Skip to main content

Changing source code at run-time with Service Locator pattern

I have a service to get some data but the result can be different basing on where the implementation is. Technically, I have two or more concrete implementation of an interface and I am able to switch using these concrete classes at run-time. That means I have a place to configure it without re-deploying the application. In order to overcome this issue, I use Service Locator design pattern and here I only care about two advantages below:
  • Encapsulating the specific implementation, we just declare the name and don't care about the implementation of the service.
  • Changing the implementation at run-time.



Client: an object that invokes the services via Service Locator
Business services: services that is used by Client.


Once again, I used the JFS Helloworld example from previous post for this example.

1. Create a service interface "CountryService" and two concrete classes "CountryService1" and "CountryService2" (Business services)

package vn.nvanhuong.servicelocator.service;

import java.util.List;

public interface CountryService{
 public List<String> getCountries();
}

The first concrete service:
package vn.nvanhuong.servicelocator.service.impl;

import java.util.ArrayList;
import java.util.List;

import vn.nvanhuong.servicelocator.service.CountryService;

public class CountryService1 implements CountryService{

 public List<String> getCountries() {
  List<String> result = new ArrayList<String>();
  result.add("Vietname");
  result.add("Switzerland");
  result.add("Japan");
  return result;
 }

}

The second concrete service:
package vn.nvanhuong.servicelocator.service.impl;

import java.util.ArrayList;
import java.util.List;

import vn.nvanhuong.servicelocator.service.CountryService;

public class CountryService2 implements CountryService{

 public List<String> getCountries() {
  List<String> result = new ArrayList<String>();
  result.add("Vietname");
  result.add("America");
  result.add("China");
  return result;
 }

}

2. Create class "InitialContext" that is used for looking up and creating classes basing on the provided names
package vn.nvanhuong.servicelocator;

import java.lang.reflect.Constructor;

public class IntitialContext {
 
 public Object lookup(String serviceName){
    
  if(serviceName != null){
   try {
    Class<?> clazz = Class.forName(serviceName);
    Constructor<?> ctor = clazz.getConstructor();
    Object object = ctor.newInstance();
    return object;
   } catch (Exception ex) {
    ex.printStackTrace();
   } 
  }
  return null;
 }

}


3. Create class "ServiceLocator", I used Singleton to cache the object.
package vn.nvanhuong.servicelocator;

public class ServiceLocator {
 private static ServiceLocator instance;
 
 private ServiceLocator(){}
 
 public static synchronized ServiceLocator getInstance(){
  if(instance == null){
   return new ServiceLocator();
  }
  return instance;
 }

 public Object getService(String serviceName) {
  IntitialContext initialContext = new IntitialContext();
  return initialContext.lookup(serviceName);
 }

}

4. Create a resource file "services.properties" in order to configure the changing implementation at run-time. :)
country = vn.nvanhuong.servicelocator.service.impl.CountryService1
language = vn.nvanhuong.servicelocator.service.impl.LanguageService1

5. Calling SeviceLocator in Managed bean (Client)
package vn.nvanhuong.servicelocator.bean;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import java.util.Properties;

import javax.faces.bean.ManagedBean;

import vn.nvanhuong.servicelocator.ServiceLocator;
import vn.nvanhuong.servicelocator.service.CountryService;

@ManagedBean(name = "helloBean")
public class HelloBean {
 private List<String> countries;
 
 public HelloBean() throws IOException {
  Properties prop = new Properties();
     InputStream input = null;
     
     String filename = "services.properties";
  input = HelloBean.class.getClassLoader().getResourceAsStream(filename);

  prop.load(input);
  CountryService countryService = (CountryService) ServiceLocator.getInstance()
              .getService(prop.getProperty("country"));
        this.countries = countryService.getCountries();
  
 }

 public List<String> getCountries() {
  return countries;
 }

 public void setCountries(List<String> countries) {
  this.countries = countries;
 }
 
}
6. GUI code: index.xhtml
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml"
 xmlns:f="http://java.sun.com/jsf/core"
 xmlns:h="http://java.sun.com/jsf/html"
 xmlns:ui="http://java.sun.com/jsf/facelets">
<h:head>
 <title>Service Locator</title>
</h:head>
<h:body>
 <h3>Lis of countries:</h3>
 <h:dataTable value="#{helloBean.countries}" var="country">
   <h:column>
         <h:outputText value="#{country}" />
     </h:column>
 </h:dataTable>

</h:body>
</html>

7. Test (on Tomcat v7.0)

http://localhost:8080/service_locator/

Use the "CountryService1" by changing in "services.properties", don't need to restart the server.

country = vn.nvanhuong.servicelocator.service.impl.CountryService1




Use the "CountryService2" by changing in "services.properties", don't need to restart the server.

country = vn.nvanhuong.servicelocator.service.impl.CountryService2

Here we also can add more concrete classes of CountryService and just declare in file "services.properties" for using. For this reason, I use Java Reflection to create the objects basing on the provided names in general way otherwise we have to change the InitialContext whenever we want to add new concrete class of "CountryService".

