Hashtable synchronized?

[Knute Johnson]

The docs say that Hashtable is synchronized. Does that mean that I
don't have to prevent concurrent gets/puts with my own synchronization?
I'm not concerned about manipulating the Iterators just storing and
retrieving elements.

Thanks,

 

[garskof]

Yes, Hashtable is sync.

 

[Thomas Fritsch]

The docs say that Hashtable is synchronized. Does that mean that I don't
have to prevent concurrent gets/puts with my own synchronization?

Yes, exactly.
Looking into the source of Hashtable, you see that all its data-accessing
methods (get, put, isEmpty, size, clear, remove, ......) are synchronized by
themselves. So there is no need for you to guard them with your own sync.

 

[Thomas Hawtin]

The docs say that Hashtable is synchronized. Does that mean that I
don't have to prevent concurrent gets/puts with my own synchronization?
I'm not concerned about manipulating the Iterators just storing and
retrieving elements.

Each get and put will be correctly synchronised.

However, sequences of operations will not be. Consider:

Value value = table.get(key);
if (value == null) {
value = new Value();
table.put(key, value);
}

Between the get and put another thread may have inserted an entry under
the same key. So either synchronise the whole block against the
Hashtable, or from 1.5 use a ConcurrentMap and replace put with putIfAbsent.

Tom Hawtin

 

[Knute Johnson]

Each get and put will be correctly synchronised.

However, sequences of operations will not be. Consider:

Value value = table.get(key);
if (value == null) {
value = new Value();
table.put(key, value);
}

Between the get and put another thread may have inserted an entry under
the same key. So either synchronise the whole block against the
Hashtable, or from 1.5 use a ConcurrentMap and replace put with
putIfAbsent.

Tom Hawtin

Thanks Tom.

 

[Knute Johnson]

Yes, exactly.
Looking into the source of Hashtable, you see that all its data-accessing
methods (get, put, isEmpty, size, clear, remove, ......) are synchronized by
themselves. So there is no need for you to guard them with your own sync.

Thanks Tom.

 

[snp_shailesh]

Hashtable is said synchronized. what o/p u expect for following prog ? is it five 0,

"Knute Johnson" <[email protected]> wrote:
> The docs say that Hashtable is synchronized. Does that mean that I don't
> have to prevent concurrent gets/puts with my own synchronization?
Yes, exactly.
Looking into the source of Hashtable, you see that all its data-accessing
methods (get, put, isEmpty, size, clear, remove, ......) are synchronized by
themselves. So there is no need for you to guard them with your own sync.

> I'm not concerned about manipulating the Iterators just storing and
> retrieving elements.

--
"TFritsch$t-online:de".replace(':','.').replace('$','@')

Hi,
i am agree with the above statements. but still wondering why the output of following program is not like 5 zeros, 5 ones, and then 5 twos.
Can anyone help in understanding this ?
------------------------------------------------------------------------------

import java.util.Hashtable;
import java.util.Enumeration;

class Insects
{
Hashtable ht;
int i;

public Insects()
{
ht=new Hashtable();
ht.put("a",0);
ht.put("b",0);
ht.put("c",0);
ht.put("d",0);
ht.put("e",0);

}



public synchronized void showData()
{
try{
Enumeration e=ht.elements();
System.out.println();

while(e.hasMoreElements())
{

System.out.println((Integer)e.nextElement());
Thread.sleep(1000);

}
}
catch(InterruptedException e)
{
System.out.println(e);
}

}

public void modifyData(int i)
{
Enumeration e=ht.keys();
while(e.hasMoreElements())
{
String s=(String)e.nextElement();
ht.put(s,i);
}

}
}

class Flier extends Thread
{
Insects its;
static int i=0;
public Flier(String name,Insects it)
{
super(name);
try{
its=it;
Thread.sleep(2000);
start();
}
catch(InterruptedException e)
{
System.out.println(e);
}
}

public void run()
{
its.modifyData(i);
i++;
its.showData();

}

}

class Threader
{
public static void main(String a[])
{
Insects it=new Insects();
Flier f1=new Flier("f1",it);
Flier f2=new Flier("f2",it);
Flier f3=new Flier("f3",it);



}
}






Have created 3 threads for attacking on the hashtable. But with above program, i guess synchronization is not achieved. :-(