Everyone knows about the Java programming language, but fewer people know what makes it so popular. One of the reasons is what’s going to be described here today: the JVM.

The JVM primary role is to convert java code to machine code (passing through bytecode of course) , it provides a runTime environment where the bytecode gets interpreted and executed.

A few things to note:

  • The Java Virtual Machine is a specification: meaning it’s just a formal description of a required implementation. (HotSpot, GraalVM and Zulu are pretty popular implementations).
  • The Java Virtual Machine is Platform Independent: obviously because it acts as a layer between the operating system and the code.

The implementation of the Java Virtual Machine is called a Java Run Time environment (JRE) and when you execute your code (running the java command) a new Instance of the Java Virtual Machine is created.

Overview of the JVM

The figure above shows an the different parts (subsystems) of the JVM Specification, we will briefly describe few of them later on.

One must know that the JVM performs a lot of tasks to execute your Java application, but essentially it boils down to this 4 steps:

  1. Loading the Code .
  2. Verifying the Code .
  3. Executing the Code.
  4. Providing a Runtime Environment.

Description of JVM components

  • Class Loader: It’s the component responsible of loading classes into memory and make them known to the runtime.
  • Method/Class Area :It stores per-class structures such as the runtime constant pool, field and method data …, it is initialized per JVM startup.
  • The Heap: It’s a data area, where all object are stored, and it’s accessible by all the threads created by the JVM.
  • The Stack: A Java Virtual Machine stack is analogous to the stack of a conventional language such as C . it holds local variables and partial results, and plays a part in method invocation and return. When a new Thread is created and new Java Stack is associated with it’s creation , so every thread has his own stack.
  • Pc Registers: It contains the address of the Java virtual machine instruction currently being executed.
  • Native Method Stacks: It contains all the native methods used in the application.
  • The Execution Engine: The component responsible of running your code, either via the interpreter or via JIT.

That’s the overview of the components, see you in the next one.