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Beans and Form processing

 
  Forms are a very common method of interactions in web sites.  JSP makes forms processing specially easy.

The standard way of handling forms in JSP is to define a "bean".  This is not a full Java bean.  You just need to define a class that has a field corresponding to each field in the form.  The class fields must have "setters" that match the names of the form fields.  For instance, let us modify our GetName.html to also collect email address and age.

The new version of GetName.html is

<HTML>
<BODY>
<FORM METHOD=POST ACTION="SaveName.jsp">
What's your name? <INPUT TYPE=TEXT NAME=username SIZE=20><BR>
What's your e-mail address? <INPUT TYPE=TEXT NAME=email SIZE=20><BR>
What's your age? <INPUT TYPE=TEXT NAME=age SIZE=4>
<P><INPUT TYPE=SUBMIT>
</FORM>
</BODY>
</HTML>
To collect this data, we define a Java class with fields "username", "email" and "age" and we provide setter methods "setUsername", "setEmail" and "setAge", as shown.  A "setter" method is just a method that starts with "set" followed by the name of the field.  The first character of the field name is upper-cased.  So if the field is "email", its "setter" method will be "setEmail".  Getter methods are defined similarly, with "get" instead of "set".   Note that the setters  (and getters) must be public.

package user;

public class UserData {
    String username;
    String email;
    int age;

    public void setUsername( String value )
    {
        username = value;
    }

    public void setEmail( String value )
    {
        email = value;
    }

    public void setAge( int value )
    {
        age = value;
    }

    public String getUsername() { return username; }

    public String getEmail() { return email; }

    public int getAge() { return age; }
}
The method names must be exactly as shown.  Once you have defined the class, compile it and make sure it is available in the web-server's classpath.  The server may also define special folders where you can place bean classes, e.g. with Blazix you can place them in the "classes" folder.  If you have to change the classpath, the web-server would need to be stopped and restarted if it is already running.  (If you are not familiar with setting/changing classpath, see notes on changing classpath.)

Note that we are using the package name user, therefore the file UserData.class must be placed in a folder named user under the classpath entry.

Now let us change "SaveName.jsp" to use a bean to collect the data.

<jsp:useBean id="user" class="user.UserData" scope="session"/>
<jsp:setProperty name="user" property="*"/> 
<HTML>
<BODY>
<A HREF="NextPage.jsp">Continue</A>
</BODY>
</HTML>
All we need to do now is to add the jsp:useBean tag and the jsp:setProperty tag!  The useBean tag will look for an instance of the "user.UserData" in the session.  If the instance is already there, it will update the old instance.  Otherwise, it will create a new instance of user.UserData (the instance of the user.UserData is called a bean), and put it in the session.

The setProperty tag will automatically collect the input data, match names against the bean method names, and place the data in the bean! 

Let us modify NextPage.jsp to retrieve the data from bean..

<jsp:useBean id="user" class="user.UserData" scope="session"/> 
<HTML>
<BODY>
You entered<BR>
Name: <%= user.getUsername() %><BR>
Email: <%= user.getEmail() %><BR>
Age: <%= user.getAge() %><BR>
</BODY>
</HTML>
Notice that the same useBean tag is repeated.  The bean is available as the variable named "user" of class "user.UserData".  The data entered by the user is all collected in the bean.

We do not actually need the "SaveName.jsp", the target of GetName.html could have been NextPage.jsp, and the data would still be available the same way as long as we added a jsp:setProperty tag.  But in the next tutorial, we will actually use SaveName.jsp as an error handler that automatically forwards the request to NextPage.jsp, or asks the user to correct the erroneous data.

Exercise:  1)  Write a JSP/HTML set that allows a user to enter the name of a system property, and then displays the value returned by System.getProperty for that property name (handle errors appripriately.)   2)  Go back to the exercises where you manually modified boolean variables.  Instead of a boolean variable, make these come from a HIDDEN form field that can be set to true or false.
 

 

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