/*服务定位器模式(Service Locator Pattern)用在我们想使用 JNDI 查询定位各种服务的时候。考虑到为某个服务查找 JNDI 的代价很高,服务定位器模式充分利用了缓存技术。在首次请求某个服务时,服务定位器在 JNDI 中查找服务,并缓存该服务对象。当再次请求相同的服务时,服务定位器会在它的缓存中查找,这样可以在很大程度上提高应用程序的性能。以下是这种设计模式的实体。

服务(Service) - 实际处理请求的服务。对这种服务的引用可以在 JNDI 服务器中查找到。
Context / 初始的 Context - JNDI Context 带有对要查找的服务的引用。
服务定位器(Service Locator) - 服务定位器是通过 JNDI 查找和缓存服务来获取服务的单点接触。
缓存(Cache) - 缓存存储服务的引用,以便复用它们。
客户端(Client) - Client 是通过 ServiceLocator 调用服务的对象。*/
import java.util.ArrayList;
import java.util.List;

public interface Service {
 public String getName();
 public void execute();
 }

class Service1 implements Service {
 public void execute(){
 System.out.println("Executing Service1");
 }
 
 @Override
 public String getName() {
 return "Service1";
 }
 }
class Service2 implements Service {
 public void execute(){
 System.out.println("Executing Service2");
 }
 
 @Override
 public String getName() {
 return "Service2";
 }
 }

class InitialContext {
 public Object lookup(String jndiName){
 if(jndiName.equalsIgnoreCase("SERVICE1")){
 System.out.println("Looking up and creating a new Service1 object");
 return new Service1();
 }else if (jndiName.equalsIgnoreCase("SERVICE2")){
 System.out.println("Looking up and creating a new Service2 object");
 return new Service2();
 }
 return null; 
 }
 }

 
class Cache {
 
 private List<Service> services;
 
 public Cache(){
 services = new ArrayList<Service>();
 }
 
 public Service getService(String serviceName){
 for (Service service : services) {
 if(service.getName().equalsIgnoreCase(serviceName)){
 System.out.println("Returning cached "+serviceName+" object");
 return service;
 }
 }
 return null;
 }
 
 public void addService(Service newService){
 boolean exists = false;
 for (Service service : services) {
 if(service.getName().equalsIgnoreCase(newService.getName())){
 exists = true;
 }
 }
 if(!exists){
 services.add(newService);
 }
 }
 }

class ServiceLocator {
 private static Cache cache;
 
 static {
 cache = new Cache(); 
 }
 
 public static Service getService(String jndiName){
 
 Service service = cache.getService(jndiName);
 
 if(service != null){
 return service;
 }
 
 InitialContext context = new InitialContext();
 Service service1 = (Service)context.lookup(jndiName);
 cache.addService(service1);
 return service1;
 }
 }

 public class J2eeServiceLocatorPatternDemo {
 public static void main(String[] args) {
 Service service = ServiceLocator.getService("Service1");
 service.execute();
 service = ServiceLocator.getService("Service2");
 service.execute();
 service = ServiceLocator.getService("Service1");
 service.execute();
 service = ServiceLocator.getService("Service2");
 service.execute(); 
 }
 }

 //j2ee 服务定位器模式
 /*
 Looking up and creating a new Service1 object
Executing Service1
Looking up and creating a new Service2 object
Executing Service2
Returning cached Service1 object
Executing Service1
Returning cached Service2 object
Executing Service2
 */