仅用Mockito/JUnit如何Mock javax.mail.Transport类？

一、仅用Mockito/JUnit实现：重构代码解耦依赖 #

Mockito默认不支持Mock静态方法，所以要不用PowerMock，必须先重构MailSender，把对Transport的静态调用改成可注入的依赖：

1. 重构MailSender类 #

把静态方法改为实例方法，封装邮件发送逻辑为可替换的接口：

import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Session;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import java.util.Properties;

public class MailSender {
    // 定义接口封装发送逻辑
    public interface MailTransport {
        void send(MimeMessage message) throws MessagingException;
    }

    private final MailTransport transport;

    // 生产环境使用默认实现
    public MailSender() {
        this.transport = javax.mail.Transport::send;
    }

    // 测试环境注入Mock实现
    public MailSender(MailTransport transport) {
        this.transport = transport;
    }

    // 改为实例方法
    public boolean sendMail(String fromAddress, String content, String toAddress) {
        try {
            // 原有构造MimeMessage的逻辑示例
            MimeMessage mimeMessage = new MimeMessage(Session.getDefaultInstance(new Properties()));
            mimeMessage.setFrom(new InternetAddress(fromAddress));
            mimeMessage.addRecipient(Message.RecipientType.TO, new InternetAddress(toAddress));
            mimeMessage.setText(content);

            transport.send(mimeMessage);
            return true;
        } catch (MessagingException e) {
            e.printStackTrace();
            return false;
        }
    }
}

2. 编写Mockito测试 #

现在可以正常Mock自定义的MailTransport接口，完全避免实际发邮件：

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import javax.mail.MessagingException;
import javax.mail.internet.MimeMessage;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.doNothing;
import static org.mockito.Mockito.doThrow;
import static org.mockito.Mockito.any;

@ExtendWith(MockitoExtension.class)
class MailSenderTest {

    @Mock
    private MailSender.MailTransport mockTransport;

    @InjectMocks
    private MailSender mailSender;

    @Test
    void sendMail_shouldReturnTrueWhenSuccess() throws MessagingException {
        // Mock发送成功的场景
        doNothing().when(mockTransport).send(any(MimeMessage.class));

        boolean result = mailSender.sendMail("me@gmail.com", "hi", "you@gmail.com");
        assertEquals(true, result);
    }

    @Test
    void sendMail_shouldReturnFalseWhenFailure() throws MessagingException {
        // Mock发送失败的场景
        doThrow(new MessagingException("发送失败")).when(mockTransport).send(any(MimeMessage.class));

        boolean result = mailSender.sendMail("me@gmail.com", "hi", "you@gmail.com");
        assertEquals(false, result);
    }
}

二、不重构代码的方案：使用PowerMock（不推荐） #

如果必须保留原有静态方法和直接调用Transport.send的逻辑，才需要用PowerMock来Mock静态方法：

1. 添加PowerMock依赖（Maven示例） #

<dependencies>
    <dependency>
        <groupId>org.powermock</groupId>
        <artifactId>powermock-module-junit4</artifactId>
        <version>2.0.9</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.powermock</groupId>
        <artifactId>powermock-api-mockito2</artifactId>
        <version>2.0.9</version>
        <scope>test</scope>
    </dependency>
</dependencies>

2. 编写PowerMock测试代码 #

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import javax.mail.Transport;
import javax.mail.internet.MimeMessage;
import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.any;

@RunWith(PowerMockRunner.class)
@PrepareForTest(Transport.class) // 指定要Mock的静态类
class MailSenderTest {

    @Test
    void sendMail_success() throws Exception {
        // Mock静态方法Transport.send
        PowerMockito.mockStatic(Transport.class);
        PowerMockito.doNothing().when(Transport.class);
        Transport.send(any(MimeMessage.class));

        boolean result = MailSender.sendMail("me@gmail.com", "hi", "you@gmail.com");
        assertEquals(true, result);
    }
}

原有测试代码的问题总结 #

1. @Mock和@InjectMocks注解使用错误：应该给测试类MailSender加@InjectMocks，给依赖对象加@Mock
2. Mockito无法直接MockTransport的静态方法，这是核心限制
3. 静态方法sendMail不应通过实例调用，且Mockito无法Mock静态方法内部的静态调用
4. 字符串转义错误："you@gmail.com"写成了""you@gmail.com""

内容的提问来源于stack exchange，提问作者likejudo