Why is my Spring @Autowired field null?
If your `@Autowired` field is null, it's likely because the class it resides in is not managed by Spring. Annotate the class with `@Component` or `@Service`. Also, ensure that Spring's component scan includes the target package. Instead of field injection, prefer constructor injection. This avoids null fields if the class is manually instantiated:
```java
@Service
public class MyService {
private final MyDependency dependency;
@Autowired
public MyService(MyDependency dependency) {
this.dependency = dependency; // 🏀 Looks like someone's got a handle on dependencies...
}
}
Ensure MyDependency is a managed bean and your application class is annotated with @SpringBootApplication, with the correct package scan:
Getting the gist of Spring bean injection
To dodge null fields in Spring, grasp the way the Spring container manages beans. Here are a handful of key points that you should bear in mind:
Manual instantiation - Stay away!
Avoid creating objects with new. It sidesteps the Spring context and leaves dependencies unmanaged.
Bean lifecycle - Know your lifecycle events
Understanding when and how Spring beans undergo creation, initialization, and destruction.
Scoped beans - Get to know the business of beans
Use scoped proxies for specific scopings such as request or session to ensure Spring manages beans for each individual scope.
Legacy code - Build bridges not walls
To integrate non-Spring-managed legacy code, use @Configurable with AspectJ weaving or make use of a singleton adapter class that Spring can autowire.
Chasing the answers
When you encounter null Autowired fields, ask yourself the following:
- Is my class suitably adorned with Spring-based annotations like
@Component,@Service, etc? - Have I set up component scanning to include the correct packages?
- Have I been blending constructor and field injection, leading to a confusing mix?
Ways to do some detective work
- Turn on debug logs for the Spring framework.
- Use
@PostConstructto check if the dependencies have been appropriately injected. - Utilize the old-school, but still effective, breakpoints in your IDE.
The Screwdriver Isn't Enough - Advanced tactics
Let's explore some advanced strategies and norms that will keep your project afloat in the evolving Spring ecosystem.
Configuration over convention
Embrace Java configuration via @Configuration and @Bean for a more granular handling of beans.
More power to you - Bean definition manipulation
Deal with bean definitions directly using BeanDefinitionRegistry or BeanFactoryPostProcessor giving you dynamic abilities for bean registration and configuration.
AOP - Majorly underrated
Make use of Aspect Oriented Programming (AOP) and @Configurable for non-Spring-managed beans - for injecting dependencies even into manually instantiated objects!
Clean code - Not just a book!
Maintain small, purpose-driven beans, prefer constructor injection for immutable fields, and avoid circular dependencies for a cleaner, smoother working Spring context.
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