JUnit+Mockito测试时如何阻止被测方法调用其他类方法？

在使用JUnit结合Mockito测试QueryHandler类的addMessage方法时，尝试让依赖的ResponseBuilder类的addMessageResponse方法抛出AddMessageResponseException，但实际运行时抛出了NullPointerException，mock的拦截逻辑未生效。

测试类代码 #

@ExtendWith(MockitoExtension.class)
class QueryHandlerTest {

    @InjectMocks
    QueryHandler queryHandler;

    @Mock
    ResponseBuilder responseBuilder;

    @BeforeEach
    void setUp() {
        MockitoAnnotations.openMocks(this);
    }

    @Test
    void TC5() {
        doThrow(AddMessageResponseException.class).when(responseBuilder).addMessageResponse(isA(Boolean.class), isA(Boolean.class));
        assertThrows(AddMessageResponseException.class, ()-> queryHandler.addMessage("Hello",true));
    }
}

被测的addMessage方法代码 #

public void addMessage(String message, boolean lengthExceedsLimit) {
    boolean messageAdded;

    if (checkIfJarExists()) {
        if (!lengthExceedsLimit) {
            // attempt to add the message to the jar
            messageAdded = addMessageQuery(new Message(event.getMessageAuthor().getIdAsString(), message));
        } else {
            messageAdded = false;
        }
    } else {
        messageAdded = false;
    }

    responseBuilder.addMessageResponse(messageAdded, lengthExceedsLimit);

    if (messageAdded) {
        // check to see if the jar's message limit has been reached; if so, perform opening ceremony
        if (checkMessageLimit()) {
            responseBuilder.performOpeningEvent(currentJar);
            deleteJarQuery(this.serverId);
        }
    }
}

被调用的addMessageResponse方法代码 #

public void addMessageResponse(boolean messageAdded, boolean lengthExceedsLimit){

    if (lengthExceedsLimit) {

        event.getChannel().sendMessage("I'm sorry, your message is too long. Please limit your message " +
                "to 250 characters or less.");

    } else if(messageAdded){

        String nickname = getNickname();
        event.getChannel().sendMessage("Thanks, " + nickname + "! Your message has " +
                "been added to the jar!");

    } else {

        event.getChannel().sendMessage("Sorry, it looks like a jar has not been set up for your server. " +
                "If you're a server admin, you can create a jar! " +
                "Please use '!tiko help' to see a list of my commands.");

    }
}

报错信息 #

org.opentest4j.AssertionFailedError: Unexpected exception type thrown, 
Expected :class com.tikoJar.exceptions.AddMessageResponseException
Actual   :class java.lang.NullPointerException
<Click to see difference>

...

Caused by: java.lang.NullPointerException: Cannot invoke "org.javacord.api.entity.channel.TextChannel.sendMessage(String)" because the return value of "org.javacord.api.event.message.MessageCreateEvent.getChannel()" is null
    at com.tikoJar.DTO.ResponseBuilder.addMessageResponse(ResponseBuilder.java:35)
    at com.tikoJar.DTO.QueryHandler.addMessage(QueryHandler.java:74)
    at com.tikoJar.DTO.QueryHandlerTest.lambda$TC5$0(QueryHandlerTest.java:68)
    at org.junit.jupiter.api.AssertThrows.assertThrows(AssertThrows.java:53)
    ... 73 more

核心原因 #

mock拦截未生效是因为匹配器类型不匹配：你使用isA(Boolean.class)匹配包装类型，但addMessageResponse方法的参数是boolean基本类型，Mockito无法匹配到对应的方法调用，因此执行了真实的方法逻辑，而真实方法中的event对象为null，触发了NullPointerException。

修复步骤 #

1. 替换匹配器：将isA(Boolean.class)改为anyBoolean()，该匹配器专门用于匹配boolean基本类型，能正确匹配方法参数。
2. 移除冗余初始化：已经使用@ExtendWith(MockitoExtension.class)，无需再在setUp中调用MockitoAnnotations.openMocks(this)，扩展会自动完成mock对象的初始化。

修改后的测试类代码 #

@ExtendWith(MockitoExtension.class)
class QueryHandlerTest {

    @InjectMocks
    QueryHandler queryHandler;

    @Mock
    ResponseBuilder responseBuilder;

    @Test
    void TC5() {
        // 使用anyBoolean()匹配基本类型参数
        doThrow(AddMessageResponseException.class)
            .when(responseBuilder)
            .addMessageResponse(anyBoolean(), anyBoolean());
            
        assertThrows(AddMessageResponseException.class, ()-> queryHandler.addMessage("Hello", true));
    }
}

补充说明 #

如果之前尝试doNothing()未生效，也是同样的匹配器类型问题，修改为anyBoolean()后，doNothing()也能正常拦截方法调用。

内容的提问来源于stack exchange，提问作者Matthew Brown