What is the method to test an Apex REST class effectively?

Simulating a REST request and setting request properties is the most effective method to test an Apex REST class. This approach closely resembles how the RESTful service will be invoked in a real-world scenario. By creating a test that mimics an actual request, you ensure that all aspects of the HTTP request, including headers and parameters, are correctly handled by the Apex code.

In this method, you can utilize the Test.startTest() and Test.stopTest() methods to encapsulate your test execution, allowing for proper context and limits management. You can set the necessary request properties using the RestRequest object, which lets you define the HTTP method, path, and body that the REST class is expected to handle. This not only tests the functionality of the class but also verifies its response under realistic conditions, ensuring that any data transformations, validations, and error handling are functioning as intended.

Choosing other methods, such as directly calling the method, checking only database changes, or running the class without parameters, will not provide a comprehensive test of the REST class functionality. These alternatives don't validate how the class interacts with the incoming REST requests or whether it handles HTTP-specific scenarios correctly, making them less effective for thorough testing.