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javajspscriptlet

When do (jsp) scriptlets run their (Java) code?

I was working through a null pointer exception on code like the following:

<%
SessionData session = getSessionData(request);
Webpage webPage = null;
if (session!= null) {
    webPage = session.getWebPage();
}
%>

<script type="text/javascript"> 

//NullPointer happens here, webPage is null when the session is lost
<tags:ComboBox
    comboBox="<%=webPage.getComboBox()%>" />

</script>

I was surprised when I could move the ending of if (session!=null to after the javascript, which seems to ignore that code when the session was null.

<%
SessionData session = getSessionData(request);
Webpage webPage = null;
if (session!= null) {
    webPage = session.getWebPage();
//} move this to below
%>

<script type="text/javascript"> 

//NullPointer happens here, webPage is null when the session is lost
<tags:ComboBox
    comboBox="<%=webPage.getComboBox()%>" />

</script>
<% } %> //moved to here

Does the scriptlet for the ComboBox tag, inside the brackets, no longer run? I would think it would still try to get the combobox off the webpage, and still end up getting a null pointer. Am I incorrect in thinking that scriptlets all get their values before the code is actually ran?

(just thought I'd mention, there is an included script which redirects the page if there is no session. I get a NullPointer with the first section of code, and correctly redirect with the second section)

Solution

  • A JSP is compiled to a servlet on-the-fly by the servlet container.

    This compilation is actually simple kind of inversion:

    TEXT1
<% java code %>
TEXT2
<%= java expression %>
TEXT3

    That is compiled to:

    out.print("TEXT1");
java code
out.print("TEXT2");
out.print(java expression);
out.print("TEXT3");

    So when you say:

    TEXT1
<% if (true) { %>
TEXT2
<% } %>
TEXT3

    You get:

    out.print("TEXT1");
if (true) {
out.print("TEXT2");
}
out.print("TEXT3");

    The above examples are minified for clarity, e.g. newlines are ignored, the boilerplate servlet setup is not included, and the complexity of tag library execution is not covered.