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Discover how to resolve application run failures in Spring Boot caused by incorrect JPA mapping. Get practical advice for using String types and GeneratedValue.
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Solving the Application Run Failed Error in Spring Boot: Understanding JPA Mapping Issues

As a new coder venturing into the world of Spring Boot, encountering errors can be quite intimidating—especially when your application fails to run. One common error that developers face is due to incorrect Java Persistence API (JPA) mapping. This post is designed to help you understand why you might be facing the Application run failed error in Spring Boot and how to resolve it effectively.

The Problem at Hand

Imagine you've built a simple application that reads JSON data and saves it to a database using Spring Boot. You might have the following class to represent a City:

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Along with this, you created a repository as follows:

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After completing the above, let’s say you attempt to run your application, but you encounter an error. In a trial-and-error approach, you change the City class to:

Use Integer as the type:

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Change the corresponding JSON data to something numeric:

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Surprisingly, this works, and you don't see any errors when accessing your endpoint at http://localhost:8080/cities/list. But why is the application rejecting the String type? Let’s explore this issue further.

Understanding the JPA Mapping Issue

The root of your problem lies within the JPA mapping of the name field in your City class. Specifically, the @ GeneratedValue annotation is intended to automate the generation of unique identifiers (such as primary keys) for entity instances. Here’s what you need to know:

@ GeneratedValue Explanation: This annotation indicates that the persistence provider must assign primary keys for the entity using a specified strategy, in this case, GenerationType.AUTO. This strategy generates a key by incrementing a counter by 1 each time a new entry is added.

Incompatibility with Strings: The AUTO generation strategy is designed for numeric types (like Integer or Long) because they can be easily incremented. Using it with a String type—like your name field—does not make logical sense, leading to the application run failure.

The Solution

To resolve this issue, you need to change your City class to use a numeric identifier instead of a String for the name field. Here’s how you can do that:

Update the City Class:

Change the name attribute's type from String to Integer or Long.

Ensure that you're not using @ GeneratedValue with a field that doesn't adhere to the increment logic.

Here is a revised version of your class:

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Modify the JSON Input:

Ensure that your JSON input reflects the new structure. For example:

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Continue Using the Repository:

Your CourierRepository can stay the same as it now correctly maps to the City entity when using its updated structure.

Conclusion

By understanding JPA mapping issues and ensuring that correct data types and strategies are utilized, you can prevent application run failures in Spring Boot. Whenever you're using @ GeneratedValue, make sure it corresponds with a numeric primary key rather than attempts to apply it to strings. With this knowledge, you can now confidently tackle similar issues in your development journey!