Application Programming Interfaces (APIs) are crucial for creating software solutions that are robust, scalable, and seamless. APIs allow different systems to communicate with each other and ensuring these APIs work as intended is very important. This is where API testing comes into play. API testing helps you identify bugs in the software. The bugs in question can affect performance, security, and reliability. In this blog, our focus will be to learn more about the three popular API testing tools. These tools are Postman, REST Assured, and Swagger. We will also learn more about how they can be used to ensure high-quality APIs.

What are APIs? Why are  They Important in Software Development

APIs (Application Programming Interfaces) serve as intermediaries allowing two applications to communicate with each other. They define methods and data formats that programmers must adhere to when programming. 

• Commonly, APIs are used to enable the integration of new features, handling data exchange, and facilitating user functionality in applications.
• APIs are categorized into REST (Representational State Transfer), SOAP (Simple Object Access Protocol), and GraphQL.
• Each type serves different needs and follows different standards.
• REST APIs are the most popular because they are simple in nature and integrate well with web technologies.

Testing these APIs is critical for several reasons. They are:

• Functionality: Ensuring the API performs as expected under various conditions.
• Reliability: APIs should be able to handle a large number of calls and data without failure.
• Performance: APIs must deliver prompt responses and handle loads effectively.
• Security: APIs need robust security measures to protect sensitive data and prevent malicious attacks.

Top API Testing Tools

In our comparison, we will explore three different tools:

• Postman – A user-friendly tool for both developing and testing APIs.
• REST Assured – A Java library that simplifies testing of REST-based services.
• Swagger – Primarily an API design and documentation tool with features that assist in testing.

Our comparison will focus on their ease of use, automation capabilities, community support, and integration with other software development tools.

Postman for API Testing

• Postman is a popular choice for API development and testing among developers due to its user-friendly graphical interface and extensive features. 
• It allows developers to send HTTP requests, inspect responses, and even automate tests.

Key Features of Postman include:

• User-Friendly Interface: Simple to use for both developers and testers.
• Collections and Environments: Organize requests and control environments with different settings.
• Pre-request Scripts and Tests: Write JavaScript codes that run before requests are sent or after responses are received.

Example Usage:

Let’s say we need to test a simple GET request to fetch user data from a placeholder API:

Create a Request:

• Open Postman.
• Click on “New” -> “Request”.
• Name the request and save it to a collection.

Set the HTTP method to GET and enter the URL https://api.example.com/users.

Add Tests:

In the “Tests” tab, you can add JavaScript to test the response:

javascript

pm.test(“Status code is 200”, function () {

    pm.response.to.have.status(200);

});

pm.test(“Response time is less than 200ms”, function () {

    pm.expect(pm.response.responseTime).to.be.below(200);

});

Send the Request and View Results:

• Click “Send”.
• Observe the status code and response time in the response area.

Using REST Assured for API Testing

REST Assured is a Java DSL (Domain Specific Language) that simplifies testing of REST based services. It integrates seamlessly with existing Java-based testing frameworks like JUnit and TestNG.

Key Features of REST Assured include:

• Ease of Integration: Integrates smoothly with Java-based applications and testing frameworks.
• BDD Support: Supports Behavior Driven Development testing style.
• Simplicity: Abstracts away much of the boilerplate code needed to interact with HTTP.

Example Usage:

Suppose we want to verify that our API returns a list of users with correct names when a GET request is made:

java

import static io.restassured.RestAssured.*;

import static org.hamcrest.Matchers.*;

public class UserApiTest {

    @Test

    public void testUserFetchesSuccess() {

        given().

            baseUri(“https://api.example.com”).

        when().

            get(“/users”).

        then().

            statusCode(200).

            body(“users.name”, hasItems(“Alice”, “Bob”));

    }

}

In this example, we define a test method that makes a GET request to /users and asserts that the response status is 200 and contains names “Alice” and “Bob” in the returned user list.

Swagger for API Documentation and Testing

Swagger (also known as OpenAPI) is primarily an API specification framework that helps design, build, document, and consume RESTful web services. 

• It also provides tools like Swagger UI. 
• It can generate interactive API documentation.
•  This allows developers to perform API calls directly from the browser.

Key Features of Swagger include:

• Standardized Documentation: Automatically generates and updates API documentation.
• Interactive API Testing: Test APIs directly from the documentation.

Example Usage:

Here is a simple Swagger definition that documents and allows testing a GET /users endpoint:

yaml

openapi: 3.0.0

info:

  title: Sample API

  version: “1.0”

paths:

  /users:

    get:

      summary: Returns a list of users

      responses:

        ‘200’:

          description: Successfully returns a list of users

          content:

            application/json:

              schema: 

                type: array

                items:

                  type: string

                  example: [“Alice”, “Bob”]

Developers can load this specification into Swagger UI, where they can see the documentation and perform a test request directly.

Integrating API Testing Tools with CI/CD

Integrating API testing tools into CI/CD pipelines ensures that APIs are automatically tested with every code change, leading to higher quality software deliveries.

• Postman can be integrated using Newman, its command-line companion, to run collections.
• REST Assured tests can be triggered as part of Maven or Gradle scripts in Jenkins, GitHub Actions, or any other CI server.
• Swagger tests can be automated by generating server stubs and client SDKs using Swagger Codegen in a CI pipeline.

Conclusion

API testing is an important aspect of modern software development. Tools like Postman, REST Assured, and Swagger each provide unique benefits. They can enhance API testing efforts. Postman offers a user-friendly GUI for manual and automated testing. REST Assured is ideal for Java developers who need a robust framework that integrates easily with existing workflows. Swagger excels in creating interactive API documentation that can also be used for testing purposes.

By selecting the right tool and integrating it into your development and testing workflows, you can ensure that your APIs are reliable, secure, and perform well under all circumstances. Experiment with these tools to find the one that best fits your needs, and leverage their full potential to achieve superior software quality.