Components

About

The Java Virtual Machine (JVM) is the core of the Java ecosystem, responsible for executing Java applications. It abstracts hardware details and provides key features like platform independence, memory management, security, and performance optimizations.

The JVM consists of several core components that work together to execute Java programs efficiently. The primary components are:

1. ClassLoader Subsystem – Loads Java classes into memory.

2. Runtime Data Areas (Memory) – Manages various memory sections like heap, stack, and method area.

3. Execution Engine – Executes Java bytecode using an interpreter and Just-In-Time (JIT) compiler.

4. Garbage Collector (GC) – Automatically manages memory by reclaiming unused objects.

5. Native Interface (JNI) – Allows interaction with native libraries (C/C++).

6. Java Security Manager (Deprecated in Java 17) – Controlled access to system resources.

1. ClassLoader Subsystem (Class Loading Mechanism)

The ClassLoader is responsible for loading Java classes into JVM memory. It ensures that Java programs can dynamically load and use classes at runtime.

How ClassLoader Works?

1. Loading – Class files (.class) are read and loaded into memory.

2. Linking

   • Verification – Ensures bytecode follows JVM rules.

   • Preparation – Memory is allocated for static variables.

   • Resolution – Converts symbolic references to actual memory addresses.

3. Initialization – Executes static initializers and assigns values to static variables.

Types of ClassLoaders

JVM uses a hierarchical delegation model with three main ClassLoaders:

ClassLoader	Purpose	Loads Classes From
Bootstrap ClassLoader	Loads core Java classes	java.lang.*, java.util.* (from rt.jar or jmods)
Extension ClassLoader	Loads extension classes	lib/ext directory
Application ClassLoader	Loads user-defined classes	Classpath (bin, jar files)

Custom ClassLoaders

Developers can create custom ClassLoaders to load classes dynamically, modify bytecode at runtime, or implement security policies.

Example: Custom ClassLoader

public class CustomClassLoader extends ClassLoader {
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        System.out.println("Loading class: " + name);
        return super.loadClass(name);
    }
}

2. Runtime Data Areas (JVM Memory Model)

The JVM manages memory through different regions, ensuring efficient execution and garbage collection.

JVM Memory Structure

1. Heap Memory – Stores objects and class instances.

2. Stack Memory – Stores method call frames and local variables.

3. Method Area – Stores class metadata, constants, and static fields.

4. PC Register – Holds the address of the currently executing instruction.

5. Native Method Stack – Stores native method calls.

Heap Memory (Object Storage)

• Divided into Young Generation (Eden, Survivor spaces) and Old Generation.

• Garbage Collection runs frequently in the Young Generation.

Stack Memory (Method Execution)

• Stores method execution frames. Each method call creates a new frame.

• Contains local variables, method parameters, and return values.

Method Area (Class Metadata)

• Stores class-level information like method bytecode, static variables, and runtime constants.

3. Execution Engine (JVM Code Execution)

The Execution Engine is responsible for converting bytecode into machine code and executing it.

Components of Execution Engine

1. Interpreter – Reads and executes bytecode line by line.

2. JIT (Just-In-Time) Compiler – Translates bytecode into machine code for better performance.

3. Garbage Collector (GC) – Frees unused memory.

4. Native Interface (JNI) – Enables Java to call native C/C++ methods.

JIT Compilation and Optimization

• HotSpot JIT Compiler optimizes frequently executed methods.

• Uses techniques like method inlining, loop unrolling, escape analysis for better performance.

4. Garbage Collection (GC)

Garbage Collection automatically removes unused objects from Heap Memory, preventing memory leaks.

Types of Garbage Collectors in Java

Garbage Collector	Characteristics
Serial GC	Simple, single-threaded (best for small apps)
Parallel GC	Multi-threaded GC for better performance
CMS (Concurrent Mark-Sweep)	Low-latency GC, runs alongside application
G1 (Garbage First) GC	Modern GC, balances performance and latency
ZGC / Shenandoah	Ultra-low latency GCs for large applications

Garbage Collection Phases

1. Mark – Identifies reachable objects.

2. Sweep – Removes unused objects.

3. Compact – Defragments memory for efficiency.

Example: Forcing GC Manually

System.gc();  // Requests garbage collection

5. Java Native Interface (JNI)

JNI allows Java programs to call native (C/C++) code.

Why Use JNI?

• Access system resources not available in Java.

• Performance optimization using native code.

• Interoperability with existing C/C++ libraries.

Example: Calling C Code from Java

public class NativeExample {
    static {
        System.loadLibrary("nativeLib");
    }
    public native void nativeMethod();
}

C Code Implementation

#include <jni.h>
#include <stdio.h>
JNIEXPORT void JNICALL Java_NativeExample_nativeMethod(JNIEnv *env, jobject obj) {
    printf("Hello from C!\n");
}

6. Java Security Manager (Deprecated in Java 17)

The Security Manager provided a way to restrict access to system resources (file system, network, etc.).

Security Features in JVM

• ClassLoader Security – Prevents unauthorized bytecode execution.

• Bytecode Verification – Ensures code integrity before execution.

• Cryptography API (JCA/JCE) – Secure data encryption and hashing.

• JAAS (Java Authentication and Authorization Service) – Manages authentication.

Java 17 removed the Security Manager due to modern security best practices.

Example: Using Security Manager

SecurityManager sm = System.getSecurityManager();
if (sm != null) {
    sm.checkRead("/restricted/file.txt");
}