While the JCE secures cryptographic operations and key generation, it is up to the developer to actually manage their keys. More information needs to be provided here.
One commonly-accepted best practice for handling keys at runtime is to store them only as
byte arrays, and never as strings. This is because Java strings are immutable, and cannot be manually "cleared" or "zeroed out" in memory; while a reference to a string can be removed, the exact string will remain in memory until its segment of memory is garbage-collected and reused. An attacker would have a large window in which they could dump the program's memory and easily find the key. Contrarily,
byte arrays are mutable, and can have their contents overwritten in place; it is a good idea to 'zero-out' your keys as soon as you no longer need them.