JVM Tutorials - Herong's Tutorial Examples - v5.13, by Herong Yang
Class Loading Problem - JAR Hell
This section provides a tutorial example on how to use ClassLoader.getSystemResources() method to return all locations of the same class file. This is useful when you are dealing with JAR hell problem.
What Is JAR Hell? JAR hell is a class loading problem where the wrong version of a class is loaded into the JVM, because the same class exists in multiple JAR files given in the classpath.
Case 1 - When deploying an application with two JAR files, one JAR file for each version of the same class files, are listed in the classpath, and the older version is listed first. The System ClassLoader will load the older version of the class into JVM. This is a deployment mistake which can be easily corrected.
Case 2 - When deploying an application with one JAR file and one class folder, one for each version of the same class files, are listed in the classpath, and the older version is listed first. The System ClassLoader will load the older version of the class into JVM. This is a deployment mistake which can be easily corrected.
Case 3 - An application requires two libraries, which require different versions, two JAR files, of a single third library. If both versions of the third library use same class names, there is no way to load both JAR files of the third library with the same ClassLoader.
One way to find how many versions of the same class are accessible in the "classpath" is to use the ClassLoader.getSystemResources() method, which finds and returns all locations of the give class file name.
Here is tutorial example program showing you how to find all locations of a Java class:
/* ClassLoaderJarHell.java * Copyright (c) HerongYang.com. All Rights Reserved. */ class ClassLoaderJarHell { public static void main(String[] a) { java.io.PrintStream out = System.out; java.net.URL u = null; out.println(""); out.println("Find all locations of Hello.class..."); try { java.util.Enumeration e = ClassLoader.getSystemResources("Hello.class"); while (e.hasMoreElements()) { u = (java.net.URL) e.nextElement(); out.println("URL: "+u); } } catch(Exception e) { e.printStackTrace(); } out.println(""); out.println("Running Hello class..."); Hello.main(null); } }
Before running this tutorial example, I need to prepare two versions of Hello class.
Version 1 of Hello class is compiled in the current folder. This Hello.class is copied to MyZIP.zip:
class Hello { public static void main(String[] a) { System.out.println("Hello world!"); } }
Version 2 of Hello class is compiled in a sub folder ./src. This Hello.class is copied to ./src/Hello.jar:
class Hello { public static void main(String[] a) { System.out.println("Hello world! -- v2"); } }
Now run ClassLoaderJarHell.java with different "classpath" settings:
herong> java ClassLoaderJarHell Find all locations of Hello.class... URL: file:/C:/herong/jvm/Hello.class Running Hello class... Hello world! herong> java -classpath src/Hello.jar;MyZIP.zip;. ClassLoaderJarHell Find all locations of Hello.class... URL: jar:file:/C:/herong/jvm/src/Hello.jar!/Hello.class URL: jar:file:/C:/herong/MyZIP.zip!/Hello.class URL: file:/C:/herong/jvm/Hello.class Running Hello class... Hello world! -- v2 herong> java -classpath MyZIP.zip;src/Hello.jar;. ClassLoaderJarHell Find all locations of Hello.class... URL: jar:file:/C:/herong/jvm/MyZIP.zip!/Hello.class URL: jar:file:/C:/herong/jvm/src/Hello.jar!/Hello.class URL: file:/C:/herong/jvm/Hello.class Running Hello class... Hello world!
The test result confirms that:
Table of Contents
JVM (Java Virtual Machine) Specification
Java HotSpot VM - JVM by Oracle/Sun
java.lang.Runtime Class - The JVM Instance
java.lang.System Class - The Operating System
►ClassLoader Class - Class Loaders
What Is java.lang.ClassLoader Class
Accessing the ClassLoader of a Class
loadClass() Method - Loading Classes Explicitly
getSystemResource() Method - Finding Files
►Class Loading Problem - JAR Hell
ClassChecker.java - Reports Class Loader
ClassChecker.java - Reports Class Locations
"superclass access check failed" Class Load Error
Class Loading Followed by Class Initialization
Class Class - Class Reflections
JVM Stack, Frame and Stack Overflow
Thread Testing Program and Result
CPU Impact of Multi-Thread Applications
I/O Impact of Multi-Thread Applications
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