Resolving null Issues with String.format in Spring Boot when Using @ Value

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Let’s dive into what’s happening in your Spring Boot application and how you can resolve this issue effectively.

Breakdown of the Issue

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While you might expect the entranceId to be utilized successfully in URL construction, the resulting URL often contains the string 'null'. This happens due to the order in which Spring processes the annotations during the lifecycle of the object.

Exposing the Root Cause

The key detail that is overlooked here is that Spring initializes the fields of a class after its construction. This can lead to a situation where your variable is accessed before it's populated by Spring. Here’s the critical sequence that occurs:

new Controller() is called, creating a new object instance.

Spring detects the @ Value annotation and uses reflection to set the entranceId field.

Any construction callbacks, such as @ PostConstruct, are invoked after the object is constructed.

How to Fix the Null Value Issue

To resolve this problem, you can take one of two approaches:

1. Using @ PostConstruct

By using the @ PostConstruct annotation, you can ensure that the URL is constructed only after Spring has fully injected the property values:

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2. Utilize Constructor Injection

Another effective approach is to use constructor injection, allowing you to construct your URL right after the property is injected:

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Conclusion

This not only solves your immediate problem but also enhances the design of your code for better maintainability and readability.

If you have further questions or another issue arises, feel free to reach out. Happy coding!