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garbage collector

garbage collector

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  • 1. Disclaimer: This presentation is prepared by trainees of baabtra as a part of mentoring program. This is not official document of baabtra –Mentoring Partner Baabtra-Mentoring Partner is the mentoring division of baabte System Technologies Pvt . Ltd
  • 2. Garbage Collection in Java Abhishek H Menon abhishekmenon121989@gmail. com kmenon5 121989 menon
  • 3. Features of Java programming language • Platform independent • Object oriented • Automatic garbage collection • Rich standard library
  • 4. 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
  • 5. Marking Before marking After marking Alive object Unreferenced object Memory object
  • 6. 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
  • 7. 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
  • 8. Garbage collection • What is Garbage collection? – Memory management technique – Process of freeing objects – No longer referenced by the program
  • 9. 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.
  • 10. 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.
  • 11. 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()
  • 12. Garbage collection algorithms • Reference counting • Tracing • Compacting • Copying • Generational • The train algorithm
  • 13. Reference counting • Keeps counts for each object • Count increases as the number of references increases • Eligible for gc when count becomes zero.
  • 14. 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.
  • 15. 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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