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Unit test autowired field NullPointerException

Cowrie
Cowrie
Dev @ Bswen

Introduction

This post demonstrates how to resolve the NullPointerException issue when using SpringBoot unit testing.

Environments

  • SpringBoot 1.5.9
  • Java 1.8

Class under test

This is the class under test, it’s just a simple class.

LambdaCUT.java
// An interface which returns the length of a string
public interface StringLengther {
    int getLength(String s);
}


@Component
public class LambdaCUT {
    public int s1(StringLengther stringLengther, String s) {
        return stringLengther.getLength(s);
    }
}

As you can see, this class uses the StringLengther to get the string length.

The unit test code

TestLambdas.java
public class TestLambdas {
    @Autowired
    private LambdaCUT lambdaCUT;


    @Test
    public void test3() {
        assertEquals(lambdaCUT.s1((String s) -> s.length(), "comeon"), 6);
    }
}

As you can see, we use a @Test annotation to test the lambdaCUT’s s1 method, and the lambdaCUT instance is @Autowired by the Spring container.

Problem occurring

Now run the unit test, and you can see the result:

java.lang.NullPointerException
    at TestLambdas.test3(TestLambdas.java:45)

Problem debugging

We can debug the problem by adding a breakpoint at the line of the problem and rerunning it. We would get this:

image-title-here

We can see that the root cause is the lambdaCUT property being null, even though we @Autowired it! The autowire process must be disabled for some reason.

Problem resolving

After debugging, we found that the root cause is the @Autowired not working, and we discovered that the unit test is a common JUnit test case, not a SpringBoot test case. Therefore, there is no Spring container for it. This is the root cause. We then changed the code like this:

TestLambdas.java
@RunWith(SpringRunner.class)
@SpringBootTest
public class TestLambdas {
    @Autowired
    private LambdaCUT lambdaCUT;


    @Test
    public void test3() {
        assertEquals(lambdaCUT.s1((String s) -> s.length(), "comeon"), 6);
    }
}

Rerun the test, and we get this:

 .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v1.5.9.RELEASE)
2025-01-26 21:19:25.414  INFO 38582 --- [           main] java8.learn.lambda.TestLambdas           : Starting TestLambdas on -Pro.local with PID 38582 (started by  in /Users/xxx/JavaProjects/xxx_tests/test-java8)
2025-01-26 21:19:25.415 DEBUG 38582 --- [           main] java8.learn.lambda.TestLambdas           : Running with Spring Boot v1.5.9.RELEASE, Spring v4.3.13.RELEASE
2025-01-26 21:19:25.416  INFO 38582 --- [           main] java8.learn.lambda.TestLambdas           : No active profile set, falling back to default profiles: default
2025-01-26 21:19:25.956  INFO 38582 --- [           main] java8.learn.lambda.TestLambdas           : Started TestLambdas in 1.196 seconds (JVM running for 2.167)
Process finished with exit code 0

Summary

In this post, we explored how to resolve the NullPointerException issue when using SpringBoot unit testing. The key takeaway is to ensure that your unit test class is annotated with @RunWith(SpringRunner.class) and @SpringBootTest to enable the Spring context and dependency injection. By doing so, the @Autowired fields will be properly initialized, and the test will run as expected. This approach is essential for integrating SpringBoot features into your unit tests.

Final Words + More Resources

My intention with this article was to help others who might be considering solving such a problem. So I hope that’s been the case here. If you still have any questions, don’t hesitate to ask me by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!

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