The class loader
locates and reads the *.class files needed to execute the
Java program and loads the bytecodes into memory. To safeguard a secure
execution, it can assign different portions (namespaces) of memory to locally
versus remotely obtained classes. Classes are typically assembled into
libraries that are stored physically in JAR (Java Archive) files. The libraries
may have been written by the user or obtained externally. In order to locate
the classes, the class loader will first find the corresponding libraries and
then load the classes as they are needed by the program (called on demand loading).
The class loader basically has three subcomponents:
➤ Bootstrap
class loader
The bootstrap class loader loads the core Java libraries located
in <JAVA_HOME>/jre/lib.
➤ Extensions
class loader
The extensions class loader loads the classes from the extensions
directory <JAVA_HOME>/jre/lib/ext
➤ System
class loader
The system class loader loads the code from the locations
specified in the CLASSPATH environment
variable, which is defined by the operating system. The latter provides the path
to all physical directories where the system class loader can look for Java
files.
