How do you write test cases for REST APIs?

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1. Understand the API Specification

• Get details from API documentation (Swagger, Postman collection, OpenAPI).

• Note down: request method (GET, POST, PUT, DELETE), endpoint URL, required headers, parameters, body, and expected responses.

2. Identify Test Scenarios

For each API, write test scenarios like:

• ✅ Positive Cases: Correct input → expect valid response.

• ❌ Negative Cases: Invalid/missing data → expect error response.

• 🔐 Security Cases: Unauthorized/Forbidden access.

• 📈 Performance Cases: Response time and load.

Example for a User Login API:

• Valid username/password → 200 OK + JWT token.

• Wrong password → 401 Unauthorized.

• Missing fields → 400 Bad Request.

• SQL injection attempt → should fail safely.

3. Write Automated Test Cases

Using JUnit + RestAssured (Java example):

import io.restassured.RestAssured;
import org.junit.jupiter.api.Test;
import static io.restassured.RestAssured.*;
import static org.hamcrest.Matchers.*;

public class UserApiTest {

    @Test
    void testValidLogin() {
        RestAssured.baseURI = "http://localhost:8080/api";

        given()
            .contentType("application/json")
            .body("{\"username\":\"admin\", \"password\":\"admin123\"}")
        .when()
            .post("/login")
        .then()
            .statusCode(200)
            .body("token", notNullValue());
    }

    @Test
    void testInvalidLogin() {
        given()
            .contentType("application/json")
            .body("{\"username\":\"admin\", \"password\":\"wrongPass\"}")
        .when()
            .post("/login")
        .then()
            .statusCode(401)
            .body("error", equalTo("Unauthorized"));
    }
}

4. What to Validate in API Test Cases

• Status codes → 200, 201, 400, 401, 404, 500.

• Response body → Correct fields, formats, values.

• Headers → Content-Type, Authorization.

• Performance → Response time within limits.

• Database side effects (if applicable).

✅ In summary:

Writing REST API test cases involves analyzing API requirements, covering positive/negative/security scenarios, and automating with frameworks like RestAssured (Java), Pytest + Requests (Python), or Postman/Newman.

Read more :

What is Mockito?

What are the differences between JUnit and TestNG?

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