How Can I Print WSO2 HBS Attributes as JSON?

In the ever-evolving landscape of integration and API management, WSO2 stands out as a powerful and flexible platform. One common challenge developers and architects face when working with WSO2 is effectively handling and visualizing data attributes within Handlebars (Hbs) templates. Specifically, the ability to print Hbs attributes as JSON can significantly streamline debugging, logging, and data transformation tasks, making it easier to understand complex payloads and enhance the overall integration workflow.

This article delves into the nuances of rendering WSO2 Handlebars attributes in JSON format, offering insights into why this approach is valuable and how it can be leveraged within your integration projects. By exploring the relationship between WSO2’s mediation sequences and Handlebars templating, readers will gain a clearer perspective on managing dynamic data and improving transparency in message flows.

Whether you’re a seasoned WSO2 developer or just beginning your journey with API management, understanding how to print Hbs attributes as JSON opens up new possibilities for monitoring and manipulating message content. The following sections will guide you through the concepts and practical considerations necessary to harness this technique effectively.

Techniques to Print HBS Attributes as JSON in WSO2

When working with WSO2 and Handlebars (HBS) templates, printing attributes as JSON can be essential for debugging or passing structured data downstream. Since HBS itself is a logic-less templating language, it lacks native functions to stringify objects directly. However, several approaches can be employed to achieve this within WSO2’s mediation sequences or API Gateway flows.

One common technique involves using the `json-eval` expression within the mediation logic to retrieve attributes as JSON and then pass them into the Handlebars context. Alternatively, custom script mediators or extensions can be introduced to serialize attributes explicitly before template rendering.

Key approaches include:

• Using Script Mediators to Serialize Attributes:

JavaScript or Nashorn script mediators allow you to convert message context properties into JSON strings, which then can be injected into the HBS template context.

• Preprocessing Attributes in the Mediation Flow:

Extract the required attributes and assemble a JSON object in the mediation flow, storing it as a property. This property is then referenced within the HBS template.

• Leveraging HBS Helpers for JSON Stringify:

Registering a custom helper function in the WSO2 environment that wraps `JSON.stringify` to output JSON-formatted strings inside the HBS template.

Below is a comparison of these methods:

Method	Implementation Complexity	Flexibility	Performance Impact	Use Case
Script Mediator Serialization	Medium	High	Moderate	Dynamic attribute structures requiring runtime serialization
Preprocessing in Mediation Flow	Low	Medium	Low	Static or predictable attribute sets
Custom HBS Helper	High	Very High	Low to Moderate	Advanced templating scenarios with repeated JSON serialization

Implementing a Script Mediator to Serialize Attributes

To serialize message context attributes as JSON within WSO2, you can insert a script mediator using JavaScript. The script extracts the necessary properties and constructs a JSON string, which is then saved into a new property accessible by the Handlebars template.

Example script snippet:

“`javascript
var mc = mc; // Message context
var attributes = {
attr1: mc.getProperty(“attr1”),
attr2: mc.getProperty(“attr2”),
attr3: mc.getProperty(“attr3”)
};
var jsonString = JSON.stringify(attributes);
mc.setProperty(“attributesJson”, jsonString);
“`

After setting this property, the HBS template can access it as follows:

“`handlebars
{{attributesJson}}
“`

This will output the JSON string corresponding to the selected attributes.

Creating a Custom Handlebars Helper for JSON Output

If you require more control or want to perform JSON serialization directly within the template, you can create a custom Handlebars helper in WSO2. This involves adding a helper method that accepts an object and returns its JSON string.

Steps to implement:

• Develop a Java class implementing the `Helper` interface from Handlebars.java.
• Register this helper with the Handlebars engine in WSO2’s template configuration.
• Use the helper in your HBS template like `{{json this}}` or `{{json attribute}}`.

Example helper usage in HBS:

“`handlebars
{{json myObject}}
“`

This will output the serialized JSON form of `myObject`.

Best Practices When Printing JSON in HBS

To ensure clarity and maintainability when printing JSON from HBS templates in WSO2 environments, consider the following best practices:

• Sanitize Data Before Serialization: Ensure that the attributes do not contain circular references or unsupported types to prevent serialization errors.
• Use Readable Formatting for Debugging: When debugging, pretty-print JSON by adding indentation in the serialization method.
• Avoid Overloading Templates with Logic: Keep templates focused on presentation; perform complex data manipulations in mediators or scripts.
• Cache Serialized JSON if Reused: If the same JSON string is used multiple times in a template, store it in a property rather than serializing repeatedly.
• Handle Null or Values Gracefully: Provide fallback values or guards in templates to avoid rendering issues.

These guidelines help maintain efficient, clear, and error-free JSON output within your WSO2 Handlebars templates.

Rendering WSO2 HBS Attributes as JSON

WSO2 uses Handlebars (HBS) templates extensively for rendering dynamic content, especially in API Manager and Identity Server UI customizations. When dealing with HBS templates, you may need to output attributes or objects as JSON strings for debugging, logging, or client-side processing.

Challenges with Printing Attributes as JSON in HBS

• Handlebars is primarily a logic-less templating engine, which limits direct JavaScript execution or complex transformations.
• By default, HBS prints attributes as plain text, escaping special characters.
• Printing complex objects or nested attributes requires custom helpers or pre-processing.

Methods to Print Attributes as JSON in WSO2 HBS Templates

1. Using a Custom Handlebars Helper

Creating a custom helper to serialize objects to JSON is the most flexible and clean solution. WSO2 allows extending Handlebars with JavaScript helpers.

Example: Registering a `toJson` helper in JavaScript

“`js
Handlebars.registerHelper(‘toJson’, function(context) {
return JSON.stringify(context, null, 2);
});
“`

Usage in the HBS template:

“`hbs

{{{toJson this.attributes}}}

“`

• The triple braces `{{{ }}}` prevent Handlebars from escaping the JSON string.
• The helper serializes the `attributes` object with indentation for readability.

2. Inline JSON.stringify in Scripts (If Supported)

Some WSO2 environments allow limited inline JavaScript evaluation or script blocks, though this is not standard.

“`hbs

“`

This method relies on the custom `toJson` helper or pre-serialized JSON passed into the template context.

3. Pre-Serializing Attributes in Backend Before Passing to Template

If you have control over the Java backend or mediation logic, serialize the attributes to JSON before sending them to the HBS template.

Step	Description	Benefit
Serialize attributes	Use a JSON library in Java to convert objects	Offloads serialization from HBS
Pass JSON string to HBS	Include JSON string as a template variable	Simplifies template rendering
Print directly in HBS	Use triple braces to output raw JSON	Avoids escaping issues

Example in HBS:

“`hbs

{{{attributesJson}}}

“`

Important Considerations

• Escaping: Use triple braces to avoid HTML escaping of JSON strings.
• Performance: Large objects serialized in templates can impact rendering time; pre-serializing is preferable.
• Security: Ensure no sensitive data is unintentionally exposed in JSON output.
• Helper Registration Location: Custom helpers must be registered in the same runtime context where templates are rendered (e.g., in WSO2 Carbon components or custom UI apps).

Sample Custom Helper Registration in WSO2 Microservices

If developing a custom microservice or extension that uses Handlebars:

“`java
Handlebars handlebars = new Handlebars();
handlebars.registerHelper(“toJson”, new Helper