When working with Spring Framework, a common issue developers face is that an @Autowired field ends up being null, which can lead to NullPointerExceptions during runtime. To understand why this happens, it's important to first grasp how Spring's dependency injection works and some common pitfalls that can lead to this problem.

Understanding Dependency Injection in Spring

Dependency Injection (DI) is a core concept of the Spring Framework, which allows classes to define their dependencies without constructing them. Spring container is responsible for wiring these dependencies at runtime. The @Autowired annotation is used to automatically inject dependencies. When Spring initializes the beans, it resolves @Autowired annotations and fulfills the dependencies by type, name, or qualifiers.

Reasons for @Autowired Field Being Null

The following are some common reasons why an @Autowired field might be null:

1. Improper Spring Bean Configuration: The class with @Autowired fields itself must be managed by the Spring container. If the class is not a Spring bean, Spring cannot manage its dependencies. This often happens if the object is created using the new keyword instead of being fetched from the Spring application context.
2. Context Configuration Issues: If the context configuration does not include the packages where the beans are located, Spring will not know about these beans, and hence it will not inject the dependencies.
3. Component Scan Not Configured Correctly: If Spring's component scan is not configured to include the package of the dependency, Spring won't be able to find and create the bean required for injection.
4. Circular Dependencies: If you have circular dependencies in your beans, it can lead to cases where one of the beans in the cycle is not fully initialized leading to null fields.
5. Bean Creation Order: Sometimes, the order in which beans are created matters. If a bean is instantiated before another bean which it depends on, it might not be able to inject the dependency if it is not ready.
6. Using field injection with final fields: Since final fields must be initialized when an object is created, Spring cannot inject a value into a final field using field injection as it uses reflection which is done post-object creation.

Example to Demonstrate the Issue

Consider you have a BookService class which depends on a BookRepository bean:

1@Component
2public class BookService {
3    @Autowired
4    private BookRepository bookRepository;
5
6    public void performAction() {
7        bookRepository.save(new Book());
8    }
9}

Now, if BookService is not recognized as a bean (perhaps because it's directly instantiated using new BookService() somewhere in your code or not defined properly in Spring's application context), bookRepository will be null.

Corrective Actions and Best Practices

To resolve issues related to @Autowired fields being null, consider the following steps and best practices:

• Ensure Proper Bean Management: Always ensure that your beans are properly declared and managed by Spring. Use annotations like @Component, @Service, @Repository, etc., or XML/Java Config configurations to declare beans.
• Component Scan: Make sure your Spring configuration, via XML or Java config, includes proper component scanning through @ComponentScan annotation.
• Prefer Constructor Injection: Instead of field injection, use constructor injection. This not only avoids null issues but also makes your code cleaner and more testable.

1@Service
2public class BookService {
3    private final BookRepository bookRepository;
4
5    @Autowired
6    public BookService(BookRepository bookRepository) {
7        this.bookRepository = bookRepository;
8    }
9}

• Use @PostConstruct for Initialization: For off-the-line initializations, use @PostConstruct to ensure all dependencies are injected before utilisation.

Summary Table

By understanding and mitigating these common pitfalls with the correct approaches, maintaining effective Spring-based applications becomes much more manageable.