Скачать или смотреть Resolving the null Value Issue with the Spring @Value Annotation in Your Application

Discover why your @Value annotated parameters are instantiated as `null` in Spring Boot and learn how to correctly access them using the `@PostConstruct` annotation.
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This video is based on the question https://stackoverflow.com/q/78018480/ asked by the user 'vs777' ( https://stackoverflow.com/u/2101649/ ) and on the answer https://stackoverflow.com/a/78018515/ provided by the user 'johyunkim' ( https://stackoverflow.com/u/23435396/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Why Your @Value Annotation is Instantiated as null in Spring Boot

If you've been working with Spring Boot and encountered an issue where a member variable annotated with @Value is being instantiated as null, you're not alone. This common challenge often arises from a misunderstanding of the Spring Bean's lifecycle. Let’s delve into the reason behind this behavior and how to address it effectively.

The Problem: null Value in Spring Boot

In a Spring Boot application, using the @Value annotation allows you to inject values from a properties file or YAML configuration directly into your beans. However, suppose you've set up your class as follows:

[[See Video to Reveal this Text or Code Snippet]]

And your application.yml file contains:

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When you run the application, you might see the following output:

[[See Video to Reveal this Text or Code Snippet]]

The Solution: Understanding Spring Bean's Lifecycle

The key to solving this issue lies in understanding when Spring injects these values. Here’s why it happened:

Constructor Invocation: When you create a Spring Bean, the constructor is invoked before any dependency injection occurs. As a result, your username variable remains uninitialized (null) when you log its value in the constructor.

Using @PostConstruct to Initialize After Injection

To correctly log or utilize the injected property value, you should leverage the @PostConstruct annotation. This annotation allows you to define a method that will be called after the properties have been set (after dependency injection is complete).

Here's how you can modify your code:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Explained:

Constructor: The default constructor has been retained but is now empty, as the logging is moved to the @PostConstruct method.

@PostConstruct Method: The init() method, annotated with @PostConstruct, ensures that the log statement executes after the property injection, thereby reflecting the correct value of username.

Conclusion

In summary, the null issue you encountered stems from the timing of Spring's dependency injection relative to the constructor's execution. By utilizing the @PostConstruct annotation, you can ensure your injected values are accessible and appropriately logged. This approach not only helps clarify your code's logic but also aligns with Spring's architectural principles.

If you have further questions or need more examples regarding dependency injection in Spring Boot, feel free to reach out!