Class Loading

From .class to Running Code

The JVM loads classes lazily: a class is loaded the first time it is needed. Loading has three phases.

Main.class ─► LOADING ─► LINKING ───────────────────────────► INITIALIZATION
             (read bytes,  verify → prepare → resolve              (run static blocks,
              create Class (bytecode   (static     (symbolic refs    assign static fields)
              object)       is safe)    defaults)   → real refs)

Class Loader Hierarchy

Loader	Loads	Name
Bootstrap	core JDK classes (`java.lang`, `java.util`) — written in native code	shown as `null`
Platform	other JDK modules (`java.sql`, `java.xml`...)	`platform`
Application (system)	your classes and jars on the classpath	`app`
Custom	plugins, hot reload (Tomcat web apps, Spring DevTools)	any

Loaders use parent delegation: a loader first asks its parent, and only loads the class itself if the parent cannot. This stops anyone from replacing java.lang.String with their own class.

See It Yourself

Example

A class is initialized only when first used. Its static block runs exactly once.

public class Main {
    public static void main(String[] args) {
        System.out.println("String loaded by: " + String.class.getClassLoader());
        ClassLoader app = ClassLoader.getSystemClassLoader();
        System.out.println("System loader: " + app.getName() + ", parent: " + app.getParent().getName());
        System.out.println("java.sql.Date loaded by: " + java.sql.Date.class.getClassLoader().getName());

        System.out.println("main started, Config not used yet");
        System.out.println("Reading constant: " + Config.APP_NAME);   // compile-time constant: no init
        System.out.println("Calling method:   " + Config.timeout());  // triggers initialization
        System.out.println("Calling again:    " + Config.timeout());  // static block does not run again
    }
}

class Config {
    static final String APP_NAME = "ShopEase";      // inlined by the compiler
    private static int timeout;
    static {
        System.out.println("  [Config static block runs]");
        timeout = 30;
    }
    static int timeout() { return timeout; }
}

Output

String loaded by: null
System loader: app, parent: platform
java.sql.Date loaded by: platform
main started, Config not used yet
Reading constant: ShopEase
  [Config static block runs]
Calling method:   30
Calling again:    30

Common Errors

Error	Meaning
`ClassNotFoundException`	loading by name at runtime failed (`Class.forName("x")`), jar missing
`NoClassDefFoundError`	class was present at compile time but missing at runtime, or its static init failed earlier
`ExceptionInInitializerError`	a static block or static field initializer threw
`UnsupportedClassVersionError`	compiled with a newer Java than the one running it (e.g. built on 21, run on 17)