How to Unit Test Callouts in Apex?

Question:

How do I unit test Apex code that includes a SOAP or REST-based callout or indirectly calls code that makes a callout? I am getting an error saying that callouts are not supported during test execution.

Answer:

Salesforce enforces test isolation, which means you cannot make REST or SOAP callouts during test execution. This restriction applies to all code executed within a test context, whether it directly or indirectly attempts a callout. To test code that performs callouts, you must use a Mock class to simulate the behavior of the remote server and provide a mock response.

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Salesforce provides interfaces for mocking callouts:

1. HttpCalloutMock for REST callouts.
2. WebServiceMock for SOAP callouts.

You must implement these interfaces to define the mock behavior. Additionally, Salesforce offers the built-in StaticResourceCalloutMock and MultiStaticResourceCalloutMock classes, which let you use Static Resources to store mock responses for your tests.

Example 1: Mocking a REST Callout Using HttpCalloutMock

Here’s an example of how to mock a REST callout:

Production Code:

public class CalloutExample {
    public static String makeRestCall() {
        HttpRequest req = new HttpRequest();
        req.setEndpoint('https://example.com/api/resource');
        req.setMethod('GET');
        Http http = new Http();
        HttpResponse res = http.send(req);
        return res.getBody();
    }
}

Mock Class:

public class MockHttpResponse implements HttpCalloutMock {
    public HttpResponse respond(HttpRequest req) {
        HttpResponse res = new HttpResponse();
        res.setStatusCode(200);
        res.setBody('{"message": "Mock response"}');
        return res;
    }
}

Unit Test:

@IsTest
public class CalloutExampleTest {
    @IsTest
    static void testMakeRestCall() {
        Test.setMock(HttpCalloutMock.class, new MockHttpResponse());
        String response = CalloutExample.makeRestCall();
        System.assertEquals('{"message": "Mock response"}', response);
    }
}

Example 2: Mocking a SOAP Callout Using WebServiceMock

Production Code:

global class SoapCalloutExample {
    webService static String makeSoapCall(String input) {
        // Callout logic here
        return null; // Actual implementation omitted
    }
}

Mock Class:

global class MockSoapResponse implements WebServiceMock {
    global void doInvoke(
        Object stub, 
        Object request, 
        Map<String, Object> response, 
        String endpoint, 
        String soapAction, 
        String requestName, 
        String responseNS, 
        String responseName
    ) {
        response.put('result', 'Mocked Response');
    }
}

Unit Test:

@IsTest
global class SoapCalloutExampleTest {
    @IsTest
    static void testMakeSoapCall() {
        Test.setMock(WebServiceMock.class, new MockSoapResponse());
        String result = SoapCalloutExample.makeSoapCall('TestInput');
        System.assertEquals('Mocked Response', result);
    }
}

Using Static Resources for Mock Responses

To simplify mocking for REST callouts, you can use the StaticResourceCalloutMock. Store the mock response in a Static Resource file and reference it in your test.

Example:

@IsTest
public class StaticResourceMockTest {
    @IsTest
    static void testWithStaticResourceMock() {
        StaticResourceCalloutMock mock = new StaticResourceCalloutMock();
        mock.setStaticResource('MockResponse');
        mock.setStatusCode(200);
        mock.setHeader('Content-Type', 'application/json');
        Test.setMock(HttpCalloutMock.class, mock);

        String response = CalloutExample.makeRestCall();
        System.assert(response.contains('Mock data'), 'Response did not contain expected mock data');
    }
}

By using these approaches, you can test all possible scenarios, including different response bodies and status codes, without violating Salesforce’s test isolation rules.

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See also : Salesforce Testing

Guide to Become a Salesforce Tester