JVM or Java Virtual Machine

Java Virtual Machine is a specification to provide the runtime environment on which a bytecode can be executed. It provide class loader to load a class, runtime and executed the bytecode. The following is the architecture diagram of JVM.

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• Classloader − Loads the class file into the JVM.

• Class Area − Storage areas for a class elements structure like fields, method data, code of method etc.

• Heap − Runtime storage allocation for objects.

• Stack − Storage for local variables and partial results. A stack contains frames and allocates one for each thread. Once a thread gets completed, this frame also gets destroyed. It also plays roles in method invocation and returns.

• PC Register − Program Counter Register contains the address of an instruction that JVM is currently executing.

• Native method stack − It contains all the native methods used by the application.

• Execution Engine − It has a virtual processor, interpreter to interpret bytecode instructions one by one and a JIT, just in time compiler.

Parameter	Stack Memory	Heap Space
Application	Stack is used in parts, one at a time during execution of a thread	The entire application uses Heap space during runtime
Size	Stack has size limits depending upon OS, and is usually smaller than Heap	There is no size limit on Heap
Storage	Stores only primitive variables and references to objects that are created in Heap Space	All the newly created objects are stored here
Order	It's accessed using Last-in First-out (LIFO) memory allocation system	This memory is accessed via complex memory management techniques that include Young Generation, Old or Tenured Generation, and Permanent Generation.
Life	Stack memory only exists as long as the current method is running	Heap space exists as long as the application runs
Efficiency	Much faster to allocate when compared to heap	Slower to allocate when compared to stack
Allocation	This Memory is automatically allocated and deallocated when a method is called and returned, respectively	Heap space is allocated when new objects are created and deallocated by Garbage Collector when they're no longer referenced