Java方法中System.in输入的单元测试策略

public class BillingService {

    public void addBill() {
        Scanner scanner = new Scanner(System.in); // 直接依赖System.in

        System.out.print("Insert menu you want to be served: ");
        var option = scanner.nextLine(); // 阻塞等待用户输入

        try {
            switch (option) {
                case "1":
                    System.out.print("Choose menu you want to eat: ");
                    var menuOptionFood = scanner.nextLine(); // 再次阻塞
                    // ... 处理食物选项
                    break;
                case "2":
                    System.out.print("Choose menu you want to drink: ");
                    var menuOptionDrink = scanner.nextLine(); // 再次阻塞
                    // ... 处理饮品选项
                    break;
                case "e":
                    System.out.println("e");
                    break;
                default:
                    System.out.println("No option found !!!");
                    break;
            }
        } catch (NullPointerException exception) {
            System.out.println(exception.getMessage());
        }
        // 注意：实际应用中，Scanner应该被关闭，或者通过依赖注入管理
        // scanner.close();
    }
}

<dependencies>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-api</artifactId>
        <version>5.10.0</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-engine</artifactId>
        <version>5.10.0</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.mockito</groupId>
        <artifactId>mockito-junit-jupiter</artifactId>
        <version>5.6.0</version>
        <scope>test</scope>
    </dependency>
</dependencies>

import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.PrintStream;

@ExtendWith(MockitoExtension.class)
class BillingServiceTest {

    private BillingService billingService;

    // 保存原始的System.in和System.out
    private final InputStream systemIn = System.in;
    private final PrintStream systemOut = System.out;

    // 用于模拟输入的流
    private ByteArrayInputStream testIn;
    // 用于捕获System.out的输出
    private ByteArrayOutputStream testOut;

    @BeforeEach
    public void init() {
        billingService = new BillingService();

        // 在每个测试开始前，重定向System.out到ByteArrayOutputStream
        testOut = new ByteArrayOutputStream();
        System.setOut(new PrintStream(testOut));
    }

    @AfterEach
    public void restoreSystemInputOutput() {
        // 在每个测试结束后，恢复原始的System.in和System.out
        System.setIn(systemIn);
        System.setOut(systemOut);
    }

    /**
     * 辅助方法：设置模拟的用户输入
     * @param data 模拟的输入字符串，多行输入使用 '\n' 分隔
     */
    private void provideInput(String data) {
        testIn = new ByteArrayInputStream(data.getBytes());
        System.setIn(testIn);
    }

    @Test
    void whenAddNewMenuItemToBill_selectFoodAndBreakfast_shouldProcessCorrectly() {
        // 模拟用户输入 "1" (选择食物菜单) 和 "1" (选择早餐)
        provideInput("1\n1");
        billingService.addBill();

        // 进一步的断言可以检查System.out的输出，或者服务内部状态的改变
        // 例如：Assertions.assertTrue(testOut.toString().contains("1"));
    }

    @Test
    void whenAddNewMenuItemToBill_selectDrinkAndAlcohol_shouldProcessCorrectly() {
        // 模拟用户输入 "2" (选择饮品菜单) 和 "1" (选择酒精饮品)
        provideInput("2\n1");
        billingService.addBill();

        // 例如：Assertions.assertTrue(testOut.toString().contains("1.2"));
    }

    @Test
    void whenAddNewMenuItemToBill_selectExit_shouldExit() {
        // 模拟用户输入 "e" (退出)
        provideInput("e");
        billingService.addBill();

        // 例如：Assertions.assertTrue(testOut.toString().contains("e"));
    }

    @Test
    void whenAddNewMenuItemToBill_selectInvalidOption_shouldShowErrorMessage() {
        // 模拟用户输入 "invalid" (无效选项)
        provideInput("invalid\nignored"); // 第二个输入会被忽略，因为第一个已处理
        billingService.addBill();

        // 例如：Assertions.assertTrue(testOut.toString().contains("No option found !!!"));
    }
}