How to import an existing X.509 certificate and private key in Java keystore to use in SSL?

I used the following two steps which I found in the comments/posts linked in the other answers:

Step one: Convert the x.509 cert and key to a pkcs12 file

openssl pkcs12 -export -in server.crt -inkey server.key \
               -out server.p12 -name [some-alias] \
               -CAfile ca.crt -caname root

Note: Make sure you put a password on the pkcs12 file - otherwise you'll get a null pointer exception when you try to import it. (In case anyone else had this headache). (Thanks jocull!)

Note 2: You might want to add the -chain option to preserve the full certificate chain. (Thanks Mafuba)

Step two: Convert the pkcs12 file to a Java keystore

keytool -importkeystore \
        -deststorepass [changeit] -destkeypass [changeit] -destkeystore server.keystore \
        -srckeystore server.p12 -srcstoretype PKCS12 -srcstorepass some-password \
        -alias [some-alias]

Finished

OPTIONAL Step zero: Create self-signed certificate

openssl genrsa -out server.key 2048
openssl req -new -out server.csr -key server.key
openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt

Cheers!

Keytool in Java 6 does have this capability: Importing private keys into a Java keystore using keytool

Here are the basic details from that post.

1. Convert the existing cert to a PKCS12 using OpenSSL. A password is required when asked or the 2nd step will complain.

   openssl pkcs12 -export -in [my_certificate.crt] -inkey [my_key.key] -out [keystore.p12] -name [new_alias] -CAfile [my_ca_bundle.crt] -caname root
   

2. Convert the PKCS12 to a Java Keystore File.

   keytool -importkeystore -deststorepass [new_keystore_pass] -destkeypass [new_key_pass] -destkeystore [keystore.jks] -srckeystore [keystore.p12] -srcstoretype PKCS12 -srcstorepass [pass_used_in_p12_keystore] -alias [alias_used_in_p12_keystore]