I've 2 tables:

s_id(key) name cli type

sa_id(key) s_id user pwd

So in Jpa I've:

class A...{
    @JoinTable( name="A_B", 
    joinColumns={@JoinColumn(name="a_id", table="a",unique=false)}, 
    inverseJoinColumns={@JoinColumn(name="b_id", table="b", unique=true)} )
   Collection<B> getB(){...}

class b is simply a fundamental entity class without any mention of the A.

Hopefully that's obvious. My real question is: Will I actually need a join table to complete this type of simple join? Can't this be achieved having a simple joincolumn or something like that?

No you don't need a join table for OneToMany. Consider the @mappedBy annoatation

You don't need a JoinTable with this. When the class B doesn't have mention of the class A then your following will suffice

@Entity class A...{ 
Collection getB(){...} }

Generally though you might want a bidirectional relationship by which situation B includes a mention of the A. For the reason that situation you will have to lookup the @mappedBy annotation. pointed out by Paul.

The fast answer is when you've got a Many-to-Many relationship you'll need another table. For those who have a 1-to-Many or perhaps a Many-to-One relationship you won't.