Virtual machine : In general terms Virtual machine is a software that creates an environment between the computer and end user in which end user can operate programs.
Java Virtual machine: it is also Virtual machine that runs java bytecode by creating run time areas to execute class members.
when we compile a java program, the compiler will generate .class (dot class) file.
This .class file contains byte code (Special java instructions).
To execute a java program we have to provide .class file as the input to JVM.
The JVM performs following operation:
Loads code
Verifies code
Executes code
Provides runtime environment
Internal Architecture of JVM
It contains classloader, memory area, execution engine etc.
1) Classloader:
Classloader is a subsystem of JVM that is used to load class files.
2) Class(Method) Area:
Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.
3) Heap:
It is the runtime data area in which objects are allocated.
4) Stack:
Java Stack stores frames.It holds local variables and partial results, and plays a part in method invocation and return.
Each thread has a private JVM stack, created at the same time as thread.
A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
5) Program Counter Register:
PC (program counter) register. It contains the address of the Java virtual machine instruction currently being executed.
6) Native Method Stack:
It contains all the native methods used in the application.
7) Execution Engine:
It contains:
1) A virtual processor
2) Interpreter:Read bytecode stream then execute the instructions.
3) Just-In-Time(JIT) compiler:It is used to improve the performance.JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term ?compiler? refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.