what is classloader?

The classloader is a subsystem of JVM that is used to load classes and interfaces.There are many types of classloaders e.g. Bootstrap classloader, Extension classloader, System classloader, Plugin classloader etc.

Some extra knowledge…

Java class loaders can be broadly classified into below categories:

  • 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.

How a Java Class Loader Works?When a class name is given, class loader first locates the class and then reads a class file of that name from the native file system. Therefore this loading process is platform dependent.

By default java.lang.ClassLoader is registered as a class loader that is capable of loading classes in parallel. But the subclasses needs to register as parallel or not at the time of instantiation.

Classes can also be loaded from network, constructed on runtime and loaded. ClassLoader class has a method name defineClass which takes input as byte array and loads a class.


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