Walk Through the Code

Let’s inject a bean into a static field with @Autowired in Spring. Assume the following code exists.

@Component
class Something {
    public void sayHi() {
        System.out.println("Hi");
    }
}

class Someone {
    // want to use it...
    @Autowired
    public static SomeObject someObject;

    public static void say() {
        someObject.sayHi();
    }
}

// config omitted
public class InjectionTest {

    @Test
    public void method() {
        // NullPointerException
        Someone.say();
    }
}

Running this throws NullPointerException. Something is created after the Someone class is loaded, so the bean does not exist at class-load time.

More importantly, @Autowired works only for classes managed by Spring. Someone is not a Spring bean, so Spring never injects it. Even if it were managed, the Spring context is not ready at class-load time, so static injection still fails.

Fix

First, make the class Spring-managed.

@Component
class Someone {
    // ...
}

Then assign the injected instance during construction. You can use a constructor or a setter, or use @PostConstruct.

@Component
class Someone {
    public static SomeObject someObject;

    // Option 1) setter
    @Autowired
    public void setSomeObject(SomeObject someObject) {
        this.someObject = someObject;
    }
    
    // Option 2) constructor
    @Autowired
    private Someone(SomeObject someObject) {
        this.someObject = someObject;
    }

    public static void say() {
        someObject.sayHi();
    }
}

@Component
class Someone {
    @Autowired
    private SomeObject beanObject;
    public static SomeObject someObject;
    
    @PostConstruct
    private void initialize() {
        this.someObject = beanObject;
    }

    public static void say() {
        someObject.sayHi();
    }
}