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How JVM Works – JVM Architecture

Last Updated : 04 Jan, 2025

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JVM(Java Virtual Machine) runs Java applications as a run-time engine. JVM is the one that calls the main method present in a Java code. JVM is a part of JRE(Java Runtime Environment).

Java applications are called WORA (Write Once Run Anywhere). This means a programmer can develop Java code on one system and expect it to run on any other Java-enabled system without any adjustment. This is all possible because of JVM.

When we compile a .java file, .class files(contains byte-code) with the same class names present in .java file are generated by the Java compiler. This .class file goes into various steps when we run it. These steps together describe the whole JVM.

Class Loader Sub System

1. Class Loader Subsystem

It is mainly responsible for three activities.

Loading
Linking
Initialization

Loading:

The Class loader reads the “.class” file, generate the corresponding binary data and save it in the method area. For each “.class” file, JVM stores the following information in the method area.

The fully qualified name of the loaded class and its immediate parent class.
Whether the “.class” file is related to Class or Interface or Enum.
Modifier, Variables and Method information etc.

After loading the “.class” file, JVM creates an object of type Class to represent this file in the heap memory. Please note that this object is of type Class predefined in java.lang package. These Class object can be used by the programmer for getting class level information like the name of the class, parent name, methods and variable information etc. To get this object reference we can use getClass() method of Object class.

Java

// A Java program to demonstrate working // of a Class type object created by JVM // to represent .class file in memory. import java.lang.reflect.Field; import java.lang.reflect.Method;  // Java code to demonstrate use // of Class object created by JVM public class Test {     public static void main(String[] args)     {         Student s1 = new Student();          // Getting hold of Class         // object created by JVM.         Class c1 = s1.getClass();          // Printing type of object using c1.         System.out.println(c1.getName());          // getting all methods in an array         Method m[] = c1.getDeclaredMethods();         for (Method method : m)             System.out.println(method.getName());          // getting all fields in an array         Field f[] = c1.getDeclaredFields();         for (Field field : f)             System.out.println(field.getName());     } }  // A sample class whose information // is fetched above using its Class object. class Student {     private String name;     private int roll_No;      public String getName() { return name; }     public void setName(String name) { this.name = name; }     public int getRoll_no() { return roll_No; }     public void setRoll_no(int roll_no)     {         this.roll_No = roll_no;     } }

Output

Student getName setName getRoll_no setRoll_no name roll_No

Note: For every loaded “.class” file, only one object of the class is created.

Student s2 = new Student();
// c2 will point to same object where 
// c1 is pointing
Class c2 = s2.getClass();
System.out.println(c1==c2); // true

Linking

Performs verification, preparation, and (optionally) resolution.

Verification: It ensures the correctness of the .class file i.e. it checks whether this file is properly formatted and generated by a valid compiler or not. If verification fails, we get run-time exception java.lang.VerifyError. This activity is done by the component ByteCodeVerifier. Once this activity is completed then the class file is ready for compilation.
Preparation: JVM allocates memory for class static variables and initializing the memory to default values.
Resolution: It is the process of replacing symbolic references from the type with direct references. It is done by searching into the method area to locate the referenced entity.

Initialization

In this phase, all static variables are assigned with their values defined in the code and static block(if any). This is executed from top to bottom in a class and from parent to child in the class hierarchy. In general, there are three class loaders:

Bootstrap class loader: Every JVM implementation must have a bootstrap class loader, capable of loading trusted classes. It loads core java API classes present in the “JAVA_HOME/lib” directory. This path is popularly known as the bootstrap path. It is implemented in native languages like C, C++.
Extension class loader: It is a child of the bootstrap class loader. It loads the classes present in the extensions directories “JAVA_HOME/jre/lib/ext”(Extension path) or any other directory specified by the java.ext.dirs system property. It is implemented in java by the sun.misc.Launcher$ExtClassLoader class.
System/Application class loader: It is a child of the extension class loader. It is responsible to load classes from the application classpath. It internally uses Environment Variable which mapped to java.class.path. It is also implemented in Java by the sun.misc.Launcher$AppClassLoader class.

Java

// Java code to demonstrate Class Loader subsystem public class Test {     public static void main(String[] args)     {         // String class is loaded by bootstrap loader, and         // bootstrap loader is not Java object, hence null         System.out.println(String.class.getClassLoader());          // Test class is loaded by Application loader         System.out.println(Test.class.getClassLoader());     } }

Output

null jdk.internal.loader.ClassLoaders$AppClassLoader@8bcc55f

Note: JVM follows the Delegation-Hierarchy principle to load classes. System class loader delegate load request to extension class loader and extension class loader delegate request to the bootstrap class loader. If a class found in the boot-strap path, the class is loaded otherwise request again transfers to the extension class loader and then to the system class loader. At last, if the system class loader fails to load class, then we get run-time exception java.lang.ClassNotFoundException.

2. Class Loaders

There are three primary types of class loaders:

Bootstrap Class Loader: Loads core Java API classes from the JAVA_HOME/lib directory. It is implemented in native code and is not a Java object.
Extension Class Loader: Loads classes from the JAVA_HOME/jre/lib/ext directory or any directory specified by the java.ext.dirs system property. It is implemented in Java.
System/Application Class Loader: Loads classes from the application classpath, which is specified by the java.class.path environment variable. It is also implemented in Java.

