單例模式大致有五種寫法，分別為懶漢，惡漢，靜態內部類，列舉和雙重校驗鎖。

1、懶漢寫法，常用寫法

class LazySingleton{ private static LazySingleton singleton; private LazySingleton(){ } public static LazySingleton getInstance(){ if(singleton==null){ singleton=new LazySingleton(); } return singleton; } }

2、惡漢寫法，缺點是沒有達到lazy loading的效果

class HungrySingleton{ private static HungrySingleton singleton=new HungrySingleton(); private HungrySingleton(){} public static HungrySingleton getInstance(){ return singleton; }}

3、靜態內部類，優點：載入時不會初始化靜態變數INSTANCE，因為沒有主動使用，達到Lazy loading

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

4、列舉，優點：不僅能避免多執行緒同步問題，而且還能防止反序列化重新建立新的物件

enum EnumSingleton{ INSTANCE; public void doSomeThing(){ }}

5、雙重校驗鎖，在當前的記憶體模型中無效

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