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October 4th, 2013

by Ivan St. Ivanov

The community keynote

I’m not a big fan of keynotes at conferences. Prior to going to my first conference (back in 2010) I thought that this is where all the announcements and big news come. Now that I’ve been to my second JavaOne, I see that the community keynote is not for me. I’m not saying it was bad, but it was all about the different areas of life where Java is involved. Sounds good, the people that showed their achievements (James Gosling including) deserve all the best. But to be honest, I did not enjoy that session.

The most boring part was the introduction. One of the big sponsors of JavaOne (freescale) showed their vision about the Internet of Things. Most of the time I thought that the gentleman speaking got the venue wrong: instead of the Moscone center where Oracle Open World was going on, he came to show his white collar presentation to the geeks. If I compare it to the other sponsor’s talk (IBM) at the first day keynote I would say that blue giant’s was much more geek oriented. It showed IBM’s vision on how they want to improve developer’s life. Now what we saw is how freescale is going to make millions from home automation and all the likes.

Well, the community event had a really motivating part. Right after Stephan Janssen talked about Devoxx4Kids we saw on the stage a 10-year-old youngster, who had not been happy with his Minecraft experience and learned how to hack it… in Java. Really, it was very interesting to listen to that boy (his name is Aditya Gupta and he happens to be the son of Arun Gupta) talking about programming, Java and even de-obfuscating code.

The Forge gathering

As the talk to which I wanted to go was full, I decided to go to the Howard Street Caf?. There I saw Lincoln Baxter, the project lead of JBoss Forge – a project that I also contribute from time to time. We were soon joined by another contributor – Paul Bakker. Lincoln explained us in detail the architecture of Forge 2.0, the ideas behind addons and how the Furnace framework (i.e. our OSGi lite) works. Paul wants to integrate bnd tools into Forge, while I am right now doing the git tools migration to 2.0. So Lincoln’s lecture was quite valuable for us both.

You see, conferences are not just about going to talks, but are also about talking to each other (some people call it networking). You go there and meet all those kinds of geeks, whom you just follow on twitter or exchange mails from time to time. Now they are all there and you can ask them whatever is bugging you. Or just go and say Hi!.

Venkat in action

After that I went to see a talk on mixing JVM languages with Java. The JavaOne content catalog site is designed in a way that you have to click two or three times in order to get to the speakers for a session. So I did not know who was speaking there. And when I entered I found that it was Venkat Subramaniam.

You’ve got to see this guy in action. He has an infinite source of energy somewhere inside. He talks and talks and waves his hands and live-codes and at the same time never stops to talk. I have only heard of him so far, but have never seen him.

His talk was on calling Java from Groovy and Scala and vice versa. He was using Groovy shell, Scala REPL and InteliJ IDEA and his presentation was in a text file, where he just ticked the topics of the agenda, which he had passed.

Basically calling Java from other languages was not a big challenge. The only problem was with Java methods, which names had special meaning in the respective language, e.g. yield and def for Scala. You do this (in most JVM languages) by simply “escaping” the method name (usually by surrounding it with quotes). There were more things to tackle in the opposite case: when Java program was calling something from the other two. And it is natural: they are more powerful in terms of language features. But at the end they are compiled down to byte code. What Venkat showed was first how to run those: include the respective jar (scala/groovy) plus the compiled Scala or Groovy classes into your classpath. And then look at the file system to see what was generated: for example for companion objects scalac generates class files named <ComanionClass>$class.class. After that instantiate the class that you saw on the file system. But that does not seem to always work: sometimes the IDE complains that it can’t find a certain class (generated by scalac), but finally the program runs. The bottom line is: you have to know very well the internals of the language that you want to call, what is its syntax and most importantly how it maps to the Java concepts.

My five cents on mixing JVM languages. Most importantly: you shouldn’t do it just because it sounds cool. There must be a reason for that. The most compelling reason hides in Ola Bini’s programming languages pyramid. The essence is that the stable part of an application (the business logic for example) should be written in a stable language, which is statically compiled. But if you want to create a DSL that uses those APIs, Groovy or Scala are far better choices for that. And that’s where JVM language interoperability kicks in. Never did it myself, so I am just pondering here.

JVM internals

The last two sessions for me at this JavaOne were all about the JVM. IBM’s presentation on the community day JavaOne keynote touched the surface of their work on the packed objects, which try to overcome the problem that most of the memory consumed by Java objects is just header data, telling the JVM how to store, hash and synchronize on objects. The talk by an IBM architect on the last day showed exactly how much memory overhead the different Java collection structures require.

He started off by calculating how much memory an Integer object takes. On a 32-bit virtual machine it takes 4 times the data that a normal int would take. This makes 96 bits “administrative” data and just 32 bits for the int value.  The first 4 bytes are for the pointer to class object, the next 4 are for things like hashcode, then come some 4 bytes for locking and synchronization and only then come the last 4 bytes for the real value. The overhead becomes even bigger on 64-bit OSs, where it takes 224 bits for storing that same integer (all the first three groups now take 8 bytes, just the int value stays at 4 bytes). This can be decreased a little by using compressed object pointers (an option of the JVM).

The second and more interesting part of the presentation was about the collections: the memory that they use for storing objects, how fast you find those objects inside and the strategy for resizing each collection when it reaches its capacity limit. The collections that were compared were the Hash*’s (HashSet, HashMap, Hashtable), the lists (ArrayList, LinkedList) and the StringBuffer. As you might guess, there’s a tradeoff – in Hash* collections you find faster an element versus the lists, but the overhead for storing objects (i.e. the amount of memory that just contains “administrative” data for the JVM) is bigger (1.5 times more than in LinkedLists and 9 times more than in ArrayLists).

Expanding a collection is another very interesting topic. What happens if you have created a StringBuffer with size 40MBs, you fill it and then add an extra character? Well, you will get an 80MBs StringBuffer, as capacity of StringBuffer always doubles upon expansion. The same applies for Hash* structures, ArrayLists resize by the factor of 1.5. The LinkedLists are the best structure in this area as their nature is such that they always expand by one.

Briefly, my takeaways from this very informative talk are:

  • Use Hash* collections if you care more about finding an element faster than about the consumed memory
  • If possible create your collections with optimal capacity so that you avoid resizing. At the same time try to avoid additional overhead in memory consumption (empty collections also use memory)
  • If you are not sure about your memory consumption, use Eclipse Memory Analyzer

The last session that I visited happened to be about SAP JVM. But as I work for that company, I would like to keep the JavaOne 2013 series free from the least sense of corporate feelings. That’s why I am going to write about that talk in another post in the next few weeks.

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