JUnit测试中TheConfiguration Mock为空引发空指针异常问题排查

问题重现 #

待测试的Authentication类：

@Component
@Slf4j
public class Authentication {

    private final RestTemplate restTemplate;

    @Autowired
    private TheConfiguration config;

    public Authentication(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }


    public String generateAuthentification(){

        String plainCreds = config.geUser()+":"+config.getPassword();
        byte[] plainCredsBytes = plainCreds.getBytes();
        byte[] base64CredsBytes = Base64.encodeBase64(plainCredsBytes);

        return new String(base64CredsBytes);
    }

}

JUnit测试代码：

import static org.mockito.Mockito.when;
import org.apache.commons.codec.binary.Base64;
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 org.springframework.web.client.RestTemplate;

import com.example.TheConfiguration;

@ExtendWith(MockitoExtension.class)
class AuthenticationTest {

    @InjectMocks
    Authentication authentication;

    @Mock
    RestTemplate restTemplate;

    @Mock
    TheConfiguration config;

    @Test
    void testGenerateAuthentifications() {

        when(config.getUser()).thenReturn("user");
        when(config.getPassword()).thenReturn("pass");
        String plainCreds = config.getUser() + ":" + config.getPassword();
        System.out.println(plainCreds);
        byte[] plainCredsBytes = plainCreds.getBytes();
        byte[] base64CredsBytes = Base64.encodeBase64(plainCredsBytes);
        String a = new String(base64CredsBytes);

        assertEquals(authentication.generateAuthentification(),a);

    }

}

运行测试时抛出NullPointerException：

Cannot invoke "com.example.TheConfiguration.getUser()" because "this.config" is null

完整错误日志：

java.lang.NullPointerException: Cannot invoke "com.example.TheConfiguration.getUser()" because "this.config" is null
at com.example.Authentication.generateAuthentification(Authentication.java:26)
at com.example.AuthenticationTest.testGenerateAuthentifications(AuthenticationTest.java:38)
... 省略后续堆栈信息

错误原因 #

Mockito的@InjectMocks注入优先级为：构造函数注入 > setter方法注入 > 字段注入。你的Authentication类有一个带参数的构造函数（仅注入RestTemplate），Mockito会优先使用这个构造函数创建实例，不会自动注入@Autowired标注的config字段，导致config为null。

另外，代码里存在笔误：config.geUser()应该是config.getUser()，这也是后续需要修正的点。

解决方法 #

方法1：构造函数注入所有依赖（推荐，符合Spring最佳实践） #

将TheConfiguration加入构造函数，移除字段上的@Autowired：

@Component
@Slf4j
public class Authentication {

    private final RestTemplate restTemplate;
    private final TheConfiguration config;

    // Spring 4.3+支持构造函数自动装配，无需@Autowired
    public Authentication(RestTemplate restTemplate, TheConfiguration config) {
        this.restTemplate = restTemplate;
        this.config = config;
    }


    public String generateAuthentification(){
        // 修正笔误：geUser -> getUser
        String plainCreds = config.getUser()+":"+config.getPassword();
        byte[] plainCredsBytes = plainCreds.getBytes();
        byte[] base64CredsBytes = Base64.encodeBase64(plainCredsBytes);

        return new String(base64CredsBytes);
    }

}

修改后，Mockito的@InjectMocks会自动通过构造函数注入两个Mock对象，测试代码无需改动。

方法2：给config添加Setter方法 #

如果不想修改构造函数，可以给config添加Setter方法，让Mockito通过Setter注入：

@Component
@Slf4j
public class Authentication {

    private final RestTemplate restTemplate;

    private TheConfiguration config;

    public Authentication(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    @Autowired
    public void setConfig(TheConfiguration config) {
        this.config = config;
    }


    public String generateAuthentification(){
        String plainCreds = config.getUser()+":"+config.getPassword(); // 修正笔误
        byte[] plainCredsBytes = plainCreds.getBytes();
        byte[] base64CredsBytes = Base64.encodeBase64(plainCredsBytes);

        return new String(base64CredsBytes);
    }

}

方法3：测试中手动注入config #

在测试方法里通过反射手动给authentication的config字段赋值：

@Test
void testGenerateAuthentifications() {
    // 手动注入config
    ReflectionTestUtils.setField(authentication, "config", config);

    when(config.getUser()).thenReturn("user");
    when(config.getPassword()).thenReturn("pass");
    String plainCreds = config.getUser() + ":" + config.getPassword();
    byte[] plainCredsBytes = plainCreds.getBytes();
    byte[] base64CredsBytes = Base64.encodeBase64(plainCredsBytes);
    String a = new String(base64CredsBytes);

    assertEquals(authentication.generateAuthentification(),a);
}

需要导入org.springframework.test.util.ReflectionTestUtils类。

内容的提问来源于stack exchange，提问作者Marcos