Java Garbage Collection !! How it works ?

Building software in the JAVA platform has lots of advantages, Today in this post I would like to share some of my understandings on one such greatest advantage gifted by the JAVA platform, to all those developers who lost hairs from their scalp by pragmatically managing memory :), i.e Garbage collection.

Before explaining garbage collection, let me explain some things about “Objects”, If C++ is your first OO programming language then you can consider it as a pointer to a particular memory location, In JAVA too it is the same, Objects when created from class gets its memory allocated. Objects are modified and their behaviors are accessed only via variable references.

In C++-like language, Memory is managed pragmatically, ( this is what is the reason for losing hairs :) ). JAVA completely wipes out this problem by giving JVM the power to do Garbage collection. JAVA being a full-fledged OOP language, Objects referenced by variables are created in heap memory.

As long as any variable references an object, the JAVA garbage collector retains them in the memory. When all variable references of an object expire, it becomes eligible to be garbage collected.

When Do References Expire?

Variables local to functions or code blocks expire when the function is complete. Programmers can explicitly expire the references by setting them to null


String temp = "JAVA journals ";
temp = null;

Though programmers have this sort of flexibility to explicitly expire object references, programmers have no way to control the behavior of garbage collectors. The garbage collector runs on its thread. It identifies objects eligible for garbage collection and cleans them from the memory.

JDK provides certain API’s to programmers which can request garbage collection, but as per the documentation, there is no guarantee, Since garbage collector has its intelligence based on that it will prioritize and serve the request if possible. Below are the API’s which are provided by JDK to request Garbage collection

  1. System.gc()
  2. Runtime.gc()

Both these APIs do the same thing, there is a difference in terms of usage, It’s a little out of scope to explain them. Please refer to Javadocs to know more about them. Some times you might get an exception, saying “ Out Of memory “ to avoid that you can exploit some of the API’s provided by JDK to know the memory status.

As a JDK user Can I increase the HEAP size ?

Ofcourse yes, There are some commandline options which you can use

 Java -Xms        set initial Java heap size
 Java -Xmx        set maximum Java heap size
 Java -Xss        set java thread stack size

Can I disable this entire Garbage collection feature ?

OfCourse yes, you might not be able to control it but you can disable or enable it for a particular class Below are the command line options

Java -Xnoclassgc disable class garbage collection Java -Xincgc enable incremental garbage collection

How can I know what the hell this Garbage collector is doing ?

You can know by logging its status to a file.

Java -Xloggc:    log GC status to a file with time stamps\

There are still lots of magic Garbage collector does, it is really an amazing thing to play around, I hope you enjoyed reading my post.