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>();
  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>();
  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) {
  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 "" 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.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 = "";
  input = HelloBean.class.getClassLoader().getResourceAsStream(filename);

  CountryService countryService = (CountryService) ServiceLocator.getInstance()
        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=""
 <title>Service Locator</title>
 <h3>Lis of countries:</h3>
 <h:dataTable value="#{helloBean.countries}" var="country">
         <h:outputText value="#{country}" />


7. Test (on Tomcat v7.0)


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

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

Use the "CountryService2" by changing in "", 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 "" 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".

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].



Popular posts from this blog

Why Business Rules Engine?

"A good DSL minimizes the 'communication gap' between a domain concept and the code that implements it" - Robert C. Martin
A Use Case It looks like adopting a new technology is always driven by a business need.

The organizations, such as banks, have their own business processes (such as data gathering and document management). These processes are different from others but usually the differences are not big. How do we build a product which can be reused the similar features but still be adaptable with the specific requirements of each bank? Then, the answer is we should have a library/framework. There is an idea that we can implement this library/framework by using a business rules engine. Which has the following benefits:
Be able to modify implementation of business domain at runtime (such as XML, CSV)Source code is more readable for both developers and domain experts. Therefore, source code can be consider it as a document Sine Business Rules Engines support for Dom…

Strategy Design Pattern

For example, I have an program with an Animal abstract class and two sub-classes Dog and Bird. I want to add a new behavior for the class Animal, this is "fly".  Now, I face to two approaches to solve this issue:

1. Adding an abstract method "fly" into the class Animal. Then, I force the sub-classes should be implemented this method, something like:

public abstract class Animal{ //bla bla public abstract void fly(); } public class Bird extends Animal{ //bla bla public void fly(){ System.out.println("Fly high"); } } public class Dog extends Animal{ //bla bla public void fly(){ System.out.println("Cant fly"); } }
2. Creating an interfaces with method "fly" inside. The same issue to abstract class, I force the classes these implement this interface should have a method "fly" inside:

public interface Flyable{ public void fly(); } public class Bird implements Flyable{ //bla bla public void fly(){ System.out.println…

Styling Sort Icons Using Font Awesome for Primefaces' Data Table

So far, Primefaces has used image sprites for displaying the sort icons. This leads a problem if we want to make a different style for these icons; for example, I would make the icon "arrow up" more blurry at the first time table loading because I want to highlight the icon "arrow down". I found a way that I can replace these icons to Font Awesome icons.

We will use "CSS Pseudo-classes" to archive it. The hardest thing here is that we should handle displaying icons in different cases. There is a case both "arrow up" and "arrow down" showing and other case is only one of these icons is shown.

.ui-sortable-column-icon.ui-icon.ui-icon-carat-2-n-s { background-image: none; margin-left: 5px; font-size: 1.1666em; position: relative; } .ui-sortable-column-icon.ui-icon.ui-icon-carat-2-n-s:not(.ui-icon-triangle-1-s)::before { content: "\f106"; font-family: "FontAwesome"; position: absolute; t…

Attribute 'for' of label component with id xxxx is not defined

I got the warning in the log file when I have used the tag <h:outputLabel> without attribute "for" in xhtml file. It was really polluting my server log files.

The logged information actually makes sense anyway! We could find an answer as the following:

"Having h:outputLabel without a "for" attribute is meaningless. If you are not attaching the label, you should be using h:outputText instead of h:outputLabel."

However, these solutions are not possible just for my situation. Instead of using h:outputText for only displaying text, my team has used h:outputLabel too many places. We were nearly in our release time (next day) so it is quite risky and takes much efforts if we try to correct it. Because the style (with CSS) is already done with h:ouputLabel. The alternative by adding attribute "for" the existing h:outputLabel is not reasonable either. I really need to find another solution.
Fortunately, I came across a way if I change to use p:out…

AngularJS - Build a custom validation directive for using multiple emails in textarea

AngularJS already supports the built-in validation with text input with type email. Something simple likes the following:
<input name="input" ng-model="email.text" required="" type="email" /> <span class="error" ng-show="myForm.input.$"> Not valid email!</span>
However, I used a text area and I wanted to enter some email addresses that's saparated by a comma (,). I had a short research and it looked like AngualarJS has not supported this functionality so far. Therefore, I needed to build a custom directive that I could add my own validation functions. My validation was done only on client side, so I used the $validators object.

Note that, there is the $asyncValidators object which handles asynchronous validation, such as making an $http request to the backend.

This is just my implementation on my project. In order to understand that, I supposed you already had experiences with Angular…