Interview Question : How garbage collector works?

How garbage collector works?

Before going to understand the working of garbage collection functionality of Java, I hope you already gone through the previous post i.e. What is garbage collection? if not please go through it. 

Let's start how garbage collection works, in general 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. As you’ll see, this fundamental misunderstanding can lead to many performance problems. Before moving ahead let's recall few important points about garbage collection in Java.

• Objects are created on the heap in Java irrespective of their scope e.g. local or member variable. while it's 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.
• Garbage collection is a mechanism provided by Java Virtual Machine to reclaim heap space from objects which are eligible for Garbage collection.
• Garbage Collection in Java is carried by a daemon thread called Garbage Collector.
• 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 if programmer wants to do something by its own.
• We can't force garbage collection in Java, We can trigger JVM for garbage collection by using 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 immediately.

Heap Generations for Garbage Collection in Java

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 generation, Tenured or 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. So what are these spaces, let's go one by one.

Eden Space: When an instance is created, it is first stored in the eden space in young generation of heap memory area. 

Survivor Space (S0 and S1): As part of the minor garbage collection cycle, objects that are live (which is still referenced) are moved to survivor space S0 from eden space. Similarly the garbage collector scans S0 and moves the live instances to S1.

Instances that are not live (dereferenced) are marked for garbage collection. Depending on the garbage collector (there are four types of garbage collectors available and we will see about them in the next tutorial) chosen either the marked instances will be removed from memory on the go or the eviction process will be done in a separate process.

Old Generation: Old or tenured generation is the second logical part of the heap memory. When the garbage collector does the minor GC cycle, instances that are still live in the S1 survivor space will be promoted to the old generation. Objects that are dereferenced in the S1 space is marked for eviction.

Major GC: Old generation is the last phase in the instance life cycle with respect to the Java garbage collection process. Major GC is the garbage collection process that scans the old generation part of the heap memory. If instances are dereferenced, then they are marked for eviction and if not they just continue to stay in the old generation. 

Memory Fragmentation: Once the instances are deleted from the heap memory the location becomes empty and becomes available for future allocation of live instances. These empty spaces will be fragmented across the memory area. For quicker allocation of the instance it should be defragmented. Based on the choice of the garbage collector, the reclaimed memory area will either be compacted on the go or will be done in a separate pass of the GC.

REFERENCES :

http://www.jubiliation.net/2018/01/interview-question-what-is-garbage.html