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Learn how to ensure that null values in JSON outputs from Apache Freemarker templates appear as `null` instead of the string "null".
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How to Omit Quotes for Null Values in JSON Using Apache Freemarker

When working with JSON in your applications, it's crucial to ensure that your JSON output reflects the correct data types. One common issue encountered when using Apache Freemarker is how to output a null value without it being represented as the string "null". In this guide, we will explore how to correctly handle null values in Freemarker JSON templates.

The Problem

Consider the following JSON template:

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If the variable fname is set to null, many users encounter this situation in their output:

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Instead of getting the expected JSON representation with a null value:

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This discrepancy occurs because Freemarker, by default, treats null values as strings when they are used in this manner. Let's dive into how we can resolve this issue.

Solution Overview

Starting from Apache Freemarker version 2.3.32, there is a straightforward solution. By utilizing the ?cn built-in function, you can effectively control how null values are represented. Here’s how it works.

Using the ?cn Built-in

The ?cn (nullable) operator allows you to output a null literal without it being quoted. To implement this in your JSON output, you should use:

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Key Points to Consider

No Quotes: It’s important to omit quotes around the ${...} when using ?cn to ensure the value is treated as a null rather than a string.

Automatic Handling: The ?cn operator automatically handles both strings and other data types (like numbers and booleans) so you don't have to worry about specific handling depending on the type.

Compatibility: By default, Freemarker's string formatting is JSON compatible, but you may want to check the c_format setting to optimize results further.

Alternative Solutions for Older Versions

If you are using Freemarker version 2.3.31 or older, you can still achieve the desired output, though arguably with more complexity. Below is a workaround that provides a function to check for nulls and return the appropriate value:

Using a Macro and Function

You can define a function along with a macro to handle null values gracefully. Here is a potential implementation:

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To use this function, simply call:

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Explanation of Logic

In this method:

The jsonString function checks if s is null and returns 'null' without quotes if it is.

The then clause specifies that when s has a value, it should be formatted correctly as a string within quotes.

Conclusion

Handling null values in JSON outputs using Apache Freemarker can seem daunting, especially for those new to the framework. However, as demonstrated, with the right tools and understanding, it can be done effectively.

For users on Freemarker version 2.3.32 onwards, the ?cn built-in function is the simplest and most efficient method. For those on older versions, consider using the custom function and macro approach.

By applying these techniques, you'll ensure that your JSON outputs are accurate and reflect the intended data types, thereby improving the robustness of your applications.