三十六、服务定位器模式 (Service Locator Pattern)

服务定位器模式(Service Locator Pattern)用于想使用 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

实现

创建 ServiceLocatorInitialContextCacheService 作为表示实体的各种对象

Service1Service2 表示实体服务

ServiceLocatorPatternDemo ,我们的演示类在这里是作为一个客户端,将使用 ServiceLocator 来演示服务定位器设计模式

 

范例

1. 创建服务接口 Service

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public interface Service
   public String getName();
   public void execute();

2. 创建实体服务

Service1

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public class Service1 implements Service {
   public void execute(){
      System.out.println("Executing Service1");
   }
   @Override
   public String getName() {
      return "Service1";
   }

Service2

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public class Service2 implements Service {
   public void execute(){
      System.out.println("Executing Service2");
   }
   @Override
   public String getName() {
      return "Service2";
   }

3. 为 JNDI 查询创建 InitialContext

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public 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;      
   }

4. 创建缓存 Cache

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
import java.util.ArrayList;
import java.util.List;
public 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);
      }
   }

5. 创建服务定位器 ServiceLocator

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public 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;
   }

6. 使用 ServiceLocator 来演示服务定位器设计模式

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public class ServiceLocatorPatternDemo
   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();        
   }

编译运行以上 Java 范例,输出结果如下

$ javac -d . src/main/com.pottercoding/gof/ServiceLocatorPatternDemo.java
$ java  com.pottercoding.gof.ServiceLocatorPatternDemo
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