w Garbage Collection works in Java

I have read many articles on
Garbage Collection in Java, some of them are too complex to understand and some of them don’t contain enough information required to understandgarbage collection in Java. Then I decided to write my own experience as an article or you call
tutorial about How
Garbage Collection works in Java or
what is Garbage collection in Java in simple word which would be easy to understand and have sufficient information to understand
how garbage collection works in Java.



This
article is in continuation of my previous articles
How Classpath works in Java and
How to write Equals method in java and before moving ahead let's recall few important points about garbage collection in java:

1) objects are created on heap in Java irrespective of there scope e.g. local or member variable. while its worth noting that class variables or static members are created in method
area of
Java memory space and both heap and method area is shared between different thread.
2) Garbage collection is a mechanism provided by Java Virtual Machine to
reclaim heap space from objects which are eligible for Garbage collection.
3) Garbage collection
relieves java programmer from memory management which is essential part of C++ programming and gives more time to focus on
business logic.
4) Garbage Collection in Java is carried by a daemon thread called
Garbage Collector.
5) Before removing an object from memory
Garbage collection thread invokes finalize () method of that object and gives an opportunity to perform any sort of cleanup required.
6) You as Java programmer
can not force Garbage collection in Java; it will only trigger if JVM thinks it needs a garbage collection
based on
Java heap size.
7) There are methods like
System.gc () and Runtime.gc () which is used to send request of Garbage collection to JVM but it’s
not guaranteed that garbage collection will happen.
8) If there is no memory space for creating new object in Heap
Java Virtual Machine throws OutOfMemoryError or
java.lang.OutOfMemoryError heap space
9) J2SE 5(Java 2 Standard Edition) adds a new feature called
Ergonomics goal of ergonomics is to provide good
performance from the JVM with minimum of command line
tuning.

When an Object becomes Eligible for Garbage Collection

An Object becomes
eligible for Garbage collection or GC if its not reachable from any live threads or 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 reference so if Object A has reference of object B and object B has reference of Object A and they don't have any other live reference then both Objects A and B will be
eligible for Garbage collection.
Generally an object becomes
eligible for garbage collection in Java on following cases:
1) All references of that object explicitly set to null e.g. object = null
2) Object is created inside a block and reference goes out scope once control exit that block.
3)
Parent object set to null, if an object holds reference of another object and when you set container object's reference null, child or contained object automatically becomes eligible for garbage collection.
4) If an object has only live references via
WeakHashMap it will be eligible for garbage collection. To
learn more about HashMap see here
How HashMap works in Java.

Heap Generations for Garbage Collection in Java

Java objects are created in Heap and
Heap is divided into three parts or generations for sake of garbage collection in Java, these are called as
Young generation, Tenured or Old Generation and Perm Area of 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 object survives its gets moved to survivor 1 and then Survivor 2 before
Major Garbage collection moved that object to Old or tenured generation.

Permanent generation of Heap or Perm Area of Heap is somewhat special and it is used to store Meta data related to classes
and method in JVM, it also hosts String pool provided by JVM as discussed in my string tutorial

why String is immutable in Java. There are many opinions around
whether garbage collection in Java happens in perm area of java heap or not, as per my knowledge this is something which is JVM dependent and happens at least in Sun's implementation of JVM. You can also
try this by just creating millions of String and watching
for Garbage collection or OutOfMemoryError.

Types of Garbage Collector in Java

Java Runtime (J2SE 5) provides various
types of Garbage collection in Java which you can choose based upon your application's performance requirement. Java 5 adds three additional
garbage collectors except serial garbage collector. Each is
generational garbage collector which has been implemented to increase throughput of the application or to reduce
garbage collection pause times.

1) Throughput Garbage Collector: This garbage collector in Java uses a parallel version of the
young generation collector. It is used if the -XX:+UseParallelGC option is passed to
the JVM via command line options . The tenured generation collector is same as the serial collector.

