソッドが呼び出されたときに、特定の値を返したり例外をスローしたりするよう設定します。 ブラウザで直接実行するハンズオンコードでTesting Mastery: JUnit, Mockito & Integration Testsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
Testing Mastery: JUnit, Mockito & Integration Testsを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのTesting Mastery: JUnit, Mockito & Integration Testsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン1/4です。
「戻り値のスタブ化」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このTesting Mastery: JUnit, Mockito & Integration Testsレッスンでコードを書いて実行できますか？
はい。すべてのTesting Mastery: JUnit, Mockito & Integration Testsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。