Thursday, May 19, 2011

Free resources associated with web clients

Imagine we have the following problem. Resources are kept open for web clients as long as the clients are using them. When they leave, the resources have to be closed.

Technically, we can say as long as the HttpSession is alive in the web container, the resources are kept open. When the HttpSession is invalidated, we need to call a method that frees the resources.

A solution is to use the HttpSessionBindingListener to solve this problem.

The idea is to create a class that implements this interface. The unbound method contains or refers to the clean-up code. Whenever resources are opened, an instance of this class is created and saved to the corresponding session. When the session invalidates, which can happen by timeout, the web container automatically calls the method of the object in order to free open resources.

The class:

package com.javaeenotes;

import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

public class Janitor implements HttpSessionBindingListener {

public void valueBound(HttpSessionBindingEvent arg0) {

public void valueUnbound(HttpSessionBindingEvent arg0) {
// Start some cleaning here.

Use the HttpSession.setAttribute() method to save an instance of this class to the session.

Saturday, May 14, 2011

Creating a Modal Window page with Apache Wicket

This post aims to clarify creating a modal window with Apache Wicket. Before continuing, minimum basic knowledge about Apache Wicket is required. A modal window is a child window on top of the main window. The modal window requires the user to interact with it, before the user can return to the main window.

The application we're developing consists of two pages:

  • The main page ( and LaunchPage.html)
  • The modal window page ( and ModalWContentPage.html)

The main page defines and displays the content of a variable and it also has a link to open the modal window page.

The modal window page displays the content of the same variable. But when the modal window is closed, the variable is changed by the modal window page.

Let's start by creating the HTML-file of the main page.

<title>Launch Page</title>
<div wicket:id="modal"></div>
<span wicket:id="passValueLabel">Value of passValue variable.</span>
<a wicket:id="showModalLink">Open modal window.</a>

This page defines the modal page with id "modal". The next line is the location where the content of the variable "passValue" is displayed. Finally, a link is defined that opens the modal window.

Next, the corresponding Java class:

package com.javaeenotes;

import org.apache.wicket.Page;
import org.apache.wicket.ajax.AjaxRequestTarget;
import org.apache.wicket.ajax.markup.html.AjaxLink;
import org.apache.wicket.extensions.ajax.markup.html.modal.ModalWindow;
import org.apache.wicket.markup.html.WebPage;
import org.apache.wicket.markup.html.basic.Label;
import org.apache.wicket.model.PropertyModel;

public class LaunchPage extends WebPage {

private String passValue;

public LaunchPage() {

passValue = "This value is passed to the modal window.";

// Display the current content of the passValue variable. The
// PropertyModel must be used, as the value can be changed.
final Label passValueLabel;
add(passValueLabel = new Label("passValueLabel",
new PropertyModel<String>(this, "passValue")));

// Create the modal window.
final ModalWindow modal;
add(modal = new ModalWindow("modal"));

modal.setPageCreator(new ModalWindow.PageCreator() {
public Page createPage() {
// Use this constructor to pass a reference of this page.
return new ModalContentPage(LaunchPage.this.getPageReference(),
modal.setWindowClosedCallback(new ModalWindow.WindowClosedCallback() {
public void onClose(AjaxRequestTarget target) {
// The variable passValue might be changed by the modal window.
// We need this to update the view of this page.
modal.setCloseButtonCallback(new ModalWindow.CloseButtonCallback() {
public boolean onCloseButtonClicked(AjaxRequestTarget target) {
// Change the passValue variable when modal window is closed.
setPassValue("Modal window is closed by user.");
return true;

// Add the link that opens the modal window.
add(new AjaxLink<Void>("showModalLink") {
public void onClick(AjaxRequestTarget target) {;

public String getPassValue() {
return passValue;

public void setPassValue(String passValue) {
this.passValue = passValue;

The class file displays the content of "passValue" and creates the modal window class with its methods. Look carefully at the code and comments in the modal window methods. We can see that the passValueLabel is expected to change in the window close callback method. We can also see that the variable is actually changed using a setter in the close button callback method. This happens when the user clicks on the close button of the modal window. Finally, an AJAX link is defined and added to the page.

The second part is the modal window page:

<title>Modal Content Page</title>
<span wicket:id="passValueLabel">Current content of passValue variable.</span>

The modal page only displays the content of the "passValue" variable. The variable is owned by the main page.

package com.javaeenotes;

import org.apache.wicket.PageReference;
import org.apache.wicket.extensions.ajax.markup.html.modal.ModalWindow;
import org.apache.wicket.markup.html.WebPage;
import org.apache.wicket.markup.html.basic.Label;

public class ModalContentPage extends WebPage {

public ModalContentPage(final PageReference modalWindowPage,
final ModalWindow window) {

// Retrieve the passValue content for display.
String passValue = ((LaunchPage) modalWindowPage.getPage())
add(new Label("passValueLabel", passValue));

// You can use the
// ((LaunchPage)modalWindowPage.getPage()).setPassValue() method to
// change the passValue variable of the launch/caller page.

This class is pretty simple. The first parameter refers to the main page that created the modal window page. Go back, and look how the modal window page is constructed in the main page. The main page is casted and the variable is retrieved using the getter-method. The content is then displayed in the modal window page.

Now run the code and the main page will look like this:

After clicking the link, the modal window page is opened:

We can see the same variable is displayed in the modal window page. Now, we close the modal window page. Because of this event, the variable is changed as instructed in the callback code:

As expected, the variable is changed in the main window.

Thursday, May 12, 2011

Java Practices

A came across a very nice site with a collection of best practices in Java development:

Definitely worth a read!