单例模式分为两种，懒汉式和饿汉式。区别在于创建实例的时机。饿汉式在该类加载时创建实例，懒汉式则在需要获取时才进行创建。
package Create.f.Singleton;
public class Singleton {
private Singleton(){
}
protected static Singleton getInstance(){
System.out.println("获取Singleton单例对象");
return instance;
}
private static Singleton instance = new Singleton(); //饿汉式 注意此时线程安全
}
package Create.f.Singleton;
//懒汉式
public class Singleton2 {
private static Singleton2 instance = null;
private Singleton2(){
}
protected static synchronized Singleton2 getInstance(){
if(instance == null){
System.out.println("初始化Singleton2单例对象");
instance = new Singleton2();
}
return instance;
}
}
package Create.f.Singleton;
public class Client {
public static void main(String[] args) {
Singleton singleton = Singleton.getInstance();
Singleton s = Singleton.getInstance();
System.out.println("是否同个对象:" + (singleton == s));
Singleton2 s2 = Singleton2.getInstance();
Singleton2 s3 = Singleton2.getInstance();
Singleton2 s4 = Singleton2.getInstance();
System.out.println("是否同个对象:" + (s2 == s3 && s3 == s4));
}
}
原文：http://www.cnblogs.com/ciade/p/5095328.html