What is ClassLoader?

Direct Answer

A ClassLoader finds or generates the binary definition for a named class and asks the JVM to define it. Java's built-in bootstrap, platform, and system loaders usually delegate upward before searching themselves; loading is separate from linking and class initialization.

Deep Dive & Architecture

Defining and initiating are different roles

A loader that calls the JVM's definition machinery for a class is its defining loader. A different loader can initiate the request and receive the result through delegation. Runtime class identity includes the binary name and the defining loader, so two definitions named com.example.Plugin can be incompatible types.

Class loaders in a plugin boundary

An application can give each plugin its own child loader. The child finds plugin classes while delegating shared API types to a parent. This allows two plugins to carry different private library versions, yet both can implement one interface defined by the parent. If each child also defined its own copy of that interface, casts would fail even though the source names look identical.

Loader lifetime becomes class lifetime

Classes are unloaded as part of unloading their defining loader and associated classes. A static registry, running thread, thread-context class loader, cache key, or callback can accidentally retain an old plugin loader. Then its class metadata, static fields, and related resources remain reachable after redeployment.

Resources have an extra boundary

Resource lookup resembles class lookup but is not identical in every module context. Named modules can encapsulate resources, so a visible class does not mean every resource path is freely visible. When debugging, ask separately: which loader received the request, which loader defined the type, whether delegation found a parent copy, and whether module rules permit the resource.

Code Example

class ClassLoaderRoles {
    public static void main(String[] args) {
        ClassLoader app = ClassLoaderRoles.class.getClassLoader();
        System.out.println("application: " + app);
        System.out.println("parent: " + app.getParent());
        System.out.println("String loader: " + String.class.getClassLoader());
    }
}