Concurrent Modification Exception

I ran into a ConcurrentModificationException (CME) during stress testing.
What does CME actually mean?
It means that you've modified (add, remove, update) your Collection while you've been iterating over it (usually in a multi-threaded fashion, but it can occur in a single thread that modifies while iterating).

A few more things to note about CME:
Best effort detection - If you see a CME printout, first off, consider yourself lucky, CMEs are thrown only in best effort. In another universe, the concurrent modification would not have been detected, causing your collection to become corrupted, instead of fast-failing with a CME.

IDing the problem - Like deadlocks, CME's are easy to pinpoint once you inspected the exception's stack trace.

Avoiding CME:

1. ListIterator
   To modify a collection by the same thread that is currently iterating on it, use a ListIterator that will allow you to perform both.
   Drawbacks - single thread solution only.


2. Naive solution: Synchronizers
   Use locks to for mutually excluding traversal and modification operations.
   Advantages - easy to code.
   Drawbacks - very long lock periods while iterating.


3. CopyOnWrite
   Take advantage of the Java.util.concurrent collections like: CopyOnWriteArrayList, CopyOnWriteArraySet. If you require a map then grab CopyOnWriteMap from Apache (this guys have been doing Sun's dirty work for years now).
   Advantages - very good reading performance (no locks are used, instead visibility is obtained via map member volatility).
   Drawbacks - very bad write performance on large maps.
   Conclusion - use for seldom mutating collections.


4. toArray()
   toArray will create a new array holding a copy of your Set (Map.keySet() for a Map).
   You can then iterate over the array, freely modifying the original collection (the array doesn't change of course).
   Advantages - write operations are cheap.
   Disadvantages - copying the entire set could be expensive if it occurs too often, and/or the set is very large.


5. Concurrent Collections
   If you want to go heavyweight, consider using: ConcurrentHashMap (or one of its package friends).
   Once you create an iterator over a ConcurrentHashMap (CHM), it does not freeze the collection for traversal, updates to the collection may or may not appear during the traversal (weakly consistent).

The approach I ended up taking
My use case was seldom modifying a ~ten items cache. A copyonwrite map was what I used.
In other cases I had, ConcurrentHashMap was the easiest solution (though make sure your code can live in peace with the CHM weak consistency property).

[caption id="attachment_227" align="alignleft" width="200" caption="Best pic idea I could think of to visualize Threads :)"][/caption]

NAT in VMWare vSphere/ESX – In a nut shell

This post is about NATing an ESX VM, but first, why do I need NAT:

The SIP protocol is not NAT oblivious. To traverse NAT our application has to replace the DNS in the SIP message contact header to the external FQDN that the message receiver will be sending responses to (A NAT with static routing configured).
Therefore I needed to test our software in a NAT topology.

In the past, when we used VMWare player/workstation, it had a build-in NAT network. But, unfortunately, the ESX hypervisor does not provide a NATed network option.
Seeking alternatives at VMWare's appliance marketplace, I found and downloaded the Vyatta's community edition (VC5) router appliance (also downladble from sourceforge), and comes under the GPL license.
After 3-4 hours - guided by the official quick start guide -  I had a working NAT configuration in the ESX. Hurray!
Overall, not a hard nut to crack ;), though I wish VMWare will wise up and just add an build-in NAT option to vSphere.

Left to do:
Obtain some static IPs, so the config won't break each time the vm reboots and the DHCP lease expires.
Tip #1:
If you want want to access your NATed VM by RDP/VNC, without setting up extra NAT routing rules, consider adding the VM an additional un-NATed NIC, but when doing so, make sure that the OS routing tables are set to route through the NIC that is NATed.
Tip #2:
This short vyatta user installation report also helped me a bit.

Here's the complete configuration script I ended up feeding to the appliance console (network topology is similar to the one presented in the Vyatta's getting stated guide):
Where:
1.2.3.4 is your department's DNS server
192.168.1.199 is the VMs NATed private IP address (provided by the DHCP).
The script contains a NAT forward rule for VNC (port 5900)

configure
set system host-name vyatta-nat
set interfaces ethernet eth0 address dhcp
set service ssh
set service https
commit;
save;
# restart the appliance to switch from console remote desktop to SSH:

#login with user and password
configure
show interfaces

set interfaces ethernet eth1 address 192.168.1.254/24

commit;

delete service dhcp-server
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 start 192.168.1.100 stop 192.168.1.199
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 default-router 192.168.1.254
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 dns-server 1.2.3.4
commit;
show service dhcp-server

set service nat rule 1 source address 192.168.1.0/24
set service nat rule 1 outbound-interface eth0
set service nat rule 1 type masquerade
commit;
show service nat
save;
exit
show nat rules
configure
set service nat rule 20 type destination
set service nat rule 20 inbound-interface eth0
# use a negative fake address to so that all incoming communication will be nated
#set service nat rule 20 destination address !192.168.50.0
#Forward traffic to address 192.168.1.199
set service nat rule 20 inside-address address 192.168.1.199
set service nat rule 20 protocol tcp
set service nat rule 20 destination port 5900
commit;
save;
exit

Myth busting - String.intern() object allocations are never garbage collected

Java is becoming quite old (version 1.0 came out in 1996 if I'm not mistaken). When something turns old, legends, myths, and other perceived truths are quick to form around it (just imagine an old Gothic mansion with its stack of scare tales).
Most of the accumulated knowledge is beneficial and helpful, but some of it is not relevant anymore or just plain wrong.
Remembering that Java is 14 yeas old (2010), when I google for something, for Java info/answers, I always inspect the date of the article I landed on.
If you stumble upon somebody claiming that java can/can't do something, always check his comment's date. If I see something from 2001, you better search for newer references, instead of accepting it as is.

