What Is Class Loader?

This section describes what is Java Class Loader - Part of the Java Runtime Environment (JRE) that dynamically loads Java classes into the Java Virtual Machine (JVM).

What Is Class Loader? Class Loader is a part of the Java Runtime Environment (JRE) that dynamically loads Java classes into the Java Virtual Machine (JVM).

Class Loader is designed as a hierarchical delegation tree with these basic concepts:

Here is a simple diagram that illustrates the ClassLoader delegation tree:
ClassLoader Delegation Tree

Based JDK 1.7 documentations, here are my understandings on Bootstrap, Extensions and System ClassLoaders:

1. Bootstrap ClassLoadder - The root ClassLoader: Created by the JVM and represented as "null". Responsible to load JVM system classes located in <JAVA_HOME>/lib folder.

2. Extensions ClassLoadder - Created by the JVM and represented as a sun.misc.Launcher$ExtClassLoader object. Responsible to load JVM extension classes located in <JAVA_HOME>/lib/ext folder.

3. System ClassLoadder - Created by the JVM and represented as a sun.misc.Launcher$AppClassLoader object. Responsible to load Application classes specified in the "classpath".

Last update: 2014.

Table of Contents

 About This Book

 Downloading and Installing JDK 1.8.0 on Windows

 Downloading and Installing JDK 1.7.0 on Windows

 java.lang.Runtime Class - The JVM Instance

 java.lang.System Class - The Operating System

ClassLoader Class - Class Loaders

What Is Class Loader?

 What Is java.lang.ClassLoader Class?

 Accessing the ClassLoader of a Class

 JVM "-verbose:class" Option

 loadClass() Method - Loading Classes Explicitly

 getSystemResource() Method - Finding Files

 Class Loading Problem - JAR Hell

 Class Class - Class Reflections

 Sun's JVM - Java HotSpot VM

 JRockit JVM 28.2.7 by Oracle Corporation

 JVM Runtime Data Areas

 Memory Management and Garbage Collectors

 Garbage Collection Tests

 JVM Stack, Frame and Stack Overflow

 Thread Testing Program and Result

 CPU Impact of Multi-Thread Applications

 I/O Impact of Multi-Thread Applications

 CDS (Class Data Sharing)

 Micro Benchmark Runner and JVM Options

 Micro Benchmark Tests on "int" Operations

 Micro Benchmark Tests on "long" Operations

 Micro Benchmark Tests in JIT Compilation Mode

 Micro Benchmark Tests on "float" and "double" Operations

 Outdated Tutorials

 References

 PDF Printing Version