Dropwizard metrics (http://metrics.dropwizard.io/4.0.0/) is an excellent open source library for measuring the performance of your Java web application. Paired with Spring Boot, this can bring a lot of value by measuring the performance of the REST API, measuring the performance of cache layer and database, and so on. Dropwizard provides handy annotations to mark methods to be monitored. It supports counters, timers, and so on.