How does class loading work in the JVM?

Loading finds the bytecode and creates a Class object. Linking verifies bytecode safety, prepares static storage and resolves symbolic references. Initialization runs static initializers and static field assignments exactly once, on first active use.

The class loader hierarchy

• Bootstrap loader loads core JDK classes and is represented as null
• Platform loader loads modules such as java.sql and java.xml
• Application loader loads the classpath or module path
• Custom loaders support plugins, isolation and hot deployment
• Parent delegation asks the parent first, then the child tries locally

Class<?> type = Class.forName("com.example.Plugin");
Object instance = type.getDeclaredConstructor().newInstance();

ClassLoader loader = Thread.currentThread().getContextClassLoader();
Class<?> other = loader.loadClass("com.example.Plugin");

Parent delegation prevents application code from replacing core classes and keeps a class consistent within a loader. The same class name loaded by two different loaders produces two distinct Class objects, which causes ClassCastException even though the names match. That is common in plugin and application-server environments.

ClassNotFoundException means no loader could find the class by name. NoClassDefFoundError means the class was found earlier but its definition is unavailable now, often because a static initializer failed or a dependency is missing at runtime.