In a microservices architecture, one of the most important parts of a service is the module in charge of communicating with other services.
You often need to test end-to-end how a service communicates with other services. Mocking is not a solution since it does not test the communication stack, and skips everything related to network protocol (i.e HTTP).
Running the dependent service is not an option because of the effort required to prepare the process each time.
Service virtualization is a technique used to simulate the behavior of dependencies of service by creating a proxy service, hence the test runs against a service (i.e. the full stack is tested) without booting up the real service.
Hoverfly is an open source, lightweight, service virtualization API simulation tool written in the Go programming language and tightly integrated with Java.