Mocking Callouts with HttpCalloutMock
By the end of this lesson, you'll be able to:
- Explain why real HTTP callouts aren't allowed during Apex tests
- Implement HttpCalloutMock to simulate an external API response
Prerequisites: Testing Bulk Operations
Why callouts are blocked in tests
Tests need to be fast, deterministic, and independent of whether some external system happens to be up right now. A real callout during a test run would be slow, flaky, and could have real side effects on an external system — so Salesforce throws an exception if a test attempts a real callout without a mock registered.
Implementing HttpCalloutMock
Create a class implementing HttpCalloutMock, with one method — respond(HttpRequest req): HttpResponse — that builds and returns a fake HttpResponse. Register it before the code under test runs with Test.setMock(HttpCalloutMock.class, new YourMockClass()).
A simple HttpCalloutMock implementation
@isTest
global class WeatherApiMock implements HttpCalloutMock {
global HTTPResponse respond(HTTPRequest req) {
HttpResponse res = new HttpResponse();
res.setHeader('Content-Type', 'application/json');
res.setBody('{"tempCelsius": 22}');
res.setStatusCode(200);
return res;
}
}This mock never leaves Salesforce's servers — it just hands back a canned response any time the code under test performs a callout during this test.
Exercise
Write a test method that registers WeatherApiMock and calls WeatherService.getTemperature(), asserting it returns 22.
Show hint
Test.setMock(HttpCalloutMock.class, new WeatherApiMock()); must run before the callout happens.
Mocking Callouts with HttpCalloutMock — Quick Check
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Summary
Apex tests can't make real HTTP callouts at all — Salesforce blocks them outright — so testing integration code means implementing the HttpCalloutMock interface to return a fake, predictable response instead.