What is the garbage collector in Java

Question

I am new to Java and confused about the garbage collector in Java  What does it actually do and when does it comes into action  Please describe some of the properties of the garbage collector in Java

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garbage collector implies that objects that are no longer needed by the program are  garbage  and can be thrown away

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An object becomes eligible for Garbage collection or GC if it s not reachable from any live threads or by any static references    In other words  you can say that an object becomes eligible for garbage collection if its all references are null  Cyclic dependencies are not counted as the reference so if object A has a reference to object B and object B has a reference to Object A and they don t have any other live reference then both Objects A and B will be eligible for Garbage collection     Heap Generations for Garbage Collection -  Java objects are created in Heap and Heap is divided into three parts or generations for the sake of garbage collection in Java  these are called as Young New  generation  Tenured Old  Generation and Perm Area of the heap      New Generation is further divided into three parts known as Eden space  Survivor 1 and Survivor 2 space  When an object first created in heap its gets created in new generation inside Eden space and after subsequent minor garbage collection if an object survives its gets moved to survivor 1 and then survivor 2 before major garbage collection moved that object to old or tenured generation   Perm space of Java Heap is where JVM stores Metadata about classes and methods  String pool and Class level details     Refer here for more    Garbage Collection     You can t force JVM to run Garbage Collection though you can make a request using System gc   or Runtime gc   method   In  java lang System  public static void gc         Runtime getRuntime   gc          In java lang Runtime  public native void gc        note native  method     Mark and Sweep Algorithm -  This is one of the most popular algorithms used by Garbage collection  Any garbage collection algorithm must perform 2 basic operations  One  it should be able to detect all the unreachable objects and secondly  it must reclaim the heap space used by the garbage objects and make the space available again to the program   The above operations are performed by Mark and Sweep Algorithm in two phases    Mark phase  Sweep phase   read here for more details - Mark and Sweep Algorithm

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Garbage collection refers to the process of automatically freeing memory on the heap by deleting objects that are no longer reachable in your program  The heap is a memory which is referred to as the free store  represents a large pool of unused memory allocated to your Java application

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Garbage collector can be viewed as a reference count manager  if an object is created and its reference is stored in a variable  its reference count is increased by one  during the course of execution if that variable is assigned with NULL  reference count for that object is decremented  so the current reference count for the object is 0  Now when Garbage collector is executed  It checks for the objects with reference count 0  and frees the resources allocated to it   Garbage collector invocation is controlled by garbage collection policies   You can get some data here  http   www oracle com technetwork java gc-tuning-5-138395 html

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As objects are dynamically allocated by the new operator  you can ask how these objects are destroyed and how busy memory is freed  In other languages such as C    you need to free manually allocated objects dynamically by the delete operator  Java has a different approach  it automatically handles deallocation  The technique is known as Garbage Collection   It works like this  when there are no references to an object  it is assumed that this object is no longer needed and you can retrieve the memory occupied by the object  It is not necessary to explicitly destroy objects as in C    Garbage collection occurs sporadically during program execution  It does not simply happen because there are one or more objects that are no longer used  In addition  several Java runtime implementations have different approaches to garbage collection  but most programmers do not have to worry about this when writing programs

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To put it in the most simple terms that even a non-programmer can understand  when a program processes data it creates intermediate data and storage space  variables  arrays  certain object metadata etc   for that data     When these objects are accessed across functions or over a certain size  they are allocated from a central heap    Then when they are no long needed  they need to be cleaned up   There are some very good articles online about how this works  so I ll just cover the very basic definition   The GC is basically the function that does this cleanup   To do this is clears table entries that aren t referenced by any active objects  effectively deleting the objects  than copies and compacts the memory  It s a little more complicated than this  but you get the idea     The big problem is some parts this process often requires the entire Java VM to require to be stopped temporarily to take place  as well as this entire process being very processor and memory bandwidth intensive    The various options as of GCs and tuning options for each one are designed to balance these various issues with the whole GC process

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Garbage Collector is part of JRE that makes sure that object that are not referenced will be freed from memory  It usually runs when you app runs out of memory  AFAIK it holds a graph that represents the links between the objects and isolated objects can be freed  To save performance the current objects grouped into generations  each time GC scans an object and finds that it is still referenced its generation count incremented by 1  to some max maximum value  3 or 4 i think    and the new generation are scanned first  the shortest the object in memory the more probably it is no longer needed  so not all objects being scanned every time GC run  read this for more information

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The garbage collector is a program which runs on the Java Virtual Machine which gets rid of objects which are not being used by a Java application anymore  It is a form of automatic memory management   When a typical Java application is running  it is creating new objects  such as Strings and Files  but after a certain time  those objects are not used anymore  For example  take a look at the following code   for  File f   files        String s   f getName        In the above code  the String s is being created on each iteration of the for loop  This means that in every iteration  a little bit of memory is being allocated to make a String object   Going back to the code  we can see that once a single iteration is executed  in the next iteration  the String object that was created in the previous iteration is not being used anymore -- that object is now considered  garbage    Eventually  we ll start getting a lot of garbage  and memory will be used for objects which aren t being used anymore  If this keeps going on  eventually the Java Virtual Machine will run out of space to make new objects   That s where the garbage collector steps in   The garbage collector will look for objects which aren t being used anymore  and gets rid of them  freeing up the memory so other new objects can use that piece of memory   In Java  memory management is taken care of by the garbage collector  but in other languages such as C  one needs to perform memory management on their own using functions such as malloc and free  Memory management is one of those things which are easy to make mistakes  which can lead to what are called memory leaks -- places where memory is not reclaimed when they are not in use anymore   Automatic memory management schemes like garbage collection makes it so the programmer does not have to worry so much about memory management issues  so he or she can focus more on developing the applications they need to develop

