Tutorials123: JSON Schema Validation

What is JSON Schema?

JSON Schema is a specification that defines the structure of a JSON document.
It is used to validate that a given JSON response (or request) follows a predefined format.

A JSON Schema defines:

Data types (string, number, object, array, boolean, null)
Required vs optional fields
Field constraints (min/max length, patterns, ranges, enum values, etc.)
Nested objects and arrays

Why validate JSON Schema?

To check that API responses always follow the contract agreed between client and server.
To avoid breaking changes when backend updates occur.
To make automated testing more robust and reliable.

How to validate JSON Schema in Rest Assured

Rest Assured provides built-in support for JSON Schema validation using the json-schema-validator library.

This includes:

1. Prepare a JSON Schema file

Create a .json file that defines the expected schema of the API response.
Example: define required fields, their data types, and structure.

2. Add JSON Schema Validator dependency

Rest Assured integrates with the json-schema-validator library, which enables schema-based validation.

3. Call the API using Rest Assured

Perform a request (GET/POST/etc.) to get the API response.

4. Validate Response against JSON Schema

Use Rest Assured’s built-in schema validator methods to check whether the actual JSON response matches the schema.
If the response matches → test passes.
If not → test fails with details about what part of the schema is violated.

In short:

To validate a JSON schema using Rest Assured in a GET API response, you can use the JsonSchemaValidator provided by Rest Assured. It helps ensure that the structure of the JSON returned from the API matches an expected schema.

Steps to Use JSON Schema Validation in Rest Assured

1. Add Dependencies

<dependencies>
    <!-- Rest Assured -->
    <dependency>
        <groupId>io.rest-assured</groupId>
        <artifactId>rest-assured</artifactId>
        <version>5.4.0</version>
        <scope>test</scope>
    </dependency>

    <!-- JSON Schema Validator -->
    <dependency>
        <groupId>io.rest-assured</groupId>
        <artifactId>json-schema-validator</artifactId>
        <version>5.4.0</version>
        <scope>test</scope>
    </dependency>
</dependencies>

Just visit this page to create JSON schema or use existing below schema for API:

https://reqres.in/api/users?page=2

2. Create JSON Schema File

We can create schema from any json schema creator website. One of the best website is: http://json-schema.org/

Create a json schema file named as users-schema.json for GET API: https://reqres.in/api/users?page=2 and placed in the src/test/resources directory.

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "page": { "type": "integer" },
    "per_page": { "type": "integer" },
    "total": { "type": "integer" },
    "total_pages": { "type": "integer" },
    "data": {
      "type": "array",
      "items": {
        "type": "object",
        "properties": {
          "id": { "type": "integer" },
          "email": { "type": "string" },
          "first_name": { "type": "string" },
          "last_name": { "type": "string" },
          "avatar": { "type": "string" }
        },
        "required": ["id", "email", "first_name", "last_name", "avatar"]
      }
    },
    "support": {
      "type": "object",
      "properties": {
        "url": { "type": "string" },
        "text": { "type": "string" }
      },
      "required": ["url", "text"]
    }
  },
  "required": ["page", "per_page", "total", "total_pages", "data", "support"]
}

3. Java Code to Validate Schema

import io.restassured.RestAssured;
import io.restassured.http.ContentType;
import static io.restassured.module.jsv.JsonSchemaValidator.matchesJsonSchemaInClasspath;
import static io.restassured.RestAssured.*;

public class JsonSchemaValidationTest {

    public static void main(String[] args) {

        RestAssured.baseURI = "https://reqres.in";

        given()
            .contentType(ContentType.JSON)
        .when()
            .get("/api/users?page=2")
        .then()
            .assertThat()
            .statusCode(200)
            .body(matchesJsonSchemaInClasspath("users-schema.json"));

        System.out.println("Schema validated successfully.");
    }
}

Code explanation:

(a) Define the base URI

(b) Send request for GET API as mentioned in code and get response