Saturday, November 21, 2015

Java Garbage Collector

If you are a java developer, java garbage collection (gc) sometime pop up from time to time in javadoc, online article or online discussion. It is such a hot and tough topic because that is entirely different paradigm than what programmer usually do, that is coding. Java gc free heap for the object you created in class in the background. In the past, I also cover a few article which related to java gc and today I am thinking to go through several blogs/articles which I found online, learn the basic and share what I've learned  and hopefully for java programmer, java gc will become clearer.

When you start a java application, with the parameters that are assigned to the java, the operating system will reserved some memory for java application known as heap. The heap further divided into several regions collectively known as eden, survivor spaces, old gen and perm gens. In oracle java8 hotspot, perm gen has been removed, be sure to always check official documention on garbage collector for changes. Below are a few links for hotspot implementation for java gc.
Survivor spaces are divided into two, survivor 0 and survivor 1. Both eden and survivor spaces collectively known as Young generation or new generation whilst old gen also known as tenured generation. Garbage collections will happened on young generation and old generations. Below are two diagrams show the heap regions are divided.

While the concept of Garbage Collection is the same, the implementation is not and neither are the default settings or how to tune it. The well known jvm includes the oracle sun hotspot, oracle jrockit and ibm j9. You can find the other jvm lists here. Essentially garbage collection will perform on young generation and old generation to remove object on heap that has no valid reference.

common java parameters settings. For full list, issue the command java -X

-Xms initial java heap size
-Xmx maximum java heap size
-Xmn the size of the heap for the young generation

There are a few type of GC
- serial gc
- parallel gc
- parallel old gc
- cms gc 

You can specify what gc implementation to run on the java heap region.

If you run a server application, the metric exposed by gc is definitely to watch out for. In order to get the metric, you can use

That's it for this brief introduction.

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