如何模拟 System.in？

Question

你能解释一下如何System.in正确模拟吗？

我有一个简单的测试，我将值分配给

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

在引擎盖下我什么都不做 new Scanner(System.in);

需要注意的是，有时我的测试是绿色的，但在 80% 时它会抛出一个 Exception "java.util.NoSuchElementException: No line found".

所以，如果我在测试中按下运行，它就会出现异常。

你能给我任何提示吗？

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);
    }

}

异常发生在 run 方法中：

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

Answer 1

这个测试对我来说效果很好：

@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，如果我添加以下测试，则会失败：

@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，单元测试应该让 JVM 保持与执行测试之前相同的状态。这不是你的情况，因为你已经改变了单例状态，并且在测试后你不会回滚修改。如果您有多个使用 的测试System.in，则第一个测试可能会消耗掉所有测试，而不会为另一个测试留下任何内容。

通常，我建议您InputStream在类中注入 an ，然后您可以自由地做任何您想做的事情，而不会影响系统常量。既然您告诉我您无法编辑代码，那么您应该确保自己清理状态。

像这样的事情就可以解决问题：

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，您可能需要考虑实施JUnit 规则。