Question regarding keytool

I first generated jks using

keytool -genkey -alias keyAlias-keyalg RSA  -keypass changeit  -storepass changeit keystore keystore.jks

Then i generated the csr using

keytool -certreq -alias $addr  -sigalg SHA256withRSA -keystore $addr.jks -file $addr.csr 

Below is how i self sign and generate the .cer from jks

Export the generated certificate to the server.cer file (or client.cer if you prefer), using the following command format:
     
keytool -export -alias keyAlias-storepass changeit  -file server.cer  -keystore keystore.jks

If you ask me why i need to generate the cer from csr instead of jks then the reason is -sigalg SHA256withRSA does not work with -genkey (jks) so i have to use .csr and then use that csr to generate the self signed .cer

Question:
How can i self sign and generate the .cer from the .csr file instead of the .jks using keytool?