Java Memory Management

Memory Management - PART - 1

Memory Management in java is responsibility of garbage collector. Garbage Collection is not the only form of Memory Management in Java. Real-time Specification for Java (RTSJ) is also being used for Memory Management. These efforts were mainly dedicated to real-time and embedded programming in Java for which GC was not suitable - due to performance overhead.

Understanding JVM Memory Model is very important if we want to know the working process of Java Garbage Collection.

Types of Garbage Collector : -

• Do nothing : - It might just decide never to run and never to do anything, no memory gets free but it do stills gurantee to not collecting live objects.

• Reference Counting garbage collector : - COM programming environment is the best example of Reference Counting Garbage collector. COM application may call 2 functions. First is Add Ref and second is Release. Add Ref increments the count of the object and Release decrease the count. When count goes to zero then ref is no longer been used or we can say that A reference count is maintained for each object on the heap. When an object is first created and a reference to it is assigned to a variable, the object's reference count is set to one. When any other variable is assigned a reference to that object, the object's count is incremented. When a reference to an object goes out of scope or is assigned a new value, the object's count is decremented. Any object with a reference count of zero can be garbage collected. When an object is garbage collected, any objects that it refers to have their reference counts decremented. In this way the garbage collection of one object may lead to the subsequent garbage collection of other objects.

• Mark and Sweep : - To determine which objects are no longer in use, the JVM intermittently runs mark-and-sweep algorithm. Garbage collector runs in 2 phases, In mark phase it is marking that memory is still alive or this algorithm traverses all object references, starting with the GC roots, and marks every object found as alive and in sweep phase all of the heap memory that is not occupied by marked objects is reclaimed. It is simply marked as free, essentially swept free of unused objects.

• Copying - Copying garbage collectors move all live objects to a new area and the old area is known to be all free space. This is not following any separate Mark and Sweep phases Objects are copied(these objects are discovered by the traversal from the root nodes) to the new area on the fly and forwarding ponters are left in their old locations and these pointers allows the garbage collector to detect references to objects that have already been moved. The garbage collector can then assign the value of the forwarding pointer to the references so they point to the object's new location.

• Generational - Copying collectors spend much of their time for copying the same long-lived objects again and again. In order to address this inefficiency Generational collectors work with grouping objects by age and garbage collecting younger objects more often than older objects. This approach works by dividing the heap into two or more sub-heaps, each of which serves one "generation" of objects. The youngest generation is garbage collected most often. As most objects are short-lived, only a small percentage of young objects are likely to survive their first collection. Once an object has survived a few garbage collections as a member of the youngest generation, the object is promoted to the next generation: it is moved to another sub-heap.

• Incremental - Rather than attempting to find and discard all unreachable objects at each invocation an incremental garbage collector just attempts to find and discard a portion of the unreachable objects. Because only a portion of the heap is garbage collected at each invocation, each invocation should in theory run in less time. A garbage collector that can perform incremental collections, each of which is guaranteed to require less than a certain maximum amount of time, can help make a Java virtual machine suitable for real-time environments.  

Reference taken from other sources