ガベージコレクションの対象となるタイミングについて

class Foo {

    public static void main(String[] args) {
        String a = "unibon";
        String b = new String("unibon");
    }
}

public class UnLoadedClass {
   public String toString() {
      return "Test";
   }
}

import java.lang.ref.WeakReference;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
public class ClassUnloadTest {

   public static void main(String[] args)
      throws
         ClassNotFoundException,
         InstantiationException,
         IllegalAccessException,
         MalformedURLException {

      // URLクラスローダーを使ってクラスを読み込む
      URLClassLoader loader =
         new URLClassLoader(
            new URL[] { new URL("file:///D:/hogehoge/")},		// (1)
            ClassUnloadTest.class.getClassLoader());
      
      Class class1 = loader.loadClass("UnLoadedClass");
      Object object = class1.newInstance();
      String string = object.toString();
      
      System.out.println("Is this Class loaded from my ClassLoader? "  
         + (class1.getClassLoader() == loader));			// (2)
      System.out.println("Is this String an interned string? " 
         + (string == string.intern()));					// (3)
      
      // 弱参照を保持する
      WeakReference classRef = new WeakReference(class1);
      WeakReference loaderRef = new WeakReference(loader);
      WeakReference objectRef = new WeakReference(object);
      WeakReference stringRef = new WeakReference(string);
      
      // 参照にnullを代入する
      loader = null;
      class1 = null;
      object = null;
      string = null;

      executeGC(classRef, loaderRef, objectRef, stringRef);
   }

   // GCを実行、状態を表示  
   private static void executeGC(
      WeakReference classRef,
      WeakReference loaderRef,
      WeakReference objectRef,
      WeakReference stringRef) {

      System.out.println();
      
      System.gc();
      System.out.println("GC executed!");

      System.out.println("ClassLoader : " + loaderRef.get());
      System.out.println("Class : " + classRef.get());
      System.out.println("Object : " + objectRef.get());
      System.out.println("String : " + stringRef.get());
   }
}