Jackson Fails Due to FAIL_ON_TRAILING_TOKENS: Understanding the Issue and Finding Solutions

Are you frustrated with the error message “FAIL_ON_TRAILING_TOKENS” popping up when working with Jackson, the popular Java library for JSON processing? You’re not alone! In this article, we’ll delve into the reasons behind this error, its implications, and most importantly, provide you with clear instructions on how to resolve it.

What is FAIL_ON_TRAILING_TOKENS?

FAIL_ON_TRAILING_TOKENS is a feature in Jackson that is enabled by default. It’s designed to prevent JSON parsing issues caused by trailing tokens, which are extra characters or tokens at the end of a JSON string. When this feature is enabled, Jackson will throw an exception if it encounters any trailing tokens during parsing.

{
    "name": "John",
    "age": 30,
    " occupation": "Developer",  // trailing comma
}

In the example above, the trailing comma after the “occupation” key will cause Jackson to fail with a FAIL_ON_TRAILING_TOKENS error.

Why Does Jackson Fail with FAIL_ON_TRAILING_TOKENS?

There are several reasons why Jackson might fail with a FAIL_ON_TRAILING_TOKENS error:

• Trailing commas or tokens: As mentioned earlier, trailing commas or tokens can cause Jackson to fail. This can occur when you accidentally add an extra character or token at the end of a JSON string.
• Invalid JSON syntax: If your JSON string contains invalid syntax, such as mismatched brackets or quotes, Jackson will throw a FAIL_ON_TRAILING_TOKENS error.
• Configurations and settings: Jackson’s configuration and settings can also cause the FAIL_ON_TRAILING_TOKENS error. For instance, if you’ve enabled the `FAIL_ON_TRAILING_TOKENS` feature explicitly, or if you’re using an outdated version of Jackson.

Resolving the FAIL_ON_TRAILING_TOKENS Error

Now that we’ve covered the reasons behind the FAIL_ON_TRAILING_TOKENS error, let’s dive into the solutions:

Disable FAIL_ON_TRAILING_TOKENS Feature

One way to resolve the error is to disable the FAIL_ON_TRAILING_TOKENS feature altogether. You can do this by creating a custom `ObjectMapper` instance and configuring it to ignore trailing tokens:

ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_TRAILING_TOKENS);

This approach is useful when you’re working with JSON data that contains trailing tokens or commas. However, be aware that disabling this feature can lead to unexpected behavior or errors if your JSON data is malformed.

Use a Custom JSON Parser

An alternative approach is to use a custom JSON parser that’s more lenient when it comes to trailing tokens. For example, you can use the `JsonParser` class to parse your JSON data:

JsonParser parser = mapper.getFactory().createParser("{}");
JsonNode node = parser.readValueAsTree();

This method gives you more control over the parsing process, allowing you to handle trailing tokens or errors manually.

Validate Your JSON Data

The most straightforward solution is to ensure that your JSON data is valid and free of trailing tokens or commas. You can use online tools or libraries to validate your JSON data before attempting to parse it with Jackson.

JSON Data	Validation Result
{ “name”: “John”, “age”: 30, ” occupation”: “Developer”, }	Invalid (trailing comma)
{ “name”: “John”, “age”: 30, ” occupation”: “Developer” }	Valid

By validating your JSON data, you can identify and fix any issues before they cause problems with Jackson.

Upgrade Jackson to the Latest Version

If you’re using an outdated version of Jackson, it’s possible that the FAIL_ON_TRAILING_TOKENS error is due to a bug or issue that’s been fixed in newer versions. Make sure to upgrade to the latest version of Jackson to take advantage of any bug fixes or improvements.

1. Check the Jackson version in your project: mvn dependency:tree | grep jackson
2. Update the Jackson version in your pom.xml file: <version>2.12.3</version>
3. Run mvn clean package to rebuild your project

Conclusion

The FAIL_ON_TRAILING_TOKENS error in Jackson can be frustrating, but with the right knowledge and tools, you can overcome it. By understanding the reasons behind this error and implementing the solutions outlined in this article, you’ll be well on your way to parsing JSON data like a pro.

Remember to validate your JSON data, disable the FAIL_ON_TRAILING_TOKENS feature if necessary, and upgrade to the latest version of Jackson. With these tips and techniques, you’ll be able to tackle even the most challenging JSON parsing tasks with confidence.

Additional Resources

• Jackson GitHub Repository
• JSON Official Website
• Stack Overflow: Jackson Deserialization – Fail on trailing tokens

By following the instructions and explanations in this article, you should be able to resolve the FAIL_ON_TRAILING_TOKENS error in Jackson and take your JSON parsing skills to the next level. Happy coding!

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