Apex Callouts and Test.setMock: Testing an External REST Integration Without Live Credentials

Testing Multiple REST Endpoints in Apex

p>When testing external integrations in Apex, the Test.setMock method is the standard mechanism for intercepting HttpRequest objects. This allows you to simulate various API behaviors without requiring live credentials or Remote Site Settings in your test environment.

Structuring Mocks: Single vs. Multiple Classes

p>For services that contact multiple endpoints or use different methods, you have two primary architectural choices:

• The Branching Mock (Single Class): You implement a single HttpCalloutMock class. Inside the respond method, you use request.getEndpoint() and request.getMethod() to determine which HttpResponse to return. This is often preferred for simple integrations where the logic remains clear and centralized.
• The Specialized Mock (Multiple Classes): You create a separate mock class for each callout path (e.g., GetAccountMock, PostOrderMock). This is cleaner for complex services where the response payloads vary significantly, as it prevents a single mock class from becoming a massive, nested conditional block.

Runtime Rules and Limits

There are common misconceptions regarding how mocked calls interact with Salesforce platform constraints. Here are the confirmed runtime behaviors:

• Remote Site Settings: Mocked callouts do not require Remote Site Settings or Named Credentials. Because the test framework intercepts the request locally before it attempts to leave the Salesforce org, the whitelist check is never triggered.
• API and Concurrent Limits: Mocked requests do not count against your organization's concurrent callout limits or daily API request limits. Since the execution happens entirely within the test context's memory, it does not consume the resources associated with actual external network calls.

Implementation Example

To implement a branching mock, use a structure similar to this:

public class MyServiceMock implements HttpCalloutMock {
    public HttpResponse respond(HttpRequest request) {
        HttpResponse response = new HttpResponse();
        if (request.getEndpoint().contains('//v1/users/')) {
            response.setStatusCode(200);
            response.setBody('{"name": "Test User"}');\n        } else if (request.getMethod() == 'POST') {
            response.setStatusCode(201);
            response.setBody('{"status": "success"}');
        }
        response.setHeader('Content-Type', 'application/json');
        return response;
    }
}

Note: If your service class relies on specific headers for authentication, ensure your mock also sets or validates those headers to avoid logic failures in the code under test.