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feliciachou edited this page Jun 16, 2022 · 5 revisions

Virtual CPUO

CPU 0

JVM

A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required in a JVM implementation. Having a specification interoperability of Java programs across different implementations so that program authors ensures using the Java Development Kit (JDK) need not worry about idiosyncrasies of the underlying hardware platform.

The JVM reference implementation is developed by the OpenJDK project as open source code and includes a JIT compiler called HotSpot. The commercially supported Java releases available from Oracle Corporation are based on the OpenJDK runtime. Eclipse OpenJ9 is another open source JVM for OpenJDK.

  • 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.
  • An implementation Its implementation is known as JRE (Java Runtime Environment).
  • Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.

References : https://en.wikipedia.org/wiki/Java_virtual_machine https://www.javatpoint.com/jvm-java-virtual-machine

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 classloader, memory area, execution engine etc.

  1. Classloader Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.
  • Bootstrap ClassLoader: This is the first classloader which is the super class of Extension classloader. 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.
  • Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loades the jar files located inside $JAVA_HOME/jre/lib/ext directory.
  • System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the classfiles from classpath. By default, classpath is set to current directory. You can change the classpath using "-cp" or "-classpath" switch. It is also known as Application classloader.
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());  
    }  
}

  1. Class(Method) Area Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.

  2. Heap It is the runtime data area in which objects are allocated.

  3. 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.

  4. Program Counter Register PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.

  5. Native Method Stack It contains all the native methods used in the application.

  6. Execution Engine It contains:

  • A virtual processor
  • Interpreter: Read bytecode stream then execute the instructions.
  • 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.
  1. 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.

References : https://www.javatpoint.com/jvm-java-virtual-machine https://en.wikipedia.org/wiki/Java_virtual_machine

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