说明：
以下均为线程安全的实现方式。
懒汉、饿汉的非线程安全的意义不大，未实现。

1. 饿汉模式

类初始化时直接创建对象

public class Singleton {private static Singleton singleton = new Singleton();//饿汉模式，类初始化时直接创建public Singleton getInstance() {return singleton;}
}

2. 懒汉模式

懒汉模式，按需加载

public class Singleton {private Singleton singleton;public synchronized Singleton  getInstance() {//懒汉模式，需要的时候再创建if(singleton == null) {singleton = new Singleton();}return singleton;}
}

3. 懒汉模式优化版

优化懒汉模式，减少锁的消耗

public class Singleton {private Singleton singleton;public Singleton getInstance() {if (singleton == null) {synchronized (singleton) {//懒汉模式，需要的时候再创建。double check,防止重复创建if(singleton == null) {singleton = new Singleton();}}}return singleton;}
}

4. 懒汉模式——无锁实现

利用静态内部类来实现

public class Singleton {//Singleton类无法继承private Singleton(){
//		if(SingletonHolder.singleton == null) { 
//			throw new RuntimeException();
//		}}private static class SingletonHolder {private static Singleton singleton = new Singleton();}public Singleton getInstance() {return SingletonHolder.singleton;}}

5. 枚举实现

利用枚举来实现，

public class Singleton {//Singleton类无法继承private Singleton(){}public static Singleton getInstance() {//返回内部enum的单例引用return SingletonEnum.INSTANCE.singleton;}private static enum SingletonEnum { INSTANCE;private Singleton singleton;private SingletonEnum() {singleton = new Singleton();}}public static void main(String[] args) {Singleton singleton = new Singleton();System.out.println(singleton);}
}