Class Loaders

Example:

public class Test {
    public static void main(String[] args) {
        System.out.println(String.class.getClassLoader());
        System.out.println(Test.class.getClassLoader());
    }
}

3. JVM Memory Areas

Method area: In the method area, all class level information like class name, immediate parent class name, methods and variables information etc. are stored, including static variables. There is only one method area per JVM, and it is a shared resource.
Heap area: Information of all objects is stored in the heap area. There is also one Heap Area per JVM. It is also a shared resource.
Stack area: For every thread, JVM creates one run-time stack which is stored here. Every block of this stack is called activation record/stack frame which stores methods calls. All local variables of that method are stored in their corresponding frame. After a thread terminates, its run-time stack will be destroyed by JVM. It is not a shared resource.
PC Registers: Store address of current execution instruction of a thread. Obviously, each thread has separate PC Registers.
Native method stacks: For every thread, a separate native stack is created. It stores native method information.

JVM Memory Area

4. Execution Engine

Execution engine executes the “.class” (bytecode). It reads the byte-code line by line, uses data and information present in various memory area and executes instructions. It can be classified into three parts:

Interpreter: It interprets the bytecode line by line and then executes. The disadvantage here is that when one method is called multiple times, every time interpretation is required.
Just-In-Time Compiler(JIT) : It is used to increase the efficiency of an interpreter. It compiles the entire bytecode and changes it to native code so whenever the interpreter sees repeated method calls, JIT provides direct native code for that part so re-interpretation is not required, thus efficiency is improved.
Garbage Collector: It destroys un-referenced objects. For more on Garbage Collector, refer Garbage Collector.

5. Java Native Interface (JNI)

It is an interface that interacts with the Native Method Libraries and provides the native libraries(C, C++) required for the execution. It enables JVM to call C/C++ libraries and to be called by C/C++ libraries which may be specific to hardware.

6. Native Method Libraries

These are collections of native libraries required for executing native methods. They include libraries written in languages like C and C++.

For more information refer to this YouTube video link: How Java Works?

JDK in Java

G

Gaurav Miglani

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The Queue Interface is a part of java.util package and extends the Collection interface. It stores and processes the data in order means elements are inserted at the end and removed from the front. Key Features: Most implementations, like PriorityQueue, do not allow null elements.Implementation Clas

12 min read
PriorityQueue in Java

The PriorityQueue class in Java is part of the java.util package. It implements a priority heap-based queue that processes elements based on their priority rather than the FIFO (First-In-First-Out) concept of a Queue. Key Points: The PriorityQueue is based on the Priority Heap. The elements of the p

9 min read
Deque Interface in Java

Deque Interface present in java.util package is a subtype of the queue interface. The Deque is related to the double-ended queue that supports adding or removing elements from either end of the data structure. It can either be used as a queue(first-in-first-out/FIFO) or as a stack(last-in-first-out/

10 min read
Map Interface in Java

In Java, the Map Interface is part of the java.util package and represents a mapping between a key and a value. The Java Map interface is not a subtype of the Collections interface. So, it behaves differently from the rest of the collection types. Key Features: No Duplicates in Keys: Keys should be

12 min read
HashMap in Java

In Java, HashMap is part of the Java Collections Framework and is found in the java.util package. It provides the basic implementation of the Map interface in Java. HashMap stores data in (key, value) pairs. Each key is associated with a value, and you can access the value by using the corresponding

15+ min read
Java LinkedHashMap

LinkedHashMap in Java implements the Map interface of the Collections Framework. It stores key-value pairs while maintaining the insertion order of the entries. It maintains the order in which elements are added. Stores unique key-value pairs.Maintains insertion order.Allows one null key and multipl

7 min read
Hashtable in Java

Hashtable class, introduced as part of the Java Collections framework, implements a hash table that maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method an

13 min read
Java Dictionary Class

Dictionary class in Java is an abstract class that represents a collection of key-value pairs, where keys are unique and used to access the values. It was part of the Java Collections Framework and it was introduced in Java 1.0 but has been largely replaced by the Map interface since Java 1.2. Store

5 min read
SortedSet Interface in Java with Examples

The SortedSet interface is present in java.util package extends the Set interface present in the collection framework. It is an interface that implements the mathematical set. This interface contains the methods inherited from the Set interface and adds a feature that stores all the elements in this

9 min read
Java Comparator Interface

The Comparator interface in Java is used to sort the objects of user-defined classes. The Comparator interface is present in java.util package. This interface allows us to define custom comparison logic outside of the class for which instances we want to sort. The comparator interface is useful when

6 min read
Java Comparable Interface

The Comparable interface in Java is used to define the natural ordering of objects for a user-defined class. It is part of the java.lang package and it provides a compareTo() method to compare instances of the class. A class has to implement a Comparable interface to define its natural ordering. Exa

4 min read
Java Comparable vs Comparator

In Java, both Comparable and Comparator interfaces are used for sorting objects. The main difference between Comparable and Comparator is: Comparable: It is used to define the natural ordering of the objects within the class.Comparator: It is used to define custom sorting logic externally.Difference

5 min read
Java Iterator

An Iterator in Java is an interface used to traverse elements in a Collection sequentially. It provides methods like hasNext(), next(), and remove() to loop through collections and perform manipulation. An Iterator is a part of the Java Collection Framework, and we can use it with collections like A

7 min read

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