Threads access to object and performance/object in ServletContext: value or reference

[Brahim Bakayoko]

Hello all!
Let's suppose I have an object:

public class HTAccess{
   static Hashtable ht = ... (get a large hashtable from somewhere)
   
   public static String getElem(String elem){
      return (String)ht.get(elem);
   }
}

1. What would be the performance issues if the getElem method of this object is accessed statically from many servlet threads (1000s of threads)? (also considering that a hashtable is synchronized)
2. Are attributes objects retrieved from the ServletContext returned by value or is it by reference? (i.e, a large hashtable stored in the ServletContext)
[ March 28, 2004: Message edited by: Brahim Bakayoko ]

[Raj Chila]

Hi Brahin,
I think the Hashtable itself would have to be declared static ( which you have not, may be its just a typing mistake). I would consider this not a performance issue because the hashtable is functioning like a repository. If you are not using it to put some new Objects into it then I think it would be a performance issue as the other threads will have to wait if you synchronize the Hashtable.
I think all the Objects in the ServletContext are returned by value when you call through the getAttribute() method (otherwise what would happen if another servlet is calling the same Object?).
Hope this info is good enough.

[William Brogden]

1. Compared to all the other time consuming operations that take place in servlets, the extra time consumed would be miniscule.
2. Of course you only get a reference when you retrieve something from a ServletContext. There are no collections in Java that return a copy of an object.
All Threads servicing requests to a given servlet share the same ServletContext - if this causes a problem you need to redesign the application. If you are used to programming single user applications, this change of viewpoint is one of the major mental adjustments you have to make on moving to servlets.
Bill

[Brahim Bakayoko]

Originally posted by RajaniKanth Bhargava:
Hi Brahin,
I think the Hashtable itself would have to be declared static ( which you have not, may be its just a typing mistake).

Oops! sorry for the typo...

If you are not using it to put some new Objects into it then I think it would be a performance issue as the other threads will have to wait if you synchronize the Hashtable.

It think you meant if I am using it to store objects, then it would be a performance issue whenever there is rehashing.
Yes, that's true and I plan to create the Hashtable such that its capacity is greater than its fully loaded size to minimize the potential for rehashing. The Hashtable will be mainly used as a source with occasional updates.
Does the Hashtable lock itself whenever there is an insert like database rows? If yes, then what is the transaction mode? (dirty read? read committed? ...)

I think all the Objects in the ServletContext are returned by value when you call through the getAttribute() method (otherwise what would happen if another servlet is calling the same Object?).
Hope this info is good enough.

[edit]: Well I did a simple test (so much for being lazy) and clearly attributes are returned by reference.
My question came from the fact that Hashtables are passed by reference and that when copied only a shalow copy is made (normally).
[edit]: OK, only collection object attributes are returned by reference, else they are returned by value.
[ March 28, 2004: Message edited by: Brahim Bakayoko ]

[Brahim Bakayoko]

Originally posted by William Brogden:
1. Compared to all the other time consuming operations that take place in servlets, the extra time consumed would be miniscule.

Well I am glad to hear that...

2. Of course you only get a reference when you retrieve something from a ServletContext. There are no collections in Java that return a copy of an object.

Well, the simple test I did shows that attributes are returned:
- by reference if the object is a Hashtable (probably HashMap too)
- by value if it is a Vector or any other object
The trick is to note the behavior of the clone() method of the object in question.
- if shallow copy: then reference
- if deep copy: then value
Thanks for the responses...
[ March 28, 2004: Message edited by: Brahim Bakayoko ]

[William Brogden]

Well, the simple test I did shows that attributes are returned:
- by reference if the object is a Hashtable (probably HashMap too)
- by value if it is a Vector or any other object

You are trying to tell me that if a ServletContext contains a reference to a Vector then when you do a get, what it returns is NOT a copy of the reference but a clone?
I am eagerly awaiting your proof of this.
Bill

[Tim Holloway]

Actually, I believe that HashTable, like Vector is already synchronized and that no special mechanisms should be attached for multi-threading purposes (at best, it's extra overhead, at worst, a potential source of deadlocks).
The newer Collection classes are not synchronized, but I think all the original ones were, so I'd RTFM.