Some sites like http://Javaworld.com, have been there from the get go, were big then, but after losing popularity, are now a grave yard for old Java skeletons (I myself have a not that relevant article there).

The story with String.intern() is the same, you'll find people all around the place, claiming that over using it will finish up the perm area, because the perm area is never garbage collected. As discussed here, that's just not true.

Something I enjoy doing is not taking so called "facts" as granted, and re-validating on my IDE.
Thinking that those intern() allocations will never be GCed, I was planing a presentation on how to use weakHashMap based solution can serve as an alternative cache repository for Strings, wrote a program to demonstrate an OMME caused by intern() only to find out that intern() is not so bad  as I originally thought.
Try stuff yourself. You be surprised...

Other myths I'll should wright about some day are:

1. Regular expressions in Java are slow - FALSE! I've tested this myself, and after compiling the regex, I was able to run over than 1 million matches per second (small strings of course).


2. Always use StringBuffer to concatenate strings - dead wrong! if you have all concatenations in a single line, like the following, the compiler auto does it for you:
   s= "Hi my name is: "+myName+ ". my lucky number is: "+num;
   Run Javap on a class file using and not using StringBuffer to see that the byte code is the same.
   Though this piece of code could benefit from StringBuffer to prevent rapid object creation:
   for (...) {
   s += strOfThisCycle;
   }
   In any case, Java5 introduces StringBuilder which is the unsynchronized tween of the synchronized StringBuffer class. I guess you will rarely access the same builder from different threads, therefore StringBuilder should be the default choice for ya.

New Java blog out there

A new baby blog was born: Java Tech Sharing.
Proud father: Guy Moshkovich.

I recommend adding to your RSS/Atom reader.

Utility Frenzy #1 – The log summarizer

Here's a post I wrote (in the Hebrew language) which tells the story of the log summarizer utility that I've wrote. This story is the first in a line of "utilities stories" I'm planning on writing.
My apologies for those of you whom won't be able to read it. Posts in this site do appear in English..

Google is pregnant again - Noop

After the zillion new dynamic languages that had flooded the earth
(groovy, ruby, ...), Google is concocting Noop; a new type-safe language to join Java, Scala, and the rest.

The new language sets out to excel in testability, dependency injection, and readable code (see the proposed features). More interesting than whether Noop will gain a crowd of enthusiasts, is the language's dynamic and lucid development process; made available through Google code (Sun's JSRs, are also transparent but here it's a whole new language).

What do you think?

Why is Thread.sleep() inherently inaccurate

Avi Ribchinsky, a friend and a college of mien, is transitioning from C++ to the Java world. He had been playing with Thread.sleep(), when he noticed that the sleep method might oversleep more than ordered, and moreover, it could also under sleep (see Fig 1). Coming from the C++ world, that surely caught him surprised ;)

Fig 1.

[caption id="attachment_173" align="alignleft" width="584" caption="Thread.sleep() under sleeping"][/caption]

How is sleep implemented in Java anyway?

Avi came asking me if I knew anything about it, I was wondering myself how such a common and important method could be faking in the way shown above. Is it the OS? a Bug in the specific JRE version used? Maybe the API doesn't guarantee milliseconds precision to begin with?
Thinking about all of these factors, we realized that we don't really know how the JVM implements the sleep method functionality, my best guess would have been that the process registers itself in the OS for a wake up call, and the OS wakes the process via a software interrupt. OK, time to search the web.

The following article gives a very detailed answer, explaining that sleep is implemented by a thread giving up its OS scheduling quantum back to the scheduler, on the next execution quantum the thread gets, it has the chance to wake up and continue processing, or again continue sleeping.
Therefore, the accuracy resolution of sleep is directly dependent on the process scheduling resolution of the operating system in usage. Since windows XP process scheduling resolution is roughly 10ms, the sleep mechanism, in the Avi's example, might had preferred to under sleep "a little" rather than oversleeping "a lot", by waking himself in the current scheduling cycle quantum, rather than in the next, future, quantum.

The article also mentions that the inaccuracies are worsened when a process with a higher scheduling priority, than the sleeping process, is in a runnable state.

I assume that, running on a Hypervisor with course grained process scheduling would also produce greater inaccuracies.

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Conclusion

You can't rely on the millisecond accuracy of the sleep method. Take a before and after time measurament to find the actual time spent sleeping, in order to avoid ever increasing inacurracies.
Sleep tight :)