Garbage collection works by employing several GC algorithm e.g. Mark and Sweep. There are different kinds of garbage collector available in Java to collect different area of heap memory e.g. you have serial, parallel and concurrent garbage collector in Java. A new collector called G1 (Garbage first) are also introduced in JDK 1.7. First step to learn about GC is to understand when an object becomes eligible to garbage collection.

Since JVM provides memory management, Java developers only care about creating object, they don't care about cleaning up, that is done by garbage collector, but it can only collect objects which has no live strong reference or it's not reachable from any thread. If an object, which is suppose to be collected but still live in memory due to unintentional strong reference then it's known as memory leak in Java. ThreadLocal variables in Java web application can easily cause memory leak.

Coming to Garbage Collection lets know few Important points about Garbage Collector

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.

Now the Question is when does Object becomes eligible for Garbage Collection

An object becomes eligible for Garbage collection or GC if its 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 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.

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 runtime via JVM 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.