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.