设计模式笔记 – 服务器定位模式

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。

示例

我们将创建 ServiceLocator、InitialContext、Cache、Service 作为表示实体的各种对象。Service1 和 Service2 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。

Java 实现

创建服务接口 Service

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

创建实体服务

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

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

为 JNDI 查询创建 InitialContext

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;
    }
}

创建缓存 Cache

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);
        }
    }
}

创建服务定位器

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;
    }
}

使用 ServiceLocator 来演示服务定位器设计模式

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();
    }
}

Python 实现

import abc

class Service(abc.ABC):
    @abc.abstractmethod
    def get_name(self) -> str: ...

    @abc.abstractmethod
    def execute(self): ...

class Service1(Service):
    def execute(self):
        print("Executing Service1")

    def get_name(self) -> str:
        return "Service1"

class Service2(Service):
    def execute(self):
        print("Executing Service2")

    def get_name(self) -> str:
        return "Service2"

class InitialContext:
    def lookup(self, jndi_name: str) -> Service:
        if jndi_name.upper() == "SERVICE1":
            print("Looking up and creating a new Service1 object")
            return Service1()
        if jndi_name.upper() == "SERVICE2":
            print("Looking up and creating a new Service2 object")
            return Service2()
        return None

class Cache:
    def __init__(self):
        self.services = []

    def get_service(self, service_name: str) -> Service:
        for service in self.services:
            if service_name.upper() == service.get_name().upper():
                print(f"Returning cached {service_name} object")
                return service
        return None

    def add_service(self, new_service: Service):
        exists = False
        for service in self.services:
            if new_service.get_name().upper() == service.get_name().upper():
                exists = True
        if not exists:
            self.services.append(new_service)

class ServiceLocator:
    cache = Cache()

    @classmethod
    def get_service(cls, jndi_name: str) -> Service:
        service = cls.cache.get_service(jndi_name)
        if service is None:
            context = InitialContext()
            service = context.lookup(jndi_name)
            cls.cache.add_service(service)
        return service

if __name__ == "__main__":
    service = ServiceLocator.get_service("Service1")
    service.execute()
    service = ServiceLocator.get_service("Service2")
    service.execute()
    service = ServiceLocator.get_service("Service1")
    service.execute()
    service = ServiceLocator.get_service("Service2")
    service.execute()