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The garbage collector allows your computer to simulate a computer with infinite memory  The rest is just mechanism   It does this by detecting when chunks of memory are no longer accessible from your code  and returning those chunks to the free store   EDIT  Yes  the link is for C   but C  and Java are identical in this regard

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Garbage collector is a component of jvm   It is used to collect garbage when ever cpu gets free   Here garbage means unused objects it runs in Background of main program  to monitor the status of the main program

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Many people think garbage collection collects and discards dead objects   In reality  Java garbage collection is doing the opposite  Live objects are tracked and everything else designated garbage   When an object is no longer used  the garbage collector reclaims the underlying memory and reuses it for future object allocation  This means there is no explicit deletion and no memory is given back to the operating system  To determine which objects are no longer in use  the JVM intermittently runs what is very aptly called a mark-and-sweep algorithm   Check this for more detail information  http   javabook compuware com content memory how-garbage-collection-works aspx

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It frees memory allocated to objects that are not being used by the program any more - hence the name  garbage   For example   public static Object otherMethod Object obj        return new Object       public static void main String   args        Object myObj   new Object        myObj   otherMethod myObj              more code           I know this is extremely contrived  but here after you call otherMethod   the original Object created is made unreachable - and that s  garbage  that gets garbage collected    In Java the GC runs automatically  but you can also call it explicitly with System gc   and try to force a major garbage collection  As Pascal Thivent points out  you really shouldn t have to do this and it might do more harm than good  see this question    For more  see the wikipedia entry on Garbage collection and Tuning Garbage Collection  from Oracle

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The basic principles of garbage collection are to find data objects in a program that cannot be accessed in the future  and to reclaim the resources used by those objects  https   en wikipedia org wiki Garbage collection  28computer science 29  Advantages  1  Saves from bugs  which occur when a piece of memory is freed while there are still pointers to it  and one of those pointers is dereferenced  https   en wikipedia org wiki Dangling pointer  2  Double free bugs  which occur when the program tries to free a region of memory that has already been freed  and perhaps already been allocated again   3  Prevents  from certain kinds of memory leaks  in which a program fails to free memory occupied by objects that have become unreachable  which can lead to memory exhaustion   Disadvantages  1  Consuming additional resources  performance impacts  possible stalls in program execution  and incompatibility with manual resource management  Garbage collection consumes computing resources in deciding which memory to free  even though the programmer may have already known this information    2  The moment when the garbage is actually collected can be unpredictable  resulting in stalls  pauses to shift free memory  scattered throughout a session  Unpredictable stalls can be unacceptable in real-time environments  in transaction processing  or in interactive programs     Oracle tutorial http   www oracle com webfolder technetwork tutorials obe java gc01 index html     Garbage collection is the process identifying which objects are in use and which are not  and deleting the unused objects    In a programming languages like C  C    allocating and freeing memory is a manual process   int   array   new int size   processArray array     do some work  delete array    Free memory      The first step in the process is called marking  This is where the garbage collector identifies which pieces of memory are in use and which are not       Step 2a  Normal deletion removes unreferenced objects leaving referenced objects and pointers to free space    To improve performance  we want to delete unreferenced objects and also compact the remaining referenced objects  We want to keep referenced objects together  so it will be faster to  allocate new memory      As stated earlier  having to mark and compact all the objects in a JVM is inefficient  As more and more objects are allocated  the list of objects grows and grows leading to longer and longer garbage collection time    Continue reading this tutorial  and you will know how GC takes this challenge   In short  there are three regions of the heap  YoungGeneration for short life objects  OldGeneration for long period objects  and PermanentGeneration for objects that live during the application life  for example  classes  libraries

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Garbage Collection in Java  and other languages platforms as well  is a way for the java run-time environment  JRE  to reuse memory from java objects that are no longer needed   Simplistically  when the JRE initially starts up it asks the Operating System  O S  for a certain amount of memory  As the JRE runs your application s  it uses that memory  When your application is done using that memory  the JRE s  Garbage Collector  comes along and reclaims that memory for use by different parts of your existing application s    The JRE s  Garbage Collector  is a background task that is always running and tries to pick times when the system is idle to go on its garbage runs   A real world analogy would be the garbage men that come to your house and pick up your recyclable garbage    eventually  its reused in other ways by yourself and or other people

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Automatic garbage collection is the process of looking at heap memory  identifying which objects are in use and which are not  and deleting the unused objects  An in use object  or a referenced object  means that some part of your program still maintains a pointer to that object  An unused object  or unreferenced object  is no longer referenced by any part of your program  So the memory used by an unreferenced object can be reclaimed   In a programming language like C  allocating and deallocating memory is a manual process  In Java  process of deallocating memory is handled automatically by the garbage collector  Please check the link for a better understanding  http   www oracle com webfolder technetwork tutorials obe java gc01 index html

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Automatic garbage collection is a process where the JVM gets rid of or keeps certain data points in memory to ultimately free up space for the running program  Memory is first sent to heap memory  that is where the garbage collector  GC  does its work  then is decided to be terminated or kept  Java assumes that the programmer cannot always be trusted  so it terminates items it thinks it doesn t need

[java] What is the garbage collector in Java?

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