User.hbm.xml:
...
   <set name="role" table="User-Role">
     <key column="user_id"/>
     <many-to-many column="role_id" class="Role"/>
  </set>
...

Role.hbm.xml:
...
  <set name="user" table="User_Role">
     <key column="role_id"/>
     <many-to-many column="user_id" class="User"/>
  </set>
...

create table Roles (
   role_id VARCHAR(40) not null,
   rolename VARCHAR(20),
   brief VARCHAR(40),
   primary key (role_id)
)
create table User (
   id VARCHAR(40) not null,
   login VARCHAR(20) not null unique,
   password VARCHAR(20) not null,
   valid BIT,
   primary key (id)
)
create table User-Role (
   user_id VARCHAR(40) not null,
   role_id VARCHAR(40) not null,
   primary key (user_id, role_id)
)
...

...
create table User_Role (
   role_id VARCHAR(40) not null,
   user_id VARCHAR(40) not null,
   primary key (role_id, user_id)
)
create table Roles (
   role_id VARCHAR(40) not null,
   rolename VARCHAR(20),
   brief VARCHAR(40),
   primary key (role_id)
)
create table User (
   id VARCHAR(40) not null,
   login VARCHAR(20) not null unique,
   password VARCHAR(20) not null,
   valid BIT,
   primary key (id)
)
create table User-Role (
   user_id VARCHAR(40) not null,
   role_id VARCHAR(40) not null,
   primary key (user_id, role_id)
)
...