keytool

as root: $JAVA_HOME/bin/keytool -genkey -alias tomcat -keyalg RSA

#call this file DumpPrivateKey.java
import java.io.FileInputStream;
import java.security.KeyStore;
import java.security.Key;
public class DumpPrivateKey {
        static public void main(String[] args) {
                try {
                        KeyStore ks = KeyStore.getInstance("jks");
                        ks.load(new FileInputStream("/root/.keystore"),
                                 "password".toCharArray());
                        Key key = ks.getKey("tomcat",
                                 "password".toCharArray());
                        System.out.write(key.getEncoded());
                } catch (Exception e) {
                        e.printStackTrace();
                }
        }
}

javac DumpPrivateKey.java

# cat dump_private_key.sh 
#!/bin/sh
ALIAS=tomcat
PKEY_8=privatekey.pkcs8
PKEY_64=privatekey.b64
CERT_64=certificate.b64
CERT_12=certificate.p12
keytool -alias ${ALIAS} -export -rfc >${CERT_64}
java -classpath . DumpPrivateKey >${PKEY_8}
(echo "-----BEGIN PRIVATE KEY-----" ;
 openssl enc -in ${PKEY_8} -a;
 echo "-----END PRIVATE KEY-----") >${PKEY_64}
openssl pkcs12 -inkey ${PKEY_64} -in ${CERT_64} -out ${CERT_12} -export 
#rm ${PKEY_8} ${PKEY_64} ${CERT_64}
echo ${CERT_12}