//餓漢式（開(kāi)發(fā)時(shí)常用）
class Single//類一加載，對(duì)象就已經(jīng)存在了。
{
  private static Single s = new Single();
 
  private Single(){}
 
  public static Single getInstance()
  {
    return s;
  }
}
 

//懶漢式（面試時(shí)常問(wèn)，在多線程并發(fā)訪問(wèn)時(shí)候有可能導(dǎo)致不能保證不了對(duì)象的唯一性,存在安全隱患?。?
class Single2//類加載進(jìn)來(lái)，沒(méi)有對(duì)象，只有調(diào)用了getInstance方法時(shí)，才會(huì)創(chuàng)建對(duì)象。
      //延遲加載形式。 
{
  private static Single2 s = null;
 
  private Single2(){}
 
  public static Single2 getInstance()
  {
    if(s==null)
      s = new Single2();
    return s;
  }
}
 
//調(diào)用類
class SingleDemo
{
  public static void main(String[] args) 
  {
    Single s1 = Single.getInstance();
    Single s2 = Single.getInstance();
 
    System.out.println(s1==s2);
     
//   Single ss = Single.s; //此處不采用這個(gè)是因?yàn)椴豢煽?，采?Single.getInstance();可以傳參數(shù)進(jìn)行相應(yīng)調(diào)用。
 
  }
}

package pattern.singleton;import java.util.HashMap;import java.util.Map;
 //登記式單例類.
 //類似Spring里面的方法，將類名注冊(cè)，下次從里面直接獲取。
public class Singleton3 {
 private static Map<String,Singleton3> map = new HashMap<String,Singleton3>();
 static{
 Singleton3 single = new Singleton3();
 map.put(single.getClass().getName(), single);
 }
 //保護(hù)的默認(rèn)構(gòu)造子
protected Singleton3(){}
 //靜態(tài)工廠方法,返還此類惟一的實(shí)例
public static Singleton3 getInstance(String name) {
 if(name == null) {
 name = Singleton3.class.getName();
 System.out.println("name == null"+"--->name="+name);
 }
 if(map.get(name) == null) {
 try {
 map.put(name, (Singleton3) Class.forName(name).newInstance());
 } catch (InstantiationException e) {
 e.printStackTrace();
 } catch (IllegalAccessException e) {
 e.printStackTrace();
 } catch (ClassNotFoundException e) {
 e.printStackTrace();
 }
 }
 return map.get(name);
 }
 //一個(gè)示意性的商業(yè)方法
public String about() {
 return "Hello, I am RegSingleton.";
 }
 public static void main(String[] args) {
 Singleton3 single3 = Singleton3.getInstance(null);
 System.out.println(single3.about());
 }
 }