[Brahim Bakayoko]

Originally posted by William Brogden:

You are trying to tell me that if a ServletContext contains a reference to a Vector then when you do a get, what it returns is NOT a copy of the reference but a clone?
I am eagerly awaiting your proof of this.
Bill

Well below is your proof my friend.

  
import java.io.IOException;
import java.util.Enumeration;
import java.util.Hashtable;
import java.util.Vector;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * @version 1.0
 * @author
 */
public class Test extends HttpServlet implements Servlet {

ServletContext ctx;

public void init(ServletConfig servletConfig) throws ServletException {
this.ctx = servletConfig.getServletContext();

initMoboSpecs();

Hashtable ht = (Hashtable)ctx.getAttribute("specs_ht");
Integer value = (Integer)ctx.getAttribute("value");
Vector v = (Vector)ctx.getAttribute("v");

initMoboSpecs2(); //comment this and see behavior...

Enumeration enum = ht.elements();

while(enum.hasMoreElements()){
System.err.println((String)enum.nextElement());
}

System.err.println(value.intValue());
Integer value2 = (Integer)ctx.getAttribute("value");
System.err.println(value2.intValue());

for(int i=0; i< v.size(); i++){
System.err.println((String)v.elementAt(i));
}

Vector v2 = (Vector)ctx.getAttribute("v");

for(int i=0; i< v.size(); i++){
System.err.println((String)v2.elementAt(i));
}
}

public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
}

private void initMoboSpecs(){
Hashtable ht = new Hashtable(24);
ht.put(new Integer(1), "ProductName");
ht.put(new Integer(2), "FormFactor");
ht.put(new Integer(3), "Chipset");
ht.put(new Integer(4), "ProcessorClass");

ctx.setAttribute("specs_ht", ht);
ctx.setAttribute("value", new Integer(1));
ctx.setAttribute("v", new Vector(ht.values()));
}

private void initMoboSpecs2(){
Hashtable ht = (Hashtable)ctx.getAttribute("specs_ht");
ht.put(new Integer(5), "newProductName");
ht.put(new Integer(6), "newFormFactor");
ht.put(new Integer(7), "newChipset");
ht.put(new Integer(8), "newProcessorClass");

//ctx.setAttribute("specs_ht", ht);
ctx.setAttribute("value", new Integer(2));
ctx.setAttribute("v", new Vector(ht.values()));
}
}

[Brahim Bakayoko]

As I said above, the implementation of the clone() method for a particular object should be a give away.

[Ram Prasadh Anandaraj]

Hi Brahmin I think ur assumption is wrong.. try doing this,
private void initMoboSpecs2(){
Hashtable ht = (Hashtable)ctx.getAttribute("specs_ht");
ht.put(new Integer(5), "newProductName");
ht.put(new Integer(6), "newFormFactor");
ht.put(new Integer(7), "newChipset");
ht.put(new Integer(8), "newProcessorClass");
//ctx.setAttribute("specs_ht", ht);
ctx.setAttribute("value", new Integer(2));
Vector v = (Vector) ctx.getAttribute("v");
v.addElement("hi brahmin");
// ctx.setAttribute("v", new Vector(ht.values()));
}
"hi brahmin" will be reflected in both the iterations. If vector were to be cloned at this point,
Vector v = (Vector) ctx.getAttribute("v");
then "hi brahmin" should not appear in the first iteration of the vector.
prasadh

[William Brogden]

Well, that is strange and marvelous code to be sure but I don't see what it has to do with the question. It is your code that is mucking about with the servlet context, not the servlet context returning a clone. Looking at the source code for
org.apache.catalina.core.ApplicationContext
which implements ServletContext in Tomcat, we find:

 
    /**
     * The context attributes for this context.
     */
    private HashMap attributes = new HashMap();
//..... lotsa code skipped
   public Object getAttribute(String name) {
        synchronized (attributes) {
            return (attributes.get(name));
        }
    }
  

No sign of any clone operation there.

Bill