Java中单例模式实现方法

/**
 * 一、懒汉，常用的写法
 */
class LazySingleton{
    private static LazySingleton singleton;
    private LazySingleton(){
    }
    public static LazySingleton getInstance(){
        if(singleton==null){
            singleton=new LazySingleton();
        }
        return singleton;
    }  
}
/**
 * 二、恶汉,缺点：没有达到lazy loading的效果
 */
class HungrySingleton{
    private static HungrySingleton singleton=new HungrySingleton();
    private HungrySingleton(){}
    public static HungrySingleton getInstance(){
        return singleton;
    }
}
/**
 * 三、静态内部类 优点：加载时不会初始化静态变量INSTANCE，因为没有主动使用，达到Lazy loading
 */
class InternalSingleton{
    private static class SingletonHolder{
        private final static  InternalSingleton INSTANCE=new InternalSingleton();
    }  
    private InternalSingleton(){}
    public static InternalSingleton getInstance(){
        return SingletonHolder.INSTANCE;
    }
}
/**
 * 四、枚举，《Effective Java》作者推荐使用的方法，优点：不仅能避免多线程同步问题，而且还能防止反序列化重新创建新的对象
 */
public enum FacHelper {
    
    facHelper;
    
    private SqlSessionFactory factory;
    
    private FacHelper(){
        try {
            singletonOperation();
        } catch (IOException e) {
            e.printStackTrace();
        }
    };
    
     public void singletonOperation() throws IOException{ 
        //装载配置文文件 
         String resource = "Configuration.xml";
            Reader reader = Resources.getResourceAsReader(resource);
            Properties prop = new Properties();
            prop.put("username", "zxf");
            prop.put("password", "zxf");
            //产生工厂
            factory = new SqlSessionFactoryBuilder().build(reader, prop);
            //测试验证此方法只执行一次
            System.err.println("55555555555555555555555555555555");
     }
 
    public SqlSessionFactory getFactory() {
        return factory;
    }
 
    public void setFactory(SqlSessionFactory factory) {
        this.factory = factory;
    }
}
/**
 *五、 双重校验锁，在当前的内存模型中无效
 */
class LockSingleton{
    private volatile static LockSingleton singleton;
    private LockSingleton(){}
    
    //详见：http://www.ibm.com/developerworks/cn/java/j-dcl.html
    public static LockSingleton getInstance(){
        if(singleton==null){
            synchronized(LockSingleton.class){
                if(singleton==null){
                    singleton=new LockSingleton();
                }
            }
        }
        return singleton;
    }
    
}