单例模式浅理解

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//single 实例一次只能有一个打印任务，但是却有n台打印机，我们随机的安排打印机
class LoadBalancer
{
private Random rd=new Random();
private static LoadBalancer ld=null;
ArrayList<String> serverLs=new ArrayList<String>();
protected LoadBalancer()
{
   serverLs.add("server1");
   serverLs.add("server2");
   serverLs.add("server3");
   serverLs.add("server4");
}
public static synchronized LoadBalancer getInstance()
{
   if(ld==null)
   {
    ld=new LoadBalancer();
   }
   return ld;
}

public String getServer()
{
  int i= rd.nextInt(serverLs.size());
  return serverLs.get(i).toString();
}

}

我们的测试类：

public class MotherTest {

public static void main(String[] args)
{
  MotherTest mt=new
    MotherTest();
  mt.TestSin3();
}

public void TestSin3()
   {
    LoadBalancer l1= LoadBalancer.getInstance();
    LoadBalancer l2= LoadBalancer.getInstance();
    LoadBalancer l3= LoadBalancer.getInstance();
    LoadBalancer l4= LoadBalancer.getInstance();
    if((l1==l2)&& (l2==l3)&& (l3==l4)&&(l4==l1))
    {
     System.out.println("ok");
    }
    System.out.println(l1.getServer());
    System.out.println(l2.getServer());
    System.out.println(l3.getServer());
    System.out.println(l4.getServer());
   }

}