Apache Tomcat is a web server written in 100% Pure Java.

Apache Tomcat version 8.5 implements the Servlet 3.1, JavaServer Pages 2.3,
Java Unified Expression Language 3.0 and Java WebSocket 1.1 specifications
from the Java Community Process, and includes many additional features that
make it a useful platform for developing and deploying web applications and
web services. Tomcat 8.5 is aligned with Java EE 7.

WWW: http://tomcat.apache.org/