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Solving NullPointerException with OneToMany Eager Fetch in Spring Boot

Learn how to troubleshoot and fix `NullPointerException` errors caused by OneToMany Eager Fetch in Spring Boot applications, ensuring your code runs smoothly and efficiently. --- This video is based on the question https://stackoverflow.com/q/72328830/ asked by the user 'Nortap' ( https://stackoverflow.com/u/12365667/ ) and on the answer https://stackoverflow.com/a/72346914/ provided by the user 'Nortap' ( https://stackoverflow.com/u/12365667/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: NullPointerException and OneToMany Eager Fetch Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Understanding NullPointerException in OneToMany Eager Fetch If you've been developing applications using Spring Boot and Hibernate, you may have encountered a frustrating issue: a NullPointerException triggered during database operations. In particular, this can occur when you're dealing with OneToMany relationships and eager fetching. This guide will delve into a specific problem involving a budget field class, BudgetGeschaeftsfeld, and how to resolve the NullPointerException by revisiting the Spring JPA setup. Let’s break it down step by step. The Problem Imagine you have a service that needs to update a Betrag (let's assume this translates to "amount" in some financial context) in a BudgetGeschaeftsfeld. After setting this Betrag, the program attempts to call a method from the associated BudgetPlan to recalculate its own amount. However, you encounter the following error: [[See Video to Reveal this Text or Code Snippet]] This error indicates that the budgetPlan variable, which is expected to hold a reference to a BudgetPlan object, is actually null. But why does this happen in an environment set to eagerly fetch relationships? Analyzing the Code Here’s a simplified version of the relevant code for context: [[See Video to Reveal this Text or Code Snippet]] Why This Is Happening In the code above, the budgetPlan is expected to be initialized properly before the method berechnePrognose() is called. If for some reason budgetPlan is not set (think of it as not being fetched from the database), trying to call any method on it, including berechnePrognose(), will result in a NullPointerException. Solution Breakdown To resolve this issue, we need to make two key adjustments. Change the Fetch Type: Instead of fetching the budgetPlan eagerly, we can switch to a lazy fetch strategy. This can help ensure that references are not unnecessarily loaded into memory, which could lead to confusion if they are not set. Change the annotation from: [[See Video to Reveal this Text or Code Snippet]] to: [[See Video to Reveal this Text or Code Snippet]] Ensure BudgetPlan is Loaded: Before invoking berechnePrognose(), we should verify that budgetPlan is indeed loaded. This means fetching it explicitly when needed. You can modify your setBetrag method like this: [[See Video to Reveal this Text or Code Snippet]] Conclusion By implementing these changes, you can effectively prevent NullPointerExceptions related to OneToMany relationships in your Spring Boot applications. Remember that handling entity relationships wisely enhances the robustness of your application. If you ever run into similar issues, taking a step back to review your entity mappings and fetch strategies can often lead to a resolution. Happy coding!