There are some negatives to using CHM compared to wrapping a HashMap in
a synchronized map. It is easy to write methods for compound operations
such as removing a key-value pair if, and only if, some other key is
present.
Patricia
On 2/8/2011 8:26 AM, Dan Creswell wrote:
No friction, simplest thing that could possibly work is the only thing
running through my head.
I'm happy to see CHM used if people deem it appropriate - as is so often the
case, I'm not writing the code so my opinion should only carry so much
weight.
On 8 February 2011 15:12, Gregg Wonderly<gr...@wonderly.org> wrote:
On 2/8/2011 9:00 AM, Dan Creswell wrote:
It's all about expected usage patterns.
Locking in this case will be per instance and the likelihood of contention
is low given no one's going to call add a lot or indeed toString a lot in
a
short space of time.
So given:
Define how concurrency is handled then implement
And assuming it's in line with the above contention model I'd say
synchronized would be adequate.
I believe this is the case. But, I also know that it can come back to
bite.
CHM solves it without synchronized, and that, I believe is a better choice
because it eliminates the problem. Any additional methods or usage patterns
that fall out of the life cycle of this object will not have any new or
different behavior or synchronization requirements.
Is there an attribute of CHM that you find unattractive Dan? I'm not sure
I understand the friction I am sensing.
Gregg
On 8 February 2011 14:54, Gregg Wonderly<gr...@wonderly.org> wrote:
On 2/7/2011 5:28 PM, Peter Firmstone wrote:
Do we really need a concurrent version of this class? How often will
Entry
classes change?
My choice is always to consider concurrency first, unless there is some
element of performance that this impacts. The simple fact is that there
is
a toString() that traverses the contents. So, you either must
synchronize
all access to the contents, or use a CHM which solves the problem without
any future concurrency surprises.
I'm really serious about this, because of how I've encountered tons of
"synchronized" blocks in river which introduce huge latency in
"processing"
because they cause everyone to wait in line for stuff which very often is
quite independent.
Yes, this structure is mostly one time mutated. But, its design exposes
a
continuous mutation capability. Thus, any "read" of the contents has to
take that into account.
Gregg Wonderly
Gregg Wonderly wrote:
Yes definitely the case. CHM just makes it a lot easier to not worry
about map
and set data structures in terms of concurrent access of mutating data.
Set
and HashSet are useful, but not always in concurrent mutating data
management.
I still prefer Vector over ArrayList just because it eliminates
iteration
problems that alway require the same lines of code to resolve.
Gregg
Sent from my iPhone
On Feb 7, 2011, at 2:08 PM, Dan Creswell<dan.cresw...@gmail.com>
wrote:
All true, still need to document the concurrent contract for this
class
and
implement accordingly.
i.e. We can say "toString" is thread safe etc and then use
ConcurrentHashMap
and friends.
But we gotta do it right....
On 7 February 2011 19:57, Gregg Wonderly<gr...@wonderly.org> wrote:
On 2/7/2011 5:24 AM, Tom Hobbs wrote:
Sorry, I've got my nit-picking hat on...
The null check in the equals method isn't needed because you get a
free one by using "instanceof".
I know you've warned that the class is not thread-safe, but people
often forget that doing a sysout of an object calls toString(). I'd
still be tempted to whack some protection into toString to stop it
blowing up when people start printing lots of debug messages.
Other than that it looks good to me.
I'd suggest that it would be easy enough to just use
ConcurrentHashMap
instead of HashSet if we are going to move to Java 5 anyway. Then,
you
don't have to worry about concurrency issues with toString() etc.
Gregg Wonderly
On Mon, Feb 7, 2011 at 10:56 AM, Peter Firmstone<j...@zeus.net.au>
wrote:
Dan Creswell wrote:
Solid - probably ought to be a constructor that doesn't add
defaults
too.....and maybe a note about thread safety on add.
The finished article.
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
implied.
* See the License for the specific language governing permissions
and
* limitations under the License.
*/
package org.apache.river.api.lookup;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import net.jini.lookup.entry.Address;
import net.jini.lookup.entry.Comment;
import net.jini.lookup.entry.Location;
import net.jini.lookup.entry.Name;
import net.jini.lookup.entry.ServiceInfo;
import net.jini.lookup.entry.Status;
import net.jini.lookup.entry.UIDescriptor;
/**
* A little builder utility class that creates an array of Entry
classes
to
* be used as a parameter for StreamServiceRegistrar. All the jini
platform
* Entry's are included by default.
*
* Note: This class is not threadsafe, use external synchronization
if
required.
*
* Suggested by Dan Creswell.
* @author peter
*/
public class DefaultEntries {
private final Set<Class> entrys;
public DefaultEntries() {
entrys = new HashSet<Class>(16);
}
/**
* Add an Entry class.
* @param cl - class
* @return this
*/
public DefaultEntries add(Class cl){
entrys.add(cl);
return this;
}
/**
* All all the Jini Platform Entry's
* @return
*/
public DefaultEntries addPlatformEntries(){
add(Comment.class);
add(Location.class);
add(Name.class);
add(ServiceInfo.class);
add(Status.class);
add(UIDescriptor.class);
add(Address.class);
return this;
}
/**
* Remove all Entry's
*/
public void reset(){
entrys.clear();
}
/**
* Generate a new array containing all Entry's added since last
reset.
* @return
*/
public Class[] getEntries(){
return entrys.toArray(new Class[entrys.size()]);
}
@Override
public int hashCode() {
int hash = 3;
hash = 29 * hash + (this.entrys != null ? this.entrys.hashCode() :
0);
return hash;
}
@Override
public boolean equals(Object o){
if (o == null) return false;
if (o instanceof DefaultEntries){
if (entrys.equals(((DefaultEntries)o).entrys)) return true;
}
return false;
}
@Override
public String toString(){
String newline = System.getProperty("line.separator");
StringBuilder sb = new StringBuilder(256);
sb.append("DefaultEntries:");
sb.append(newline);
Iterator<Class> it = entrys.iterator();
while (it.hasNext()){
sb.append(it.next().getName());
sb.append(newline);
}
return sb.toString();
}
}
