//餓漢式 不推薦
public class Singleton{
    private static final  Singleton singleton = new Singleton();
    public static Singleton getInstance(){
        return singleton;
    }
}


//懶漢式 不推薦
public class Singleton{
    private static Singleton singleton;
    private Singleton(){
        
    }
    public static  sysnchronized  Singleton getInstance(){
        if(null==instance){
            instance = new Singleton();
        }
        return instance;
    }
}
//雙重檢查鎖定（DCL） 不推薦懶漢式的優(yōu)化版
public class Singleton{
    private static Singleton instance = null;
    private Singleton(){
        
    }
    public static Singleton getInstance(){
        if(null == instance){
            synchronized(Singleton.class){
                if(null==instace){
                    instance = new Singleton
                }
            } 
        }
        return instance；
    }
}

//靜態(tài)內(nèi)部單例 推薦
public class Singleton{
    private static Singleton getInstance(){
        return Singletons.singleton;
    }
    
    private static class Singletons{
        private  static final Singleton singleton = new Singleton(); 
    }
}

//枚舉單例 推薦
public enum Singleton{
    INSTANCE;
    public void doSomething(){
        System.out.println("操作單例");
    }
}
//使用容器
public class SingletonManager{
    private static Map<String,Object> objMap = new HashMap<String,Object>();
    public static void registerService(String key ,Object instance){
        if(!objMap.containsKey(key)){
            objMap.put(instance);
        }
    }
    public static Object getService(String key){
        return objMap.get(key);
    }
}

具體可參考