I am running ServiceDesk on Linux and I wanted to use LetsEncrypt with ServiceDesk and have a cron job automatically update ServiceDesk when the LE certs were renewed.

There was a lot I found on the internet but I boiled it down to this:

0 - Prerequisites:

a - install a more modern JRE than ServieDesk uses

Because the version ServiceDesk is using does not set itself up in the environment as JAVA_HOME, I felt safe installing another system based JRE.  As I am on debian I simply installed the default:

sudo apt install default-jre

Which gave me v11.

b - you already have some form of LetsEncrypt installed and certs generated

I personally use acme.sh (https://github.com/acmesh-official/acme.sh) because it allows me to use my DNS provider's API for domain verification, and allows me to set up wildcards.

1 - Use Open SSL to convert the LE certs to a PKCS12 archive

I ran this in my home directory.

openssl pkcs12 -export \
    -in <path and filename of your LE domain certificate> \
    -inkey <path and filename of your LE domain certificate key file> \
    -out ./<name of the output file. I just used my domain name>.p12 \
    -name <your domain name> \
    -CAfile <path to your LE domain certificates>ca.cer \
    -caname "Let's Encrypt Authority X3" \
    -password pass:changeit

This produces a .p12 file.

2 - import the certificates into a keystore

keytool -importkeystore \
    -srckeystore ./<the file from the previous step> \
    -srcstoretype PKCS12 \
    -srcstorepass changeit \
    -destkeystore /data/ManageEngine/ServiceDesk/conf/<whatever you want to call it>.keystore
    -deststoretype JKS \
    -deststorepass <whatever password you want to use> \
    -destkeypass <whatever password you want to use> \
    -alias <your domain name>

3 - configure ServiceDesk for SSL (if you haven't already)

Follow the instruction here: https://help.servicedeskplus.com/introduction/install-ssl.html

just below Step 3b.

- Copy the keystore to <ServiceDesk_Home>/conf

- execute <ServiceDesk_Home>/bin/changeWebServerPort.sh <https port you want> https

- update the name of the keystore and the keystore password in the file <ServiceDesk_Home>/conf/server.xml 

5 - restart ServiceDesk

Here are the most useful reference I found: