Wikipedia
Keystore
A Java KeyStore (JKS) is a repository of security certificates either authorization certificates or public key certificates plus corresponding private keys, used for instance in SSL encryption.
In IBM WebSphere Application Server and Oracle WebLogic Server, a file with extension jks serves as keystore.
The Java Development Kit maintains a CA keystore in folder jre/lib/security/cacerts. JDKs provide a tool named keytool to manipulate the keystore. keytool has no functionality to extract the private key out of the keystore, but this is possible with third-party tools like jksExportKey, CERTivity, Portecle and KeyStore Explorer.