@ManyToOne(optional=true)
@JoinColumn( nullable = true)

What is the purpose/difference of optional and nullable?

@ManyToOne(optional=true) is for a association, that means there may be not a target for the src. When you using a third table to implments the one-to-one association, that means there may be not a row to represent it.
in this case, your can't set a @JoinColumn( nullable = true) to it.

@JoinColumn( nullable = true) means you should set the column to be null.

Is it the 2 annotation also indicate the same thing? i really do not understand when should i use optional and nullable? which one is used to generate the ddl?