你作为一个Java领域的专家，请完成以下任务：手写尽可能多的单例模式，线程安全不安全都可以

1. 饿汉式（线程安全）

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return INSTANCE;
    }
}

2. 懒汉式（线程不安全）

public class Singleton {
    private static Singleton INSTANCE = null;
    private Singleton(){}
    public static Singleton getInstance(){
        if(INSTANCE == null){
            INSTANCE = new Singleton();
        }
        return INSTANCE;
    }
}

3. 懒汉式（线程安全，同步方法）

public class Singleton {
    private static Singleton INSTANCE = null;
    private Singleton(){}
    public static synchronized Singleton getInstance(){
        if(INSTANCE == null){
            INSTANCE = new Singleton();
        }
        return INSTANCE;
    }
}

4. 懒汉式（线程安全，双重校验锁）

public class Singleton {
    private static volatile Singleton INSTANCE = null;
    private Singleton(){}
    public static Singleton getInstance(){
        if(INSTANCE == null){
            synchronized(Singleton.class){
                if(INSTANCE == null){
                    INSTANCE = new Singleton();
                }
            }
        }
        return INSTANCE;
    }
}

5. 静态内部类（线程安全）

public class Singleton {
    private Singleton(){}
    private static class SingletonHolder{
        private static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getInstance(){
        return SingletonHolder.INSTANCE;
    }
}

6. 枚举（线程安全）

public enum Singleton {
    INSTANCE;
    public void doSomething(){
        //...
    }
}