Note:
There might have several ways to implement this pattern but the idea is general so that my example is only a case. We also can improve my implementation by using caching technique, see reference [3].


Reference:

Comments

Popular posts from this blog

Coders are NERDS | Learning English with Podcast

Let's learn three English vocabulary words based on real-life context through a humorous video about the life of software coders, especially at big tech companies when they work from home. Credit to Joma Tech. 🤓

Junit - Test fails on French or German string assertion

In my previous post about building a regex to check a text without special characters but allow German and French . I met a problem that the unit test works fine on my machine using Eclipse, but it was fail when running on Jenkins' build job. Here is my test: @Test public void shouldAllowFrenchAndGermanCharacters(){ String source = "ÄäÖöÜüß áÁàÀâÂéÉèÈêÊîÎçÇ"; assertFalse(SpecialCharactersUtils.isExistSpecialCharater(source)); } Production code: public static boolean isExistNotAllowedCharacters(String source){ Pattern regex = Pattern.compile("^[a-zA-Z_0-9_ÄäÖöÜüß áÁàÀâÂéÉèÈêÊîÎçÇ]*$"); Matcher matcher = regex.matcher(source); return !matcher.matches(); } The result likes the following: Failed tests: SpecialCharactersUtilsTest.shouldAllowFrenchAndGermanCharacters:32 null A guy from stackoverflow.com says: "This is probably due to the default encoding used for your Java source files. The ö in the string literal in the J...

[Snippet] CSS - Child element overlap parent

I searched from somewhere and found that a lot of people says a basic concept for implementing this feature looks like below: HTML code: <div id="parent">  <div id="child">  </div> </div> And, CSS: #parent{   position: relative;   overflow:hidden; } #child{   position: absolute;   top: -1;   right: -1px; } However, I had a lot of grand-parents in my case and the above code didn't work. Therefore, I needed an alternative. I presumed that my app uses Boostrap and AngularJs, maybe some CSS from them affects mine. I didn't know exactly the problem, but I believed when all CSS is loaded into my browser, I could completely handle it. www.tom-collinson.com I tried to create an example to investigated this problem by Fiddle . Accidentally, I just changed: position: parent; to position: static; for one of parents -> the problem is solved. Look at my code: <div class="modal-body dn-placeholder-parent-positi...

Functional programming in Java 8

In my previous post , we discussed about why we should consider to use functional programming. Now, let's delve into what functional programming in Java is. What is pure functional programming? Shortly,  f unctional programming is programming using functions. A function corresponds to a mathematical function such as log, sin. Basically, it takes zero or more arguments, give one or more result, and has no side effects. We can't completely program in pure functional style in Java Why?  For example, calling Scanner.nextLine twice typically gives different result. So, it's just called "functional-style programming". How is that? - There is no mutating structures visible to callers. That means your side effect may not be visible to a program, but it's visible to the programmer in terms of slower execution. - A function or method shouldn't throw any exceptions (follows the concept "pass arguments, return result"). We can use types like Opti...

The HelloWorld example of JSF 2.2 with Myfaces

I just did by myself create a very simple app "HelloWorld" of JSF 2.2 with a concrete implementation Myfaces that we can use it later on for our further JSF trying out. I attached the source code link at the end part. Just follow these steps below: 1. Create a Maven project in Eclipse (Kepler) with a simple Java web application archetype "maven-archetype-webapp". Maven should be the best choice for managing the dependencies , so far. JSF is a web framework that is the reason why I chose the mentioned archetype for my example. 2. Import dependencies for JSF implementation - Myfaces (v2.2.10) into file pom.xml . The following code that is easy to find from  http://mvnrepository.com/  with key words "myfaces". <dependency> <groupId>org.apache.myfaces.core</groupId> <artifactId>myfaces-api</artifactId> <version>2.2.10</version> </dependency> <dependency> <groupId>org.apache.myfaces.core<...