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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