如何模仿System.in？

Question

请你解释一下如何正确地模仿System.in好吗？

我有一个简单的测试，并将值赋给

@Test
public void testBarCorrect() {
    System.setIn(new ByteArrayInputStream("7\n2\n3".getBytes()));
    someService().consume();
}

在引擎盖下，我除了new Scanner(System.in);什么都不做

请注意，有时我的测试是绿色的，但在80%的测试中抛出了一个Exception "java.util.NoSuchElementException: No line found".。

所以，如果我在测试中按run键，它就会出错。

你能给我一些提示吗？

我的测试：

public class RecordServiceCTLIntegrationTest {

    private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
    private final PrintStream originalOut = System.out;
    private static final InputStream DEFAULT_STDIN = System.in;

    @Before
    public void setUpStreams() {
        System.setOut(new PrintStream(outContent));
        System.setIn(DEFAULT_STDIN);
    }

    @After
    public void rollbackChangesToStdin() {
        System.setOut(originalOut);
        System.setIn(DEFAULT_STDIN);
    }

    @Test
    public void testBarCorrect() {
        System.setIn(new ByteArrayInputStream("5\n1\nB\n21\n\n".getBytes()));
        buildRecordServiceCTL().run();
        assertEquals("Enter Room Id: No active booking for room id: 5\n" +
                "Enter Room Id: B:\tBar Fridge\n" +
                "R:\tRestaurant\n" +
                "S:\tRoom Service\n" +
                "Enter service typeEnter cost: Service Bar Fridge has been added for the roomNumber 1\n" +
                "Hit <enter> to continuePay for service completed\n", outContent.toString());
    }

    @Test
    public void testRestaurantCorrect() {
        System.setIn(new ByteArrayInputStream("5\n1\nR\n21\n\n".getBytes()));
        buildRecordServiceCTL().run();
        assertEquals("Enter Room Id: No active booking for room id: 5\n" +
                "Enter Room Id: B:\tBar Fridge\n" +
                "R:\tRestaurant\n" +
                "S:\tRoom Service\n" +
                "Enter service typeEnter cost: Service Restaurant has been added for the roomNumber 1\n" +
                "Hit <enter> to continuePay for service completed\n", outContent.toString());

    }

    @Test
    public void testRoomServiceCorrect() {

        System.setIn(new ByteArrayInputStream("5\n1\nS\n21\n\n".getBytes()));
        buildRecordServiceCTL().run();
        assertEquals("Enter Room Id: No active booking for room id: 5\n" +
                "Enter Room Id: B:\tBar Fridge\n" +
                "R:\tRestaurant\n" +
                "S:\tRoom Service\n" +
                "Enter service typeEnter cost: Service Room Service has been added for the roomNumber 1\n" +
                "Hit <enter> to continuePay for service completed\n", outContent.toString());
    }

    @Test(expected = NoSuchElementException.class)
    public void testRoomException() {
        System.setIn(new ByteArrayInputStream("-1\n\n".getBytes()));
        buildRecordServiceCTL().run();
    }

    @Test
    public void cancel() {
        System.setIn(new ByteArrayInputStream("5\n1\nS\n20\n".getBytes()));
        assertEquals("", outContent.toString());
    }

    private RecordServiceCTL buildRecordServiceCTL() {
        Hotel hotel = new Hotel();
        final Guest guest = new Guest("name", "address", 123);
        final Room room = new Room(1, SINGLE);
        final CreditCard card = new CreditCard(VISA, 123, 123);
        final Booking booking = new Booking(guest, room, new Date(), 10, 1, card);
        hotel.activeBookingsByRoomId.put(room.getId(), booking);
        final Map<Integer,Room> map = new HashMap<>();
        map.put(room.getId(), room);
        hotel.roomsByType.put(SINGLE, map);
        return new RecordServiceCTL(hotel);
    }

}

中的run方法中出现异常：

if (stdin == null) {
    stdin = new Scanner(System.in);
}
String ans = stdin.nextLine();

​

​

Answer 1

这个测试对我来说很好：

@Test
public void testBarCorrect() {
    System.setIn(new ByteArrayInputStream("7\n2\n3".getBytes()));
    Scanner scanner = new Scanner(System.in);
    System.out.println(scanner.nextLine());
    System.out.println(scanner.nextLine());
    System.out.println(scanner.nextLine());
}

但是，如果我添加了以下测试，这将失败：

@Test
public void testFooCorrect() {
    Scanner scanner = new Scanner(System.in);
    System.out.println(scanner.nextLine());
    System.out.println(scanner.nextLine());
    System.out.println(scanner.nextLine());
}

java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Scanner.java:1540)
    at com.amazon.adcs.service.dra.domain.A9DeviceTest.testFooCorrect(A9DeviceTest.java:15)

理论上，单元测试应该使JVM保持在执行测试之前的状态。这不是您的情况，因为您已经变异了一个单例状态，并且您不会在测试之后回滚修改。如果您有多个使用System.in的测试，那么第一个测试可能会消耗所有测试，而不会留给另一个测试。

通常，我建议您在类中注入一个InputStream，然后您可以在不影响系统常量的情况下随意进行任何操作。既然你告诉我你不能编辑代码，那么你应该确保你自己清理状态。

像这样的东西会起作用的：

private static final InputStream DEFAULT_STDIN = System.in;

@After
public void rollbackChangesToStdin() {
    System.setIn(DEFAULT_STDIN);
} 

如果您必须经常这样做，您可能需要考虑实现一个JUnit规则。