Resolving the Target type is not an interface Error in Spring Boot with CLOBs

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Understanding the Error: Target Type is Not an Interface

When developing applications using Spring Boot and Hibernate, you might encounter issues when trying to fetch complex data types from a database such as CLOB (Character Large Object). One such common problem is encountering the error message:

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This usually arises when attempting to use a projection with a native SQL query involving a CLOB field. In this post, we will dissect the root of this problem and explore the solution to properly implement projections in your Spring Boot application.

The Scenario

Suppose you have a table in an Oracle 19 database containing CLOB data that you want to fetch using a native query. Let’s take a look at your repository's code where the issue originates:

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DTO Interface for Projection

You have defined your projection interface as follows:

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The Problematic Line

The error appears when you try to access the textAnswer field in your test case:

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When you run this test, you receive the aforementioned error. This issue stems from how the JDBC driver interacts with CLOB types in this scenario.

Proposed Solution

Upon examining the error, it becomes clear that the JDBC driver treats the type as a Clob. Therefore, the projection in your MyDTO interface needs to reflect this correctly. Update the DTO as follows:

Updated DTO Interface

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Why This Works

By changing the projected return type from String to Clob, you align the data type with how JPA and the JDBC driver handle CLOB objects. This change allows Hibernate to manage the CLOB properly since it can now recognize and handle the JDBC Clob interface directly, avoiding type conversion issues.

Additional Notes

If you need to convert CLOB types to String, you can do so manually in your service layer after retrieving the data.

Ensure that other relevant annotations or settings in your entity class handling CLOB are correctly configured, such as @Lob or @Column(columnDefinition = "CLOB"), as these can aid in properly handling large text data.

Conclusion

By addressing the mismatch in data types between your projection interface and the database's CLOB type, you can effectively avoid the Target type is not an interface error. Implementing this change helps ensure smooth data retrieval and facilitates your application's interaction with large text fields in Oracle databases.

If you ever find yourself facing this issue, remember to double-check the types in your DTOs and adjust them to align with the underlying database structures. Happy coding!