2) Concurrent low pause Collector: This Collector is used if the -Xingc or
-XX:+UseConcMarkSweepGC is passed on the command line. This is also referred as
Concurrent Mark Sweep Garbage collector. The concurrent collector is used to
collect the tenured generation and does most of the collection concurrently with the execution of the application. The application is paused for short periods during the collection. A
parallel version of the young generation copying collector is sued with the concurrent collector. Concurrent Mark Sweep Garbage collector is most widely used garbage collector in java and it uses algorithm to first mark object which needs to collected
when garbage collection triggers.

3) The Incremental (Sometimes called train) low pause collector: This collector is used only if -XX:+UseTrainGC is passed on the command line. This garbage collector
has not changed since the java 1.4.2 and is currently not under active development. It will not be supported in future releases so avoid using this and please see 1.4.2 GC Tuning document for information on this collector.
Important point to not is that
-XX:+UseParallelGC should not be used with -XX:+UseConcMarkSweepGC. The argument passing in the J2SE platform starting with version 1.4.2 should only allow legal
combination of command line options for garbage collector but earlier releases may not find or detect all illegal combination and the results for illegal combination are unpredictable.
It’s not recommended to use this garbage collector in java.

JVM Parameters for garbage collection in Java

Garbage collection tuning
is a long
exercise and requires lot of profiling of application and patience to get it right.
While working with High volume low latency Electronic
trading system I have worked with some of the project where we need to increase the performance of Java application by profiling and finding what causing full GC and I found that Garbage collection tuning
largely depends on application profile, what kind of object application has and what are there average lifetime etc. for example if an application has too many short lived object then making Eden space wide enough or larger will reduces number of minor collections.
you can also control size of both young and Tenured generation using JVM parameters
for example setting -XX:NewRatio=3 means that the ratio among the young and tenured generation is 1:3 , you got to be careful on sizing these generation. As
making young generation larger will reduce size of tenured generation which will force Major collection to occur more frequently which pauses application thread during that
duration results in degraded or reduced throughput. The parameters
NewSize and MaxNewSize are used to specify the young generation size from below and above. Setting these equal to one another fixes the
young generation. In my opinion before doing garbage collection tuning detailed understanding of garbage collection in java is must and I would recommend reading Garbage collection document provided by Sun Microsystems for detail knowledge of
garbage collection in Java. Also to get a full list of JVM parameters for a particular Java Virtual machine please refer official documents on garbage collection in Java. I found this link quite helpful though http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html

Full GC and Concurrent Garbage Collection in Java

Concurrent garbage collector in java uses a single
garbage collector thread that runs concurrently with the application threads with the goal of completing the collection of the tenured generation before it becomes full. In normal operation, the concurrent
garbage collector is able to do most of its work with the application threads still running, so only brief pauses are seen by the application threads. As a fall back, if the
concurrent garbage collector is unable to finish before the tenured generation fill up, the application is paused and the collection is completed with all the application threads stopped. Such Collections with the application stopped are referred
as full garbage collections or full GC and are a sign that some adjustments need to be made to the concurrent collection parameters. Always try to avoid or minimize
full garbage collection or Full GC because it affects
performance of Java application. When you work in finance domain for electronic trading platform and with high volume low latency systems performance of java application becomes extremely critical an you definitely like to avoid full GC during
trading period.

Summary on Garbage collection in Java

1) Java Heap is divided into
three generation for sake of garbage collection. These are young generation,
tenured or old generation and Perm area.
2) New objects are created into
young generation and subsequently moved to old generation.
3) String pool is created in
Perm area of Heap, garbage collection can occur in perm
space but depends upon JVM to JVM.
4) Minor garbage collection is used to
move object from Eden space to Survivor 1 and Survivor 2 space and
Major collection is used to move object from young to tenured generation.
5) Whenever
Major garbage collection occurs application threads stops during that period which will reduce application’s
performance and throughput.
6) There are few performance improvement has been applied in
garbage collection in java 6 and we usually use JRE 1.6.20 for running our application.
7) JVM command line options –Xmx and -Xms is used to

setup starting and max size for Java Heap. Ideal ratio of this parameter is either
1:1 or 1:1.5 based upon my experience for example you can have either both
–Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.
8) There is no manual way of doing garbage collection in Java.

Read more: http://javarevisited.blogspot.com/2011/04/garbage-collection-in-java.html#ixzz2khTj09Xc