I have come across this point while i have explored Singleton Pattern
If the Singleton class is loaded by 2 different class loaders we'll have 2 different classes, one for each class loader.
I dont know how two class loader will available for a JVM and how to come it.
Multiple ClassLoaders are commonly encountered in Java EE. The Java EE application servers loads classes from deployed War/EAR by a tree of classloaders. The reason why they do it in this way is to isolate one application from other applications but still sharing classes between deployed modules. If you want your class to be truly singleton then you need to make sure that same classloader loads your singleton. You can achieve it like this
private static Class getClass(String clazz) throws ClassNotFoundException {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
if(loader == null)
loader = YourSingleton.class.getClassLoader();
return (loader.loadClass(clazz));
}
}
Note Enum in java already implement singleton pattern.
Updated Can you please explain what is multiple class Loader and how it resolve the issue.
Let say you have a library Foo. Some part of your project requires Foo_version1.jar and some other part requires Foo_version2.jar. So in your classpath you have Foo_version1.jar and Foo_version2.jar. Now the class loader need to load a class Bar from Foo, it will load it from first Foo_versionX that it finds on the classpath. In order to resolve this issue you need another class loader because remember that some part of your project require Bar class from different jar then the one loaded by classloader.
By using the code mentioned above you are making sure that if more than 1 classloader try to load your class then always the same instance will be used.
