Last week I had the pleasure attended Heinz Kabutz Java Specialists Master course to sharpen my Java skills. Java Champion Heinz, is a great trainer who manages to combine anecdotes, hard facts and deep Java knowledge with engaging exercises to a well done course. The scope was the whole spectrum of Java, but focusing on the details you normally do not use, or know how to use. Some of the material he already published as part of his newsletters , which are read all around the world.
Let me share my impressions on the course with you in this day by day review…
Day 1
The course started with discussing Threads, and how we should use them. Quite a complex topic for early morning. We played with a ThreadGroup that became a custom Thread pool. ThreadGroup is not the best designed class. Heinz called it a child’s drawing from the early years of Java. I found using the java.util.concurrent Locks really easy. Finally gone are the days of synchronized(). Before the lunch break, Heinz showed us his laws on concurrency, which he also presented in an abbreviated form at our meet the experts – performance event. We came across this code:
1private boolean running = true;
2public void dojob() {
3 while(running) {
4 // do something useful
5 }
6}
7public void shutdown() {
8 running = false;
9}
When running in a Server VM (with java -server), this might never stop, due to optimizing running would be inlined by HotSpot with being true all the time. To avoid this you would have to make it volatile. Because I asked about debugging, we tried stopping there and the debugger showed us: running = false. Still the code continued to executed, because the debugger sees the correct field value, but the running code doesn’t. It gets more interesting with this code:
1public void doJob() {
2 boolean myRunning = running;
3 while(running){
4 // do something useful
5 myRunning = running;
6 }
7}
When looking with the debugger we saw this:
1running = false; myrunning = true;
however the loop still looped. But when forcing the line to execute via F7, code terminated. This can be a nightmare to debug, so it is good to know what you should take care of when writing multithreaded programs.
Also something to remember is to check
1if (Thread.interrupted()) { 2 throw new InterruptedException() 3}
as first code in all methods declaring an InterruptedException.
We learned about CompletionService looks like an interesting interface for mass processing of asynchronous work. So, who needs closures? 🙂
Day 2
We started with Java new (yet another new?) IO, which brings quite a lot of new features, but somehow is not as widely used as it should be. One reason might be that it can easily get much more complicated to use. Perhaps one should try easy examples before writing an Async nonblocking server (which you can after attending the course :-)).
1FileChannel fc = new RandomAccessFile("test.txt", "r").getChannel(); 2MappedByteBuffer buffer = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());
Second half was on understanding Java memory management. This of course is less often really applicable, but understanding it is quite critical to solving problems. We had a look at stuff like caching and pooling, and why this creates leaks or loitering objects. This reminds me of my older post about tag pooling in servlet containers. I never understood really why tags are pooled, and even worse, do not have proper lifecycle methods to detect this when using an IDE. We used jVisualVm and HPjMeter to watch the GC at work.
Day 3
On day 3, I learned some interesting inner mechanics of Collection classes I did not use before (like PriorityQueue ), as well as some nasty tricks on classloading. Heinz explained java.lang.reflect.Proxies really well, and once understood, using them was not that difficult. Actually the best instruction is in the JavaDoc, but you must know how to read it:
1Foo f = (Foo) Proxy.newProxyInstance(
2 Foo.class.getClassLoader(), new Class[] { Foo.class },
3 new InvocationHandler() {
4 public Object invoke(Object foo, Method method, Object[] arguments) throws Throwable {
5 return method.invoke(foo, arguments);
6 }
7 });
In the afternoon we discussed about Exceptions, and I made up my mind on checked vs unchecked exceptions. Personally I will use unchecked exceptions for developer/programming errors. Catching them is not required, app may crash – Developers should fix this. However everything which is related to the Environment the app runs ins should work with checked exceptions. Ideally they provide sensible information, not just a message. Also quite important: Just rethrow Exceptions! Found yourself not able to decide what to do with an InterruptedException? Well just rethrow it 🙂 And handle it in the Thread code (calling interrupted() and exiting a loop). I never did that often, because I don’t like polluting my method signature, but it should be considered. Do not be afraid of retrowing Exceptions.
Day 4
The last day of the course started with a tough performance optimization exercise. The tough part was that we were not allowed to improve the code until we had all the numbers written down and eliminated any overhead of testing code. I especially liked this, because it thought me how eager I am sometimes fixing issues that I forget proving them first. As kind of side note, we discussed the various modes the JVM can run in and found out how slow java -Xint is. After having sped up the code down to 10% of its initial runtime, we moved on to Date and Time, which was a bit short chapter. I can recommend using jodatime and icu4j , and try to stay away from java.util.Date. Before the end of the course we covered logging including some nifty tricks. The most important lesson about logging is that you need to use code guards (which was not new to me, but I like the term, I never heard before):
1if (log.isDebugEnabled()){ 2 log.debug(complexObject.toString() + expensive.toString()); 3}
Wrap-Up
I can wholeheartedly recommend this course. 4 days packed with lots of information and exercises that are well done so that they are a challenge for every participant. You should have worked with Java already some time. This is definitely not a beginners course. You will rush past topics which you can only grasp when you have experienced the problem before. I can also recommend taking this training in German, because Heinz has a really funny accent 🙂
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