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What are Class loaders?

ClassLoader in Java is a class which is used to load class files in Java. Java code is compiled into class file by javac compiler and JVM executes Java program, by executing byte codes written in class file.

All classes in a Java application are loaded using some subclass of java.lang.ClassLoader. Loading classes dynamically must therefore also be done using a java.lang.ClassLoader subclass.

Class Loading
The steps a given class loader uses when loading classes are:

1 Check if the class was already loaded.
2 If not loaded, ask parent class loader to load the class.
3 If parent class loader cannot load class, attempt to load it in this class loader.

When a class is loaded, all classes it references are loaded too. This class loading pattern happens recursively, until all classes needed are loaded. This may not be all classes in the application. Unreferenced classes are not loaded until the time they are referenced.

ClassLoader is responsible for loading class files from file system, network or any other source. There are three default class loader used in Java, Bootstrap , Extension and System or Application class loader.

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