This techniques can not be used in distributed environments. If we use it we’ll have one injector instance on each JVM because on distributed environments for each JVM there is one ServletContext. ServletContext on one JVM is not visible from another JVM, so if we can not use guice scopes distributed on multiple machines.

Initializing Guice Injector in ServletContext

As we said there is only one instance of javax.servlet.ServletContext(as long as we don’t have a distributed application). We can trigger the initialization of the ServletContext using ServletContextListener. All we have to do is to implement the ServletContextListener.contextInitialized to create the google guice injector, and to set it as an attribute in the ServletContext:

package com.exam.web.guice;

import java.lang.reflect.InvocationTargetException;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

import com.google.inject.Guice;
import com.google.inject.Injector;
import com.google.inject.Module;

public class GuiceServletContextListener implements ServletContextListener {
    public static final String KEY = Injector.class.getName(); 

    public void contextInitialized(ServletContextEvent servletContextEvent)
    {
    	servletContextEvent.getServletContext()
    		.setAttribute(KEY, getInjector(servletContextEvent.getServletContext()));
    } 

    public void contextDestroyed(ServletContextEvent servletContextEvent) {
    	servletContextEvent.getServletContext().removeAttribute(KEY);
    } 

    @SuppressWarnings("unchecked")
    private Injector getInjector(ServletContext servletContext) {
        String className = servletContext.getInitParameter("module");
        try {
            Class module = (Class) Class.forName(className);
            return Guice.createInjector(module.getConstructor().newInstance());
        }
        catch (ClassNotFoundException e) { throw new RuntimeException(e); }
        catch (NoSuchMethodException e) { throw new RuntimeException(e); }
        catch (InvocationTargetException e) { throw new RuntimeException(e); }
        catch (IllegalAccessException e) { throw new RuntimeException(e); }
        catch (InstantiationException e) { throw new RuntimeException(e); }
    }
}

Then we have to change the web.xml to define the listener to be used by the web application container:

            com.exam.web.guice.GuiceServletContextListener
        
    

Storing module configuration in web.xml

We use web.xml to define which module to be used by Guice in block. When we create the guice injector the parameter is read and used:

String className = servletContext.getInitParameter("module");

The class name is defined like this in web.xml:

     
module
com.webapplication.bus.HibernateDaoModule
        Guice Module to be used for the servlets app
    

Finally in the servlet we can use this expression to get the injector:
(Injector) servletContext.getAttribute(GuiceServletContextListener.KEY)

We can add an injector field to the servlet and “inject” the injector into it when the servlet is initialized(a super servlet class can be created to define this code in only one place):

public class MyServlet  extends HttpServlet {

	Injector injector;
        ...

	@Override
	public void init(ServletConfig config) throws ServletException {

        HibernateUtil.Configure(true);  

	    ServletContext servletContext = config.getServletContext();
	    injector = (Injector) servletContext.getAttribute(GuiceServletContextListener.KEY);
	    injector.injectMembers(this);

	    managersRegistry = injector.getInstance(ManagersRegistry.class);

		super.init(config);
	}
}

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Context Parameters

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
  <context-param>
    <description>This is a context parameter example</description>
    <param-name>ContextParam</param-name>
    <param-value>ContextParam value</param-value>
  </context-param>
...
<web-app>

The following code can be be invoked from a servlet or a filter to retrieve the ContextParam value. The parameter can be read successfully from any servlet or filter class.

@Override
	public void init(ServletConfig config) throws ServletException {
		String contextParam = config.getServletContext().getInitParameter("ContextParam");
	}

//or
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		String contextParam = this.getServletContext().getInitParameter("ContextParam");
		...
	}
	...
}

String value = this.getServletContext().getInitParameter("ContextParam");

Init Parameters

...
<servlet>
    <servlet-name>A Servlet</servlet-name>
    <servlet-class>com.controller.TestServlet</servlet-class>
    <init-param> 
        <description>This is an init parameter example</description> 
        <param-name>InitParam</param-name> 
        <param-value>init param value</param-value> 
    </init-param> 
</servlet>
...

The following code can be be invoked to retrieve the value of InitParam. The parameter can be accessed only from com.controller.TestServlet.

public class DefaultController extends HttpServlet
{
	@Override
	public void init(ServletConfig config) throws ServletException {
		String initParam  = config.getInitParameter("InitParam");
	}

//or
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		String initParam = getServletConfig().getInitParameter("InitParam");
		...
	}
	...
}

Iterating through context-params and init-params

It’s possible to iterate through the paramters if required:

public class DefaultController extends HttpServlet
{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		Enumeration contextParams = getServletContext().getInitParameterNames();

		Sytem.out.println("context-params: ");
		while (params.hasMoreElements()) {
			String name = (String) params.nextElement();
			Sytem.out.println(name + " = " + config.getInitParameter(name));
		}

		Enumeration initParams = getServletConfig().getInitParameterNames();

		Sytem.out.println("init-params: ");
		while (params.hasMoreElements()) {
			String name = (String) params.nextElement();
			Sytem.out.println(name + " = " + config.getInitParameter(name));
		}

		...
	}
	...
}

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