This Java Keytool Tutorial will show you how to export a Certificate from a Java Keystore.

To begin you will first run (Note ensure that you update the relvnet commands to reflect your Keystore, alias and certificate names) :

keytool -list -keystore keystore.jks

to see what certificates reside in the java keystore.

Next you will run:

keytool -exportcert -alias domain -file domain.der -keystore keystore.jks

to export the Certificate.