The purpose of a Class Loader is to service the request for a class. When, JVM needs a class,
it requests the Class Loader for the class by specifying fully qualified name.
All JVM's include at least one embedded class loader
called the primordial or bootstrap class loader. Class loaders are
hierarchical. Classes are loaded into the JVM on demand as they
are referenced by name in a class that is already running in
the JVM. The very first class is loaded through static main
method.
When the JVM is started, three class loaders
are used
Bootstrap (primordial)
Bootstrap class loader written in native code, loads
internal core Java libraries. Like java.* located
in the<JAVA_HOME>/jre/lib directory )->
Extensions (Loads jar files from JDK extensions
directory <JAVA_HOME>/jre/lib/ext)->
System
System class loader loads classes from system classpath defined using
-classpath or -cp
High Level Class Loader flow:
Class
loaders are hierarchical and use delegation to load the
classes.
Class
loaders request parent to load the class first before
attempting to load it themselves.
When
a class loader loads a class, the child class loader will
never load the class again. Uniqueness is
maintained.
Classes
loaded by Child has visibility into the classes loaded by
the parent but parent doesn't have visibility into
classes loaded by parent class loader.
