NullPointerException in mockito unit test

Question

这是我尝试单元测试的SendEmail类。我在下面显示的行上收到一个 NullPointerException，但我不知道为什么。

另外，我是否正确组织了代码？我不完全知道我是否正确使用了 mockito。

public class StatsTest extends AbstractTestCase {
    @Mock
    MultiPartEmail MultiPartEmailMock;
    StatsTest statsTestObj;
    SendEmail mockedEmail;
    @Before
    public void setUp() throws Throwable {
        super.setUp();
        MockitoAnnotations.initMocks(this);
    }
    @Test(expected = ValidationException.class)
    public void testSendEmail() throws EmailException, IOException {
        MultiPartEmail multiPartEmailMock;
        SendEmail mockedEmail = Mockito.mock(SendEmail.class);
        Mockito.when((mockedEmail.getHtmlEmail()).send()) 
            .thenThrow(new ValidationException("Could not send the Email."));
        ^^ the line above is where the null pointer error is
        mockedEmail.sendEmail();
    }
}

以下是正在测试的类：

public class SendEmail {
    private StringBuilder emailBody;
    private String senderEmail;
    private ArrayList<String> receiversEmails;
    private String emailSubject;
    private String hostName;
    private MultiPartEmail htmlEmail;
    public SendEmail(StringBuilder emailBody, String senderEmail, ArrayList<String> 
        receiversEmails, String emailSubject, String hostName, MultiPartEmail htmlEmail) {
        this.setEmailBody(emailBody);
        this.setSenderEmail(senderEmail);
        this.setReceiversEmails(receiversEmails);
        this.setEmailSubject(emailSubject);
        this.setHostName(hostName);
        this.setHtmlEmail(htmlEmail);
    }
    public void sendEmail()throws IOException, EmailException{
        htmlEmail.setHostName(getHostName());
        htmlEmail.addTo(getReceiversEmails().get(0));
        for(int i=0; i<getReceiversEmails().size(); i++){
            htmlEmail.addCc(getReceiversEmails().get(i));   
        }
        htmlEmail.setFrom(getSenderEmail());
        htmlEmail.setSubject(getEmailSubject());
        htmlEmail.setMsg((getEmailBody()).toString());
        htmlEmail.send();
    }
}

Answer 1

我认为您对需要测试和模拟的内容有点困惑。Mockito提供了不同的方法来创建模拟对象，例如：@Mock或Mockito.mock()。还有不同的方法可以将这些模拟对象注入到待测试的类中，以便在该类上单元测试方法。

如果没有异常或其他类的所有细节，这将不是一个完整的答案，但我希望它有助于澄清一些概念。

注意：以下内容可能无法编译，但希望您能理解这个想法。

public class StatsTest extends AbstractTestCase {
    @Mock MultiPartEmail mockMultiPartEmail; // this mock will be used in the instantiaion of the class under test
    SendEmail sendEmail; // renamed as this is not a mock, it is the class under test
    // define some things we can make assertions against (probably in some other tests...)
    private static final StringBuilder BODY = new StringBuilder("body");
    private static final String SENDER = "[e-mail]";
    private static final Collection<String> RECIPIENTS = Arrays.asList("[e-mail]")
    private static final String SUBJECT = "subject";
    private static final String HOSTNAME = "hostname";
    @Before
    public void setUp() throws Throwable {
        super.setUp();
        MockitoAnnotations.initMocks(this); // will instantiate "mockMultiPartEmail"
        // instantiate our class under test
        sendEmail = new SendEmail(BODY, SENDER, RECIPIENTS, SUBJECT, HOSTNAME, mockMultiPartEmail);
    }
    @Test(expected = ValidationException.class)
    public void testSendEmail() throws EmailException, IOException {
        // given some condition
        Mockito.when(mockMultiPartEmail.send()).thenThrow(new ValidationException("Could not send the Email."));
        // when the method under test is called
        sendEmail.sendEmail();
        // then the exception will be thrown (and you have correctly expected this on the @Test annotation)
    }
}