Garbage Collection in Java How Java Garbage Collection Works?

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         Garbage Collection 

 In Java, the programmers don’t need to take care of destroying the objects that are out of use. The Garbage Collector takes care of it.
 
Garbage Collector is a Daemon thread that keeps running in the background. Basically, it frees up the heap memory by destroying the unreachable objects.
         
Unreachable objects are the ones that are no longer referenced by any part of the program.
        
 We can choose the garbage collector for our java program through JVM options,



     
Garbage Collector is a program that manages memory automatically wherein de-allocation of objects is handled by Java rather than the programmer. In the Java programming language, dynamic allocation of objects is achieved using the new operator. An object once created uses some memory and the memory remains allocated till there are references for the use of the object.
          
When there are no references to an object, it is assumed to be no longer  needed, and the memory, occupied by the object can be reclaimed. There is no explicit need to destroy an object as Java handles the de-allocation automatically.
         
The technique that accomplishes this is known as Garbage Collection. Programs that do not de-allocate memory can eventually crash when there is no memory left in the system to allocate. These programs are said to have memory leaks.Garbage collection in Java happens automatically during the lifetime of the program, eliminating the need to de-allocate memory and thereby avoiding memory leaks. In C language, it is the programmer’s responsibility to de-allocate memory allocated dynamically using free() function. This is where Java memory management leads.

  • Note: All objects are created in Heap Section of memory. More on this in a later tutorial.
  • Java garbage collection is the process by which Java programs perform  automatic memory management. Java programs compile to bytecode that can be run on a Java Virtual Machine, or JVM for short. When Java programs run on the JVM, objects are created on the heap, which is a portion of memory dedicated to the program. Eventually, some objects will no longer be needed. The garbage collector finds these unused objects and deletes them to free up memory.

   How Java Garbage Collection Works


Java garbage collection is an automatic process. The programmer does not need to explicitly mark objects to be deleted. The garbage collection implementation lives in the JVM. Each JVM can implement garbage collection however it pleases; the only requirement is that it meets the JVM specification. Although there are many JVMs, Oracle’s HotSpot is by far the most common. It offers a robust and mature set of garbage collection options.

While HotSpot has multiple garbage collectors that are optimized for various use cases, all its garbage collectors follow the same basic process. In the first step, unreferenced objects are identified and marked as ready for garbage collection. In the second step, marked objects are deleted. Optionally, memory can be compacted after the garbage collector deletes objects, so remaining objects are in a contiguous block at the start of the heap. The compaction process makes it easier to allocate memory to new objects sequentially after the block of memory allocated to existing objects.

All of HotSpot’s garbage collectors implement a generational garbage collection strategy that categorizes objects by age. The rationale behind generational garbage collection is that most objects are short-lived and will be ready for garbage collection soon after creation.

  Benefits of Java Garbage Collection


The biggest benefit of Java garbage collection is that it automatically handles the deletion of unused objects or objects that are out of reach to free up vital memory resources. Programmers working in languages without garbage collection (like C and C++) must implement manual memory management in their code.

Despite the extra work required, some programmers argue in favor of manual memory management over garbage collection, primarily for reasons of control and performance. While the debate over memory management approaches continues to rage on, garbage collection is now a standard component of many popular programming languages. For scenarios in which the garbage collector is negatively impacting performance, Java offers many options for tuning the garbage collector to improve its efficiency.


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