Some RPMs (e.g. jpackage.org’s tomcat7-7.0.39-1.jpp6.noarch.rpm) express their dependency upon Java by requiring a RPM that provides capability java (as opposed to, for example, depending on the existence of a file /usr/bin/java). On CentOS, this capability is normally provided by the java-*-openjdk RPM. Therefore, if you execute # yum install tomcat7 on a clean install of CentOS, yum will install OpenJDK in addition to Tomcat 7. Some people prefer to run the Oracle JRE/JDK instead of OpenJDK.