controll the hibernate session(when to close it manually)

Question

I am new in hibernate,after read the hibernate api and tutorial,it seems that the session should cloesd when not used.

Like this:

Session sess=getSession();
Transcration tx=sess.beginTranscration();
//do something using teh session
sess.save(obj);
tx.commit();
sess.close;

I have no question when using it in a standlone application. However I am not sure when using in the web app.

For exmaple, I have a servlet:TestServlet to recieve the parameters from the client,then I call a Manager to query something according to the parameters: just like this:

class TestServlet{
  doGet(HttpServletRequset,httpServletResponse){
    String para1=request.getParam...();
    String para2=.....
    new Manager().query(para1,para2);
  }
}

class Manager{
  public String query(String pa1,String pa2){
    Session=....// get the session
    //do query using para1 and 1
    session.close() //Here, I wonder if I should close it.
  }
}

Shoule I close the session in the query method?

Since someone tell me the session in hibernate just like the connection in jdbc,so open and close it so frequently is the correct way?

BTW,does the tx.commit() is required each time?

Also what's the thread problem about using session in servlet,since I saw the session is not thread safe in api.