What is JVM?
JVM(Java Virtual Machine) is the program that execute your Java applications.
As part of JRE(Java Run Environment), JVM is the one that actually calls the main() method in your Java code. It is also the JVM that loads, links and initialize the .class files which are compiled from .java by the compiler(javac).
JVM Memory Components
There are five components in JVM memory: Stack Area, PC Registers, Native Method Area, Method Area and Heap.
Heap and Method Area are shared, while Stack Area, PC Registers and Native Method Area unshared.
The following diagram shows whether each of them is shared or unshared.
Stack Area
Every time a new thread is created, JVM creates a separate Stack Area for it. The Stack Area consists of Local Variable Array, Operand Stack and Frame Data.
- Local Variable Array: Storing local variables of the method.
- Operand Stack: Storing the intermediate calculation results.
- Frame Data: Containing constant pool and Exception table.
PC Registers
Each thread has separate PC Registers to record the current executing instruction.
Native Method Area
Each thread has separate Native Method Area to store native method information.
Method Area
All the class level data including static variables are stored here.
Heap
All the objects are stored here, which means you will use the Heap every time you use the key word new. Besides, Garbage Collection mainly happens in the Heap.
The End