如何写出更好的Java单例(Singleton)模式，以及单例模式的攻防源代码

package singleton;
public class JerrySingleton {
  @SuppressWarnings("unused")
  private String name;
    private JerrySingleton(){  
      name = "Jerry";
    }  
    private  static class SingletonHolder{  
        private static final JerrySingleton INSTANCE = new JerrySingleton();  
    }  
    public static JerrySingleton getInstance()  
    {  
        return SingletonHolder.INSTANCE;  
    }  
}

package singleton;
public enum JerrySingletonAnotherApproach {
  INSTANCE ;  
  private String name = "Jerry" ;
  public String getName() {  
      return this.name;  
  }  
}

package singleton;
public class JerrySingletonImproved  
{  
    private static boolean flag = false;  
    private JerrySingletonImproved(){  
        synchronized(JerrySingletonImproved.class)  
        {  
            if(flag == false)  
            {  
                flag = !flag;  
            }  
            else 
            {  
                throw new RuntimeException("Singleton violated");  
            }  
        }  
    }  
    private  static class SingletonHolder{  
        private static final JerrySingletonImproved INSTANCE = new JerrySingletonImproved();  
    }  
    public static JerrySingletonImproved getInstance()  
    {  
        return SingletonHolder.INSTANCE;  
    }  
}

package singleton;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public class SingletonAttack {
  @SuppressWarnings("unused")
  private static void test1() throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {
    Class<?> classType = JerrySingleton.class;  
        Constructor<?> c = classType.getDeclaredConstructor(JerrySingleton.class);  
        c.setAccessible(true);  
        JerrySingleton e1 = (JerrySingleton)c.newInstance();  
        JerrySingleton e2 = JerrySingleton.getInstance();  
        System.out.println(e1 == e2);  
  }
  private static void test2() throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {
    Class<?> classType = JerrySingletonImproved.class;  
        Constructor<?> c = classType.getDeclaredConstructor();  
        c.setAccessible(true);  
        JerrySingletonImproved e1 = (JerrySingletonImproved)c.newInstance();  
        JerrySingletonImproved e2 = JerrySingletonImproved.getInstance();  
        System.out.println(e1 == e2);  
  }
  @SuppressWarnings("unused")
  private static void test3() throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {
    Class<?> classType = JerrySingletonAnotherApproach.class;   
        Constructor<?> c = classType.getDeclaredConstructor();  
        c.setAccessible(true);  
        JerrySingletonAnotherApproach e1 = (JerrySingletonAnotherApproach)c.newInstance();  
        JerrySingletonAnotherApproach e2 = JerrySingletonAnotherApproach.INSTANCE;
        System.out.println(e1 == e2);  
  }
  public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException  
    {  
        test2();
    System.out.println("Name:" + JerrySingletonAnotherApproach.INSTANCE.getName());
        // test3();
    }  
}

package singleton;
public class SingletonInnerClass {
  /*
   * 当执行getInstance()方法的时候就去调用FileIOHolder内部类里面的INSTANCE实例，
   * 此时FileIOHolder内部类会被加载到内存里，在类加载的时候就对INSTANCE实例进行初始化。
   * 和饿汉式一个道理，保证了只有一个实例，而且在调用getInstance()方法的时候才进行INSTANCE实例的初始化，
   * 又具有懒汉式的部分特性。”
   */
  private static final class FileIOHolder {
    private static final SingletonInnerClass INSTANCE = new SingletonInnerClass();
  }
  private SingletonInnerClass() {
    System.out.println("constructor of SingletonInnerClass...");
  }
  public static SingletonInnerClass getInstance() {
    String trimTest = new String("i042416").trim();
    System.out.println("GetInstance called..." + trimTest);
    return FileIOHolder.INSTANCE;
  }
  private static void objectTest(){
    Object obj = new Object();
    Class<? extends Object> clas = obj.getClass();
    System.out.println(clas.getClassLoader());
  }
  public static void main(String arg[]){
    Object obj1 = SingletonInnerClass.getInstance();
    Object obj2 = SingletonInnerClass.getInstance();
    System.out.println(obj1 == obj2);
    objectTest();
  }
}