extensions-core
A framework-agnostic SDK for developing apps with Java.
The extensions-core module is one of Canva's Java SDKs for developing server-side extensions. It streamlines the development process and helps you avoid common pitfalls, such as misspelled field names and signature verification issues.
This module contains:
  • A Signer utility for generating request signatures.
  • Plain Old Java Objects (POJOs) with JSON marshalling for API requests, responses, and models.
  • Classes for implementing common functionality, such as request validation.

Usage

  1. 1.
    Download the module via Maven Central: search.maven.org/artifact/com.canva/extensions-core
  2. 2.
    Import the relevant components:
    import com.canva.extensions.content.model.*;

Requests and responses

This snippet demonstrates how to serialize and deserialize the request and response objects for the /content/resources/find endpoint:
import com.canva.extensions.content.model.*;
import com.fasterxml.jackson.databind.ObjectMapper;
public class Example {
private static final ObjectMapper objectMapper = new ObjectMapper();
public static String handleFindResourcesRequest(String jsonBody) {
// Deserialize a FindResourcesRequest from the JSON body of the HTTP request.
var request = objectMapper.readValue(jsonBody, FindResourcesRequest.class);
// Serialize the FindResourcesResponse.
var resources = List.<Resource>of();
var findResourcesResponse = new FindResourcesSuccessResponse(resources, null);
return objectMapper.writeValueAsString(findResourcesResponse);
}
}

Models

This snippet demonstrates how to use a Resource object to create an "IMAGE" resource for a content extension:
import java.util.List;
import com.canva.extensions.content.model.*;
import com.fasterxml.jackson.databind.ObjectMapper;
public class Example {
private static final ObjectMapper objectMapper = new ObjectMapper();
public static String handleFindResourcesRequest(String jsonBody) {
// Deserialize a FindResourcesRequest from the JSON body of the HTTP request.
var request = objectMapper.readValue(jsonBody, FindResourcesRequest.class);
// Handle the request - return a list of Resource objects satisfying the request.
var imageThumbnail = new Thumbnail("picsum.photos/id/237/200", 800, 600);
var imageResource = new Resource("imageId", "An Image", ResourceType.IMAGE, "image/jpeg",
imageThumbnail, "https://url.to/image.png");
var resources = List.of(imageResource);
// Serialize the FindResourcesResponse.
var findResourcesResponse = new FindResourcesSuccessResponse(resources, "aContinuationToken");
return objectMapper.writeValueAsString(findResourcesResponse);
}
}

Signature verification

If your app has signature verification check enabled, you must verify the signatures and timestamps of all HTTP requests. To simplify this process, the extensions-core module includes a Signer class for generating request signatures.
This snippet demonstrates how to use the Signer class with the Spark framework:
import com.canva.extensions.util.Signer;
import java.util.List;
import static spark.Spark.post;
public class Example {
// Instantiated here for illustrative purposes. Ideally, provide via the constructor as a singleton.
private static final Signer signer = new Signer("clientSecret");
public static void main(String[] args) {
post("/content/resources/find", (request, response) -> {
var signatures = List.of(request.headers("X-Canva-Signatures").split(","));
var sentAtSeconds = Long.parseLong(request.headers("X-Canva-Timestamp"));
var path = "/content/resources/find";
var signature = signer.signRequest(sentAtSeconds, path, request.body());
if (!signatures.contains(signature)) {
response.status(401);
return response;
}
var receivedAtSeconds = System.currentTimeMillis() / 1000;
var leniencyInSeconds = 5 * 60; // 5 minutes
if (receivedAtSeconds - sentAtSeconds > leniencyInSeconds) {
response.status(401);
return response;
}
// If the above has passed, then the request is valid and can be handled safely.
// Here we'd handle the findResources request and return a FindResources(Success|Error)Response.
return response;
});
}
}