What is the role of the ClassLoader in the JVM, and what are the three built-in bootstrap/platform/application class loaders?

A ClassLoader is responsible for locating and loading .class files (bytecode) into the JVM at runtime, converting them into Class objects usable by running code -- the JVM has a hierarchy of built-in loaders: the Bootstrap ClassLoader (written in native code, loads core JDK classes like java.lang.* from the JDK's own modules), the Platform ClassLoader (loads other JDK platform classes), and the Application/System ClassLoader (loads classes from the application's own classpath, the one typically used for your own application code).

public class Main {
    public static void main(String[] args) {
        ClassLoader loader = Main.class.getClassLoader();
        System.out.println(loader);  // typically: jdk.internal.loader.ClassLoaders$AppClassLoader

        ClassLoader stringLoader = String.class.getClassLoader();
        System.out.println(stringLoader);  // null, since String is loaded by the Bootstrap loader
    }
}