Password Strength Checker
Visual cues are great way to inform the user of the status of things in the application. Message boxes and alerts are used much too often for this purpose, but they usually end up irritating the user. A much smoother and enjoyable user experience is provided by subtly indicating to the user the status as an application is used. In this section, we are going to create an application that indicates the strength of a typed password to the user by the use of colors and checkboxes. We are going to use check-boxes very differently than their normal usage. This is an example of using GWT widgets in new and different ways, and mixing and matching them to provide a great user experience.
Time for Action—Creating the Checker
In the current day and age, passwords are required for almost everything, and choosing secure passwords is very important. There are numerous criteria suggested for creating a password that is secure from most common password cracking exploits. These criteria run the gamut from creating 15 letter passwords with a certain number of lower case and numeric digits to creating passwords using random password generators. In our example application, we are going to create a password strength checker that is very simple, and only checks the number of letters in the password. A password string that contains less than five letters will be considered weak, while a password that contains between five and seven letters will be considered to be of medium strength. Any password containing more than seven letters will be considered as strong. The criteria were deliberately kept simple so that we can focus on creating the application without getting all tangled up in the actual password strength criteria. This will help us to understand the concepts and then you can extend it to use any password strength criteria that your application warrants. This example uses a service to get the password strength, but this could also be done all on the client without needing to use a server.
1. Create a new Java file named PasswordStrengthService.java in the com.packtpub.gwtbook.samples.client package. Define a PasswordStrengthService interface with one method to retrieve the strength of a password string provided as a parameter to the method:
public interface PasswordStrengthService extends RemoteService
public int checkStrength(String password);
2. Create the asynchronous version of this service definition interface in a new Java file named PasswordStrengthServiceAsync.java in the com.packtpub.gwtbook.samples.client package :
public interface PasswordStrengthServiceAsync
public void checkStrength
(String password, AsyncCallback callback);
3. Create the implementation of our password strength service in a new Java file named PasswordStrengthServiceImpl.java in the com.packtpub.gwtbook.samples.server package.
public class PasswordStrengthServiceImpl extends
RemoteServiceServlet implements PasswordStrengthService
private int STRONG = 9;
private int MEDIUM = 6;
private int WEAK = 3;
public int checkStrength(String password)
if (password.length() <= 4)
else if (password.length() < 8)
4. Now let's create the user interface for this application. Create a new Java file named PasswordStrengthPanel.java in the com.packtpub.gwtbook.samples.client.panels package that extends the com.packtpub.gwtbook.samples.client.panels.SamplePanel class. Create a text box for entering the password string an ArrayList named strengthPanel for holding the checkboxes that we will use for displaying the strength of the password. Also create the PasswordStrengthService object.
public TextBox passwordText = new TextBox();
final PasswordStrengthServiceAsync pwStrengthService =
public ArrayList strength = new ArrayList();
5. Add a private method for clearing all the checkboxes by setting their style to the default style.
private void clearStrengthPanel()
for (Iterator iter = strength.iterator(); iter.hasNext();)
6. Add a private method that will return the CSS name, based on the password strength. This is a nice way for us to dynamically set the style for the checkbox, based on the strength.
private String getPasswordStrengthStyle(int passwordStrength)
if (passwordStrength == 3)
else if (passwordStrength == 6)
else if (passwordStrength == 9)
7. In the constructor for the PasswordStrengthPanel class, create a HorizontalPanel named strengthPanel, add nine checkboxes to it, and set its style. As mentioned before, the styles that we are using in the sample applications in this book are available in the file Samples.css, which is part of the source code distribution for this book. We also add these same checkboxes to the strength object, so that we can retrieve them later to set their state. These checkboxes will be used for displaying the password strength visually. Create a new VerticalPanel that we will use as the container for the widgets that we are adding to the user interface. Finally, create the service target and set its entry point.
HorizontalPanel strengthPanel = new HorizontalPanel();
for (int i = 0; i < 9; i++)
CheckBox singleBox = new CheckBox();
VerticalPanel workPanel = new VerticalPanel();
ServiceDefTarget endpoint=(ServiceDefTarget) pwStrengthService;
8. In the same constructor, set the style for the password text box, and add an event handler to listen for changes to the password box.
public void onKeyDown
(Widget sender, char keyCode, int modifiers)
public void onKeyPress
(Widget sender, char keyCode, int modifiers)
public void onKeyUp(Widget sender, char keyCode,
if (passwordText.getText().length() > 0)
AsyncCallback callback = new AsyncCallback()
public void onSuccess(Object result)
int checkedStrength = ((Integer) result).intValue();
for (int i = 0; i < checkedStrength; i++)
public void onFailure(Throwable caught)
Window.alert("Error calling the password strength
service." + caught.getMessage());
9. Finally, in the constructor, add the password text box and the strength panel to the work panel. Create a little info panel that displays descriptive text about this application, so that we can display this text when this sample is selected in the list of available samples in our Samples application. Add the info panel and the work panel to a dock panel, and initialize the widget.
HorizontalPanel infoPanel = new HorizontalPanel();
"<div class='infoProse'>Start typing a password
string. The strength of the password will be
checked and displayed below. Red indicates that the
password is Weak, Orange indicates a Medium
strength password and Green indicates a Strong
password. The algorithm for checking the strength
is very basic and checks the length of the password
DockPanel workPane = new DockPanel();
10. Add the service to the module file for the Samples application—Samples.gwt.xml in the com.packtpub.gwtbook.samples package.
<servlet path="/pwstrength" class=
Here is the user interface for the password strength checking application:
Now start typing a password string to check its strength. Here is the password strength when you type a password string that is less than five characters:
What Just Happened?
The password strength service checks the size of the provided string and returns an integer value of three, six, or nine based on whether it is weak, medium, or strong. It makes this determination by using the criteria that if the password string is less than five characters in length, it is weak, and if it is more than five characters but not greater than seven characters, it is considered a medium strength password. Anything over seven characters is considered to be a strong password.
The user interface consists of a text box for entering a password string and a panel containing nine checkboxes that visually displays the strength of the typed string as a password. An event handler is registered to listen for keyboard events generated by the password text box. Whenever the password text changes, which happens when we type into the field or change a character in the field, we communicate asynchronously with the password strength service and retrieve the strength of the given string as a password. The returned strength is displayed to the user in a visual fashion by the use of colors to symbolize the three different password strengths.
The password strength is displayed in a compound widget that is created by adding nine checkboxes to a HorizontalPanel. The color of the checkboxes is changed using CSS depending on the strength of the password string. This process of combining the basic widgets provided by GWT into more complex widgets to build user interfaces is a common pattern in building GWT applications. It is possible to build quite intricate user interfaces in this way by utilizing the power of the GWT framework.
In the current day and age, passwords are required for almost everything, and choosing secure passwords is very important. In this article, we implemented a password strength checker in Google Web Toolkit (GWT) and AJAX. By going through the article, the reader can also get a general idea of implementing other interactive user forms.