Chapter 134: ClassLoader in Java

ClassLoader loads bytecode and defines Class<?> objects. The JVM uses a delegation model: child loaders ask parents first—bootstrap (null parent) loads java.*, platform loaders cover JDK modules, application loaders read CLASSPATH / module path.

Different loaders can load two classes with the same binary name—they are not assignable to each other—instanceof fails in confusing ways.

1. Topic title

ClassLoader: namespace for loaded types; delegation keeps core JDK stable

2. What it means

Class.forName(name, initialize, loader) picks which loader defines the class.

Thread.currentThread().getContextClassLoader() is the “who should load plugins?” escape hatch—frameworks set it carefully around SPI calls.

URLClassLoader (or Layer APIs) historically loaded plugins from directories—security and JPMS changed defaults—prefer documented plugin mechanisms.

3. Why it is used

OSGi, application servers, and build tools (Surefire) use loader isolation to avoid version hell.

Hot reload in dev sometimes spins throwaway loaders—never ship naive reload without leak analysis.

4. Mental sketch

ClassLoader is library branch cataloging—same book title on two shelves (loaders) means two distinct copies—do not mix pages across shelves expecting one volume.

5. Java code example

public class LoaderIntrospection {
    public static void main(String[] args) {
        ClassLoader cl = String.class.getClassLoader();
        System.out.println("String loader=" + cl); // null -> bootstrap
 
        ClassLoader app = LoaderIntrospection.class.getClassLoader();
        System.out.println("app loader=" + app);
    }
}

null loader on String signals bootstrap—normal for JDK core types.

6. Explanation of code

Parent delegation prevents user code from redefining java.lang.Object security—children cannot override bootstrap classes.

7. Common mistakes

Closing URLClassLoader while classes still live—native libraries unload painfully.

Casting instances across loaders—ClassCastException even when .class names match.

8. Best practices

Log loader identity when debugging ServiceConfigurationError.

Prefer module layers over ad-hoc URLClassLoader towers on modern JDKs.

9. Small practice task

Print getClass().getClassLoader() for com.mysql.cj.jdbc.Driver when loaded from your app—compare to java.sql.Driver interface loader chain.

Beginner tip

If NoClassDefFoundError follows ClassNotFoundException, you fixed loading once but static init failed later—read the caused-by chain fully.