Скачать или смотреть Solving the JSON parse error: Cannot deserialize instance of ArrayList in Java

Learn how to resolve the `JSON parse error` when sending data to a Java POST service. Step-by-step guide on JSON structure and Java object handling.
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This video is based on the question https://stackoverflow.com/q/63511273/ asked by the user 'Claudia Bermúdez' ( https://stackoverflow.com/u/13600245/ ) and on the answer https://stackoverflow.com/a/63512535/ provided by the user 'shinjw' ( https://stackoverflow.com/u/3810024/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding JSON Parse Errors in Java

When working with APIs, it's common to encounter errors related to JSON parsing. One such error that many developers face is the JSON parse error: Cannot deserialize instance of ArrayList. This error often occurs when sending data via a POST request that expects a certain data structure, but the provided JSON does not match that structure. Let's look at a real case scenario and see how to solve this issue.

A Practical Scenario

Imagine you are developing a web service, and you want to send JSON data to update an object in your Java backend. In this specific instance, the JSON you're trying to send looks like this:

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Your Java code is set up to handle this data like so:

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However, when you send this request, you encounter the following error:

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This indicates that your JSON structure does not match what your Java backend is expecting.

Breaking Down the Issue

The Core Problem

The primary issue here is that your API is expecting a List<UpdateDataMapping>, which translates to an array of JSON objects. However, the JSON you're sending is wrapped inside an information object. This is why Jackson (the library used to parse JSON in Java) cannot deserialize your JSON properly into a List<UpdateDataMapping> and throws the error.

Solutions to the Problem

There are a couple of ways to address this issue effectively:

1. Modify Your JSON Structure

The simplest solution is to change your JSON to directly represent a list of objects without any wrapping. Here’s how you can adjust your JSON:

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

Now, when this JSON is sent in the body of your POST request, Jackson will be able to directly map this to a List<UpdateDataMapping>.

2. Use a Wrapper Object for Your Data

If you need to keep the information object in your JSON, you will need to create a wrapper class to handle this structure. Here’s what that might look like:

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Then you can update your POST method to:

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

Conclusion

Encountering JSON parse errors can be frustrating, but understanding the reasons behind them will empower you to resolve such issues effectively. By either adjusting your JSON structure or using a wrapper object, you can ensure a smooth interaction between your frontend and backend applications. Always aim to match your JSON format with what your Java classes expect to avoid deserialization issues. Happy coding!