What is the purpose of JIT compiler?

JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed. JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).

What is JVM: It is

  1. A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Oracle and other companies.
  2. An implementation Its implementation is known as JRE (Java Runtime Environment).
  3. Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.

What it does: The JVM performs following operation:

  • Loads code
  • Verifies code
  • Executes code
  • Provides runtime environment

JVM provides definitions for the:

  • Memory area
  • Class file format
  • Register set
  • Garbage-collected heap
  • Fatal error reporting etc.

JVM Architecture: Let’s understand the internal architecture of JVM. It contains class loader, memory area, execution engine etc.

1) Classloader: Class loader is a subsystem of JVM which is used to load class files.
Whenever we run the java program, it is loaded first by the class loader.
There are three built-in class loaders in Java.
  1. Bootstrap ClassLoader: This is the first class loader which is the super class of Extension class loader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes etc.
  2. Extension ClassLoader: This is the child class loader of Bootstrap and parent class loader of System class loader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory.
  3. System/Application ClassLoader: This is the child class loader of Extension class loader. It loads the class files from class path. By default, class path is set to current directory. You can change the class path using “-cp” or “-classpath” switch. It is also known as Application class loader.
    //Let's see an example to print the class loader name  
    public class ClassLoaderExample  
    {  
        public static void main(String[] args)  
        {  
            // Let's print the classloader name of current class.   
            //Application/System classloader will load this class  
            Class c=ClassLoaderExample.class;  
            System.out.println(c.getClassLoader());  
            //If we print the classloader name of String, it will print null because it is an  
            //in-built class which is found in rt.jar, so it is loaded by Bootstrap classloader  
            System.out.println(String.class.getClassLoader());  
        }  
    }     

Output:

[email protected]
null

These are the internal class loaders provided by Java. If you want to create your own class loader, you need to extend the Class Loader class.

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 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 byte code 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.

8) Java Native Interface: Java Native Interface (JNI) is a framework which provides an interface to communicate with another application written in another language like C, C++, Assembly etc. Java uses JNI framework to send output to the Console or interact with OS libraries.

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