Singleton實現

單例模式的實現大致有以下幾種方式：

餓漢式：

/**
  * @desc: 餓漢式，類加載時即創建實例
  * @desc: feature:可以通過反射機制攻擊；線程安全(多個類加載器除外)
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class HungrySingletonOfBasic {
       public static final HungrySingletonOfBasicinstance = new HungrySingletonOfBasic();

       private HungrySingletonOfBasic() {

      }
}

餓漢式變種1：

/**
  * @desc：餓漢式變種
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class HungrySingletonTwo {
       private static HungrySingletonTwo instance =  new HungrySingletonTwo();

       public static HungrySingletonTwo getInstance() {
             return instance ;
      }

       private HungrySingletonTwo() {

      }
}

餓漢式變種2：

/**
  * @desc: 餓漢式變種2，類加載時實例化
  * @deprecated something may be wrong
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class HungrySingletonThree {
       private static HungrySingletonThree instance  = null;

       static {
             instance = new HungrySingletonThree();
      }

       private HungrySingletonThree() {

      }

       public static HungrySingletonThreegetInstance() {
             return instance ;
      }
}

懶漢式：

/**
  * @desc: 懶漢式，基本類型
  * @desc: feature:延時加載；線程不安全，多線程下不能正常工作；需要額外的serializable/transient/readResolve來實現序列化
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class LazySingletonOfBasic {
       private static LazySingletonOfBasic instance = null ;

       private LazySingletonOfBasic() {
      }

       public static LazySingletonOfBasic getInstance() {
             if (instance == null) {
                   instance = newLazySingletonOfBasic();
            }
             return instance ;
      }
}

懶漢式變種：

/**
  * @desc: 線程安全，效率較低
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class LazySingletonTwo {
       private static LazySingletonTwo instance =null ;

       private LazySingletonTwo() {

      }

       public static synchronized LazySingletonTwo getInstance() {
             if (instance == null) {
                   instance = newLazySingletonTwo();
            }
             return instance ;
      }
}

靜態內部類：

/**
  * @desc: 靜態內部類：線程安全，延遲加載
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class InnerStaticSingleton {
       private static class SingletonHolder {
             private static final InnerStaticSingleton instance = newInnerStaticSingleton();
      }

       public static final InnerStaticSingletongetInstance() {
             return SingletonHolder. instance;
      }

       private InnerStaticSingleton() {

      }
}

雙重校驗鎖：

/**
  * @desc：雙重校驗
  * @author: houkh
  * @date： Dec 2, 2014
  */
public class DoubleCheckSingleton {
       private volatile static DoubleCheckSingleton  instance;

       private DoubleCheckSingleton() {

      }

       public static DoubleCheckSingleton getInstance() {
             if (instance == null) {
                   synchronized(DoubleCheckSingleton. class) {
                         if (instance == null) {
                               instance = newDoubleCheckSingleton();
                        }
                  }
            }
             return instance ;
      }
}

枚舉類型：

/**
  * @desc: 提供序列化機制，絕對防止多次實例化，即使在面對複雜序列化或反射攻擊的時候
  * @author: houkh
  * @date： Dec 2, 2014
  */
public enum EnumSingleton {
       instance ;
}