How can I pass an object to a JSP tag?

问题:

I have a JSP page that contains a scriplet where I instantiate an object. I would like to pass that object to the JSP tag without using any cache.

For example I would like to accomplish this:

<%@ taglib prefix="wf" uri="JspCustomTag" %>

<% 
 Object myObject = new Object();
%>

<wf:my-tag obj=myObject />

I'm trying to avoid directly interacting with any of the caches (page, session, servletcontext), I would rather have my tag handle that.

回答1:

A slightly different question that I looked for here: "How do you pass an object to a tag file?"

Answer: Use the "type" attribute of the attribute directive:

<%@ attribute name="field" 
              required="true"
              type="com.mycompany.MyClass" %>

The type defaults to java.lang.String, so without it you'll get an error if you try to access object fields saying that it can't find the field from type String.

回答2:

<jsp:useBean id="myObject" class="java.lang.Object" scope="page" />
<wf:my-tag obj="${myObject}" />

Its not encouraged to use Scriptlets in JSP page. It kills the purpose of a template language.

回答3:

The original syntax was to reuse '<%= %>'

So

<wf:my-tag obj="<%= myObject %>" />

See this part of the Sun Tag Library Tutorial for an example

回答4:

For me expression language works only if I make that variable accessible, by putting it for example in page context.

<%  Object myObject = new Object();
    pageContext.setAttribute("myObject", myObject);
%>
<wf:my-tag obj="${myObject}" />

Otherwise tas receives null.

And <wf:my-tag obj="<%= myObject %>" /> works with no additional effort. Also <%=%> gives jsp compile-time type validation, while El is validated only in runtime.

回答5:

You can use "<%= %>" to get the object value directly in your tag :

    <wf:my-tag obj="<%= myObject %>"/>

and to get the value of any variable within that object you can get that using "obj.parameter" like:

<wf:my-tag obj="<%= myObject.variableName %>"/>

回答6:

Use expression language:

    <wf:my-tag obj="${myObject}" />