This document discusses garbage collection in Java. It defines garbage collection as the automatic process of freeing memory from objects that are no longer referenced. It describes the key steps of marking and deleting unreachable objects. The document outlines different garbage collection algorithms like reference counting, tracing, and compacting. It explains how objects become eligible for collection by getting de-referenced. The document also discusses the finalize method in Java that allows for cleanup before an object is collected.

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3. Garbage Collection in Java
Abhishek H Menon
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4. Features of Java programming language
• Platform independent
• Object oriented
• Automatic garbage collection
• Rich standard library

5. Automatic garbage collection
• Process of looking at heap memory, identifying which objects
are in use and which are not, and deleting the unused objects.
• In programming languages like C, CPP allocating and deallocating
is a manual process, but in Java garbage collector automatically
do allocation and deallocation.
• For performing this task of garbage collection, there are two
steps
– Marking
– Normal deletion
– Deletion with compacting

6. Marking
Before marking
After marking
Alive object
Unreferenced object
Memory object

7. Normal Deletion
Memory allocator holds a list of references to free
spaces, and searches for free spaces and allocated
memory sequentially then on.
Memory space

8. Deletion with compacting
After normal
deletion with
compacting
Memory allocation holds the reference to the
beginning of free space and allocated memory
sequentially then on

9. Garbage collection
• What is Garbage collection?
– Memory management technique
– Process of freeing objects
– No longer referenced by the program

10. Garbage collection
• Why Garbage collection?
– Free unreferenced objects.
– Relieves programmer from manual freeing the
memory.
– Helps in ensuring program integrity.
– Disadvantages
• Extra Overhead.
• Less control over scheduling of CPU time.

11. Garbage collection in java
• Purpose?
– Find and delete unreachable objects.
– Free space as much as possible.
• When?
– GC is under control of JVM.
– One can request but no guarantees.
• How?
– Discovery of unreachable objects.
– With the help of Algorithms.

12. Garbage collection in java
• Ways of making an object eligible for
collection
– Nulling a reference
– Reassigning a reference variable
– Isolating a reference variable
• Forcing Garbage collection
– Methods available to perform GC
– Only requests and no demands
– Using runtime class
– Using static methods like System.gc()

13. Garbage collection algorithms
• Reference counting
• Tracing
• Compacting
• Copying
• Generational
• The train algorithm

14. Reference counting
• Keeps counts for each object
• Count increases as the number of references
increases
• Eligible for gc when count becomes zero.

15. Tracing
• Traverses through the graph
• Starts from the root node
• Marking is done
– By setting flags in object themselves
– By setting flags in separate bitmaps
• Unmarked objects to be unreachable
• Also known as Mark and Sweep alg.

16. Finalize() method
• This method is called before the garbage
collector claims the object
• Available for all the classes in java
• Defined in java.lang.Object
• Purpose of this method is to perform clean up
activity ie, releasing system resources if
held, close the connection if open.
• After executing this method, the program gets
for garbage collection.

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