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The JRE loads classes into the JVM, most of the time. There are also custom class loaders for many online applications to allow instantiation of an object within the same namespace as a client or to instantiate an object given as reply to an HTTP request.

I do not know many class loaders.

1.bootstrap and 2.user-defined class loader. The bootstrap class loader loads classes of the Java API. User-defined class loaders loads user-defined classes.

1. bootstrap
2.extensions
3.System

One more - user Defined

there are 3 kind of loader in java....
were system loader is the parent then extension is mediator and lastly bootstrap will call...

Bootstrap Class Loader
Bootstrap class loader loads java’s core classes like java.lang, java.util etc. These are classes that are part of java runtime environment. Bootstrap class loader is native implementation and so they may differ across different JVMs.
Extensions Class Loader
JAVA_HOME/jre/lib/ext contains jar packages that are extensions of standard core java classes. Extensions class loader loads classes from this ext folder. Using the system environment propery java.ext.dirs you can add ‘ext’ folders and jar files to be loaded using extensions class loader.
System Class Loader
Java classes that are available in the java classpath are loaded using System class loader.

Very nice.. simple explanation..!!!