Why do methods defined as testmethod do not support web service callouts?

In Salesforce Apex, methods defined as testMethod (or using @isTest) do not support web service callouts because test methods are meant to run in a controlled, isolated environment without making external API requests.

Key Reasons Why Test Methods Don’t Support Callouts

 1. Governor Limits & Bulk Testing

• Salesforce enforces strict limits on callouts to prevent excessive API usage.
  
• If test methods could make real API calls, it could exceed daily limits, causing failures.
  

 2. Isolation from External Systems

• Apex test methods should run independently of external web services.
  
• A web service being slow, down, or returning different data could break tests.
  

 3. Test Data Integrity

• Test methods use mock data and cannot rely on unpredictable API responses.
  
• This ensures test results are consistent and repeatable.
  

How to Handle Callouts in Test Methods?

• Since real callouts are not allowed, Salesforce provides a solution: Mock Callouts using HttpCalloutMock.
  
• Steps to Use Mock Callouts in Tests
  

 1. Implement a Mock Class

 Create a mock class that implements HttpCalloutMock:

@isTest

public class MockCallout implements HttpCalloutMock {

    public HTTPResponse respond(HTTPRequest req) {

        HttpResponse res = new HttpResponse();

        res.setBody('{"status": "success"}');

        res.setStatusCode(200);

        return res;

    }

}

 2. Use Test.setMock() in the Test Method

 Inside the test method, use Test.setMock() to replace real callouts with mock responses:

@isTest

public static void testCalloutMethod() {

    Test.setMock(HttpCalloutMock.class, new MockCallout());

    // Call the method that performs the callout

    String response = MyClass.callExternalService();

    System.assertEquals('success', response);

}

Final Thoughts

• Real API calls are not allowed in tests to ensure isolation and prevent failures.
  
• Use HttpCalloutMock to simulate API responses in test methods.
  
• Mock callouts keep tests reliable and within Salesforce limits.