Java Runtime Data Areas

Runtime Data Area is the memory used by the Java Virtual Machine during the execution of the program.

https://sumathikotthuri.wordpress.com/2019/04/16/java-runtime-data-areas/

There are 6 java run time data areas :

• PC (Program Counter) Register
• Java Virtual Machine Stack
• Heap
• Method Area
• Run-time Constant Pool
• Native Method Stacks

PC Register

• Each JVM thread has its PC register.
• PC Register is created when a thread is created and destroyed when a thread exits.
• PC Register returns the address of the current instruction which is being executed by JVM.

Java Virtual Machine Stack

• Each JVM thread has a private Java Virtual Machine Stack
• JVM Stack is created when a thread a created and destroyed when a thread exits.
• JVM Stack stores frames.
• JVM Stack contains the local variables and partial results.
• Data passed as parameters to methods and data returned by methods are also stored in JVM Stack.
• JVM Stack memory doesn’t have to be contiguous.
• Initial Size and in case of dynamic expansion, minimum and maximum sizes can be controlled by a programmer

Heap

• All JVM threads share the Heap.
• Heap is created on JVM start-up and destroyed when JVM exits.
• Memory required for class instances and arrays is allocated in the heap.
• Initial Size and in case of dynamic expansion, minimum and maximum sizes can be controlled by a programmer
• Heap memory doesn’t have to be contiguous.

Method Area

• All JVM threads share the Method Area.
• Method Area is logically part of the heap.
• Method Area is created on JVM start-up and destroyed when JVM exits.
• Run-time Constant pool is a logical part of Method Area.
• Method Area is storage for Compiled Code i.e. it contains per-class structure (Run-time Constant Pool), field and method data.
• Method Area memory need not be contiguous.
• Initial Size and in case of dynamic expansion, minimum and maximum sizes can be controlled by a programmer

Runtime Constant Pool

• Runtime Constant pool is a logical part of Method Area.
• Runtime Constant pool is a per-class or per-interface runtime representation
• Runtime Constant pool for a Class or interface is created when a Class or interface is created by JVM

Native Method Stacks

• JVM may use conventional stacks to support native methods
• JVM need not supply Native method stacks to load native methods. If supplied, Native methods stacks are created per thread.
• Initial Size and in case of dynamic expansion, minimum and maximum sizes can be controlled by a programmer

Exceptions that will be thrown by Runtime Data Areas

• StackOverflowError
  • If the computation requires a larger stack than permitted, then JVM throws this Exception. Both stacks below throw this exception when the above-mentioned condition arises.
    • JVM Stack
    • Native Method Stack
• OutOfMemoryError
  • To create the JVM Stack , if the JVM attempts to dynamically expand the Stack Area and available memory is insufficient, then JVM throws this Exception.
  • If any computation requires more Heap than available, then JVM throws this Exception.
  • When an allocation request of Method Area doesn’t find sufficient memory, then JVM throws this exception
  • When creating a Class or Interface, if the Runtime constant pool requires more memory, then JVM throws this Exception