JSP bean tag for property that might not exist

Question

In JSP I can reference a bean's property by using the tag ${object.property}

Is there some way to deal with properties that might not exist? I have a JSP page that needs to deal with different types. Example:

public class Person {
    public String getName()
}
public class Employee extends Person {
    public float getSalary()
}

In JSP I want to display a table of people with columns of name and salary. If the person is not an employee then salary should be blank. The row HTML might look like:

<tr>
    <td><c:out value="${person.name}"></td>
    <td><c:out value="${person.salary}"></td>
</tr>

Unfortunately if person is not an employee then it can't find salary and an error occurs. How would I solve this in JSP?

Edit: Is there an instanceof check in JSP tag language?

Answer 1

You could always have a type field.

public class Person {
    public String getType() { return "Person"; }
    public String getName()
}
public class Employee extends Person {
    public String getType() { return "Employee"; }
    public float getSalary()
}

Your JSP would look like

<tr>
    <td><c:out value="${person.name}"></td>
    <td><c:if test="'Employee' eq person.type"><c:out value="${person.salary}"></c:if></td>
</tr>

Of course the Class class already has this...

<tr>
    <td><c:out value="${person.name}"></td>
    <td><c:if test="'Employee' eq person.class.simpleName"><c:out value="${person.salary}"></c:if></td>
</tr>

You could also have `isEmployee()' method.

Answer 2

One method would be to create a custom tag library and use polymorphism within it to handle the case where a Person is-a Employee.

I haven't done this in a while for JSP, but frequently use a similar technique in GSP (Groovy/Grails Server Pages).

Otherwise, you could put some logic in the JSP (not ideal) to test for Employee-ness:

<% 
   String salary
   if (person instanceof Employee) {
       salary = person.salary
   } else {
       salary = "" // or '&nbsp;'
   }
%>
<td><c:out value="${salary}"></td>

Answer 3

Just use the EL empty operator IF it was a scoped attribute, unfortunately you'll have to go with surrounding your expression using employee.salary with <c:catch>:

<c:catch var="err">
    <c:out value="${employee.salary}"/>
</c:catch>

If you really need instanceof, you might consider a custom tag.a

Answer 4

If you want the class, just use ${person.class}. You can also use ${person.class.name eq 'my.package.PersonClass'}

You can also use the "default" on c:out.

 <c:out value='${person.salary}' default="Null Value" />

Answer 5

Concise, but unchecked.

<tr>
    <td>${person.name}</td>    
    <td>${person.class.simpleName == 'Employee' ? person.salary : ''}</td>
</tr>

Is there an instanceof check in JSP tag language?

Not at the moment of this writing. I read somewhere that they have reserved it, instanceof keyword, in EL, may be for future. Moreover, there is a library available that has this specific tag. Look into that before deciding to create some custom tag for yourself. Here is the link, Unstandard Tag Library.