Introduction to JSON Schema in Java

1. Overview

JSON Schema is a declarative language for validating the format and structure of a JSON Object. It allows us to specify the number of special primitives to describe exactly what a valid JSON Object will look like.

The JSON Schema specification is divided into three parts:

• JSON Schema Core: The JSON Schema Core specification is where the terminology for a schema is defined.
• JSON Schema Validation: The JSON Schema Validation specification is the document that defines the valid ways to define validation constraints. This document also defines a set of keywords that can be used to specify validations for a JSON API. In the examples that follow, we’ll be using some of these keywords.
• JSON Hyper-Schema: This is another extension of the JSON Schema spec, wherein, the hyperlink and hypermedia-related keywords are defined.

2. Defining a JSON Schema

Now that we have defined what a JSON Schema is used for, let’s create a JSON Object and the corresponding JSON Schema describing it.

The following is a simple JSON Object representing a product catalog:

{
    "id": 1,
    "name": "Lampshade",
    "price": 0
}

We could define its JSON Schema as follow:

{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "title": "Product",
    "description": "A product from the catalog",
    "type": "object",
    "properties": {
        "id": {
            "description": "The unique identifier for a product",
            "type": "integer"
        },
        "name": {
            "description": "Name of the product",
            "type": "string"
        },
        "price": {
            "type": "number",
            "exclusiveMinimum": 0
        }
    },
    "required": ["id", "name", "price"]
}

As we can see a JSON Schema is a JSON document, and that document MUST be an object. Object members (or properties) defined by JSON Schema are called keywords.

Let’s explain the keywords that we have used in our sample:

• The $schema keyword states that this schema is written according to the draft v4 specification.
• The title and description keywords are descriptive only, in that they do not add constraints to the data being validated. The intent of the schema is stated with these two keywords: describes a product.
• The type keyword defines the first constraint on our JSON data: it has to be a JSON Object.

Also, a JSON Schema MAY contain properties that are not schema keywords. In our case id, name, price will be members (or properties) of the JSON Object.

For each property, we can define the type. We defined id and name as string and price as number. In JSON Schema a number can have an exclusiveMinimum.

Finally, the Schema tells that id, name, and price are required.

3. Validation With JSON Schema

With our JSON Schema put in place, we can validate our JSON Object.

There are many libraries to accomplish this task. For our example, we have chosen the Java json-schema library.

First of all, we need to add the following dependency to our pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>json-schema-validator</artifactId>
    <version>1.4.0</version>
</dependency>

Finally, we can write a couple of simple test cases to validate our JSON Object:

@Test
public void givenInvalidInput_whenValidating_thenInvalid() throws IOException {
    JsonSchemaFactory factory = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V4);
    JsonSchema jsonSchema = factory.getSchema(
     JSONSchemaUnitTest.class.getResourceAsStream("/schema.json"));
    JsonNode jsonNode = mapper.readTree(
     JSONSchemaUnitTest.class.getResourceAsStream("/product_invalid.json"));
    Set<ValidationMessage> errors = jsonSchema.validate(jsonNode);
    assertThat(errors).isNotEmpty().asString().contains("price: must have an exclusive minimum value of 0");
}

In this case, a validation error will be received.

The second test looks like the following:

@Test 
public void givenValidInput_whenValidating_thenValid() throws ValidationException { 
    JsonSchemaFactory factory = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V4); 
    JsonSchema jsonSchema = factory.getSchema( 
     JSONSchemaUnitTest.class.getResourceAsStream("/schema.json")); 
    JsonNode jsonNode = mapper.readTree( 
     JSONSchemaUnitTest.class.getResourceAsStream("/product_valid.json")); 
    Set<ValidationMessage> errors = jsonSchema.validate(jsonNode); 
    assertThat(errors).isEmpty(); 
}

Since we use a valid JSON Object, no validation error will be thrown.

4. Conclusion

In this article, we defined what a JSON schema is and which are some relevant keywords that help us to define our schema.

By coupling a JSON Schema with its corresponding JSON Object representation, we can perform some validation tasks.