Java Virtual Machine (JVM) & its Architecture
What is JVM?
Java Virtual Machine (JVM) is a engine that provides runtime environment to drive the Java Code or applications. It converts Java bytecode into machines language. JVM is a part of Java Run Environment (JRE). In other programming languages, the compiler produces machine code for a particular system. However, Java compiler produces code for a Virtual Machine known as Java Virtual Machine.
Here is how JVM works
First, Java code is complied into bytecode. This bytecode gets interpreted on different machines
Between host system and Java source, Bytecode is an intermediary language.
JVM is responsible for allocating memory space.
In this tutorial, you will learn-
The class loader is a subsystem used for loading class files. It performs three major functions viz. Loading, Linking, and Initialization.
2) Method Area
JVM Method Area stores class structures like metadata, the constant runtime pool, and the code for methods.
All the Objects, their related instance variables, and arrays are stored in the heap. This memory is common and shared across multiple threads.
4) JVM language Stacks
Java language Stacks store local variables, and it’s partial results. Each thread has its own JVM stack, created simultaneously as the thread is created. A new frame is created whenever a method is invoked, and it is deleted when method invocation process is complete.
5) PC Registers
PC register store the address of the Java virtual machine instruction which is currently executing. In Java, each thread has its separate PC register.
6) Native Method Stacks
Native method stacks hold the instruction of native code depends on the native library. It is written in another language instead of Java.
7) Execution Engine
It is a type of software used to test hardware, software, or complete systems. The test execution engine never carries any information about the tested product.
8) Native Method interface
The Native Method Interface is a programming framework. It allows Java code which is running in a JVM to call by libraries and native applications.
9) Native Method Libraries
Native Libraries is a collection of the Native Libraries(C, C++) which are needed by the Execution Engine.
In order to write and execute a software program, you need the following
1) Editor – To type your program into, a notepad could be used for this
2) Compiler – To convert your high language program into native machine code
3) Linker – To combine different program files reference in your main program together.
4) Loader – To load the files from your secondary storage device like Hard Disk, Flash Drive, CD into RAM for execution. The loading is automatically done when you execute your code.
5) Execution – Actual execution of the code which is handled by your OS & processor.
With this background, refer the following video & learn the working and architecture of the Java Virtual Machine.
C code Compilation and Execution process
To understand the Java compiling process in Java. Let’s first take a quick look to compiling and linking process in C.
Suppose in the main, you have called two function f1 and f2. The main function is stored in file a1.c.
The compiler will compile the three files and produces 3 corresponding .class file which consists of BYTE code. Unlike C, no linking is done.
The Java VM or Java Virtual Machine resides on the RAM. During execution, using the class loader the class files are brought on the RAM. The BYTE code is verified for any security breaches.
Next, the execution engine will convert the Bytecode into Native machine code. This is just in time compiling. It is one of the main reason why Java is comparatively slow.