Usually Java only accepts SSL certificates that can be validated with one of the CA providers in JRE’s internal cacerts keystore.

The cacerts keystore is a file located at $JAVA_HOME/lib/security/cacerts

How to import a self generated SSL certificate

First, export the self-generated key 'mywebsite.com' to a file called mywebsite.com.cert on the server
keytool -export -keystore ~/mywebsite.com.keystore -alias mywebsite.com -file mywebsite.com.cert

Then download the cert file with FTP or SFTP to your local computer.

Finally, import the certificate 'mywebsite.com.cert' into a local cacerts keystore:
keytool -import -keystore $JAVA_HOME/lib/security/cacerts -storetype jks -alias mywebsite.com -file ./mywebsite.com.cert

References

keytool export command ˆ
keytool import command ˆ

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