Jvm And Garbage Collection

20 Jan 2011 - Marcelo Costa

JVM e Garbage Collection

Oi Pessoal, tudo bem? Starting here the 1º blog post, I’m going to try to create an opuscule (i.e., a wee text) about Garbage Collection e the Java Virtual Machine. This is for those of you that never programmed even a pathetic HelloWorld class and whoever wants to reinforce existing knowledge, e.g., something you learned during the SCJA (Sun Certified Java Associate) studies. Let us jump right into it and start with the first universal truth:

The JVM is Chaotic

The Garbage Collection is not performed immediatly after an object loses its reference, the JVM waits until a few lost objects start to accumulate. Think of it as a Cleaning Agent that is watching TV and, after the children finished their fun activities in the living room, that same cleaning agent is supposed to collect the toys, sweep, vacuum the carpet (i.e., GC).

Look at the image above, on the left side we have the Stack (where the reference variable is) and on the right side we have the Heap (where the actual object is), if we add the following line of code:

my_birth = null;

Our cleaning agent will already know that, at some point in time, it is necessary to take care of that useless object that was left in ‘0x01abcdef’. We can’t do anything about it, just ask nicely:

System.gc();

In other words, “Hey cleaning agent, can you please clean that room over there?” (but that will only make the cleaning agent give more attention to that particular region, it will not be done immediately).

So, how does the GC works? Well, we know that GC will occur on objects that do not have references, so the first algorithm that we should talk about is the “Mark and Sweep”.