餓漢式單例
com.cleversoft.designpatterns.singleton1.Singleton.java:
package?com.cleversoft.designpatterns.singleton1;

/**
?*?Hungry?Singleton
?*?
?*/
public?class?Singleton?{

????private?static?Singleton?singleton?=?new?Singleton();

????private?Singleton()?{
????}

????public?static?Singleton?getInstance()?{
????????return?singleton;
????}

????public?String?demoMethod()?{
????????return?"This?is?a?hungry?singleton?demo!";
????}
}

com.cleversoft.designpatterns.singleton1.Main.java:
package?com.cleversoft.designpatterns.singleton1;

public?class?Main?{

????/**
?????*?@param?args
?????*/
????public?static?void?main(String[]?args)?{
????????//?TODO?Auto-generated?method?stub
????????System.out.println(Singleton.getInstance().demoMethod());
????}

}

懶漢式單例
com.cleversoft.designpatterns.singleton2.LazySingleton.java:
package?com.cleversoft.designpatterns.singleton2;

/**
?*?Lazy?Singleton
?*?
?*/
public?class?LazySingleton?{
????static?boolean?instance_flag?=?false;

????private?LazySingleton()?{
????}

????synchronized?public?static?LazySingleton?getInstance()?{
????????if?(!instance_flag)?{
????????????instance_flag?=?true;
????????????return?new?LazySingleton();
????????}?else
????????????return?null;
????}

}

com.cleversoft.designpatterns.singleton2.Main.java:
package?com.cleversoft.designpatterns.singleton2;

public?class?Main?{

????/**
?????*?@param?args
?????*/
????public?static?void?main(String[]?args)?{
????????//?TODO?Auto-generated?method?stub
????????LazySingleton?ls1,?ls2;
????????System.out.println("Start?to?get?first?Instance");
????????ls1?=?LazySingleton.getInstance();
????????if?(ls1?!=?null){
????????????System.out.println("This?is?the?first?Instance!\n");
????????}
????????
????????System.out.println("Start?to?get?second?Instance");
????????ls2?=?LazySingleton.getInstance();
????????if?(ls2?!=?null){
????????????System.out.println("This?is?the?second?Instance!");
????????}
????????else{????????????
????????????System.out.println("You?can?just?get?1?Instance!");
????????}
????????
????}

}

引用：
?>強烈建議大家不要再花時間在實現雙重檢查成例上
是這樣，不必在這上面鉆牛角尖，而且單例模式在實際應用中已經逐步被Ioc容器替代，實用價值不高。（banq）
(http://www.jdon.com/jive/thread.jsp?forum=91&thread=23890)

參考資料：
1. 《Java與模式》
2. http://www.java2s.com

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