There are a wide variety of free (open source) testing tools available for API testing:
- SoapUI - SoapUI is the Open Source Functional Testing tool for API Testing. It supports multiple protocols such as SOAP, REST, HTTP, JMS etc. You can download the SoapUI from https://www.soapui.org/downloads/soapui.html
- Postman - Postman is an application for interacting with HTTP APIs. It presents you with a friendly GUI for constructing requests and reading responses. You can download Postman from https://www.getpostman.com/
- Curl - cURL is a tool for working with URLs. cURL lets us query a URL from the command line. It lets us post form data. With cURL we can try out new APIs easily. You can download the curl from the command line on Linux by typing sudo yum install curl
- Apache benchmark - The ApacheBench tool (ab) can load test servers by sending an arbitrary number of concurrent requests. You can download the Apache benchmark from the command line on Linux by typing yum install httpd-tools
- Swagger – The Swagger editor can be used to validate your Swagger definition prior to importing into API Manager.