question

[Romy Huang]

Which of the given statements are correct regarding the following JSP page code?

<jsp:useBean id="mystring" class="java.lang.String" />
<jsp:setProperty name="mystring" property="*" />
<%=mystring%>

Assume that the request for this page contains a parameter mystring=hello.

A. It will print "".
B. It will print "hello"
C. It will not compile.
D. It will throw exception at runtime.

why choose A?

[Colin Fletcher]

<jsp:setProperty name="mystring" property="*" />

Tells the container to set the property named "*" (so all) to <null>

[Jose Esteban]

I think the answer to the question needs further explanation.

<jsp:setProperty name="mystring" property="*" />

Colin, I don't think that the semantics of jsp:setProperty is what you say:
"Tells the container to set the property named "*" (so all) to <null>"

I think, according to the JSP 2.0 spec (page 143), if property is set to * the tag will:
1) iterate over the current ServletRequest parameters (mystring in this example),
2) match parameter names (mystring) and value type(s) to property names and setter method type(s) (no matches in this example, since in a String there's no mystring property),
3) set each matched property to the value of the matching parameter (nothing to do, since there was no matches in 2))

So, in my opinion, the tag jsp:setProperty leave the bean mystring untouched and <%=mystring%> should print:
a) If jsp:useBean finds the bean mystring in the page scope: whatever the value of mystring is.
b) else: "", because jsp:useBean instantiates a new bean (using the no-args constructor).

So I think that there's no a totally correct answer among the options A to D.
[ February 28, 2005: Message edited by: Jose Esteban ]

[Gopal Desai]

The jsp:setProperty action in this question tries to set all the properties of the bean from the values present in the request. However, the String class does not have any method named setMystring() and so the page prints "".

[Jose Esteban]

But, what happens if there was an attribute named "mystrig" in the page scope with a value of "Hi".

Then, <%=mystring%> should print "Hi", isn't it?

[Colin Fletcher]

Jose, you are right about both.

<jsp:setProperty name="mystring" property="*" /> will match request parameters with the attributes of the mystring object and set them accordingly.

So if we did this:
html page: a.html
<form method=post action=b.jsp>
<input type=text name=myName>
<input type=submit value=' submit '>
</form>

b.jsp:

<jsp:useBean id="myobject" type="com.example.myobject">
<jsp:setProperty name="myobject" property="*" />
</jsp:useBean>

<BR><BR>
<jsp:getProperty name="myobject" property="myName" />


-C