JSON Basics for RPGLE Developers

Learning Objective

By the end of this lesson, you will be able to read a simple JSON payload and explain how its structure relates to familiar RPGLE concepts like fields and records.

Simple Explanation

JSON (JavaScript Object Notation) is a plain-text data format used by most REST APIs to represent a request or response payload. Despite the name, it is not specific to any one programming language; it is simply a widely used, human-readable way to structure data.

A JSON object is a set of named values, wrapped in curly braces:

{
  "custNbr": 100234,
  "custName": "Acme Supply Co",
  "isValid": true
}

Each name, such as custNbr, works similarly to a field name in an RPGLE record format: it labels a specific piece of data. A JSON value can be a number, text (in quotes), true/false, or even another nested object or a list of values, called an array:

{
  "custNbr": 100234,
  "orders": [1001, 1002, 1005]
}

Why It Matters

Once IBM i logic sends or receives data through an API, that data usually arrives or leaves as JSON. Being able to read a JSON payload and map it, mentally, to something like an RPGLE record makes it much easier to reason about what an API request or response actually contains, without needing to already be a JSON expert.

Practical Example

Recall the customer number validation example from earlier lessons in this batch. A request to check custNbr 100234 might be sent with a small JSON payload like the first example above. The response might come back as:

{
  "custNbr": 100234,
  "isValid": true
}

Reading this, custNbr and isValid are just two named values in the response, conceptually similar to two fields being returned from an RPGLE procedure, custNbr unchanged from the request and isValid holding the actual answer.

This is a simplified, illustrative example rather than a specific real payload, but it reflects exactly the shape of JSON used in practice.

Common Confusions

"Do I need to write JSON parsing code myself in RPGLE?" Not to understand this lesson. The concepts here, objects, named values, and arrays, are what matter first. How JSON gets converted into and out of RPGLE variables is an implementation detail that depends on the specific tools a shop uses, not something this introductory lesson depends on.

"Is a JSON array the same as an RPGLE array?" Conceptually similar: both hold a list of values. A JSON array can hold a simple list of numbers or text, or even a list of objects, which is more flexible than a typical fixed RPGLE array, but the basic idea of "more than one value under one name" is the same.

"Does the order of named values in a JSON object matter?" Generally, no. Unlike fixed-position fields in an RPGLE record format, JSON values are identified by name, not by position, so their order in the payload does not usually affect how they are read.

Quick Recap

• JSON is a plain-text format most REST APIs use to represent request and response payloads.
• A JSON object is a set of named values, similar in spirit to fields in an RPGLE record.
• A JSON array holds a list of values under one name.
• JSON values are identified by name, not position, so their order in the payload generally does not matter.

Try Asking the AI Tutor

Use the AI Tutor to ask follow-up questions about this lesson. For example, try asking:

• "Can you show me a slightly bigger JSON example with a nested object and explain each part?"
• "How would I think about mapping a JSON payload to RPGLE data structures conceptually, without worrying about exact syntax yet?"