Tuesday, March 18, 2008

Build time vs. render time

My blog has moved, please update your links. You can find this article here

Overview

There always seems to be someone posting about how MyFaces or JSF is "broken" and it turns out that they misused JSTL tags. No matter how many times a solution is posted, the issue comes up again. Therefore, I am writing this blog to help those that do not understand how facelets and JSP view handlers work.

The Problem

JSF works entirely differently than JSP. JSP, when used for JSF, is used to build a JSF component tree, not render a page. JSTL tags alter the contents of what is built, the component tree, not the HTML that is output by the component renderers.

A Background of JSP

JSP was written so that authoring servlets would be easier. JSP pages are servlets, nothing more. Using a JSP compiler, a JSP file is compiled into a Java Servlet. Before tag libraries were introduced, JSP pages simply were compiled down into System.out.println(...); statements.

Tag libraries enhanced JSP by providing an API to tag authors to write Java code that would not have to be embedded in <% ... %> tags. The tags gave developers access to the text content of the tag body so that they could alter it or use it in a custom way. After some time, Sun released the Java standard tag library API, also known as the JSTL.

The JSTL

The JSTL is a set of JSP tags to perform common JSP functionality (note that they are not related to JSF in any way at all). Many of them perform logic to alter their contents. In the case of c:if and c:choose, they choose which tag contents to be included in the response or not.

I will be mainly talking about the JSTL core tags in this article (catch, choose, forEach, forTokens, if, etc.). It is these that have such a profound, and often confusing to some people, affect on JSF.

What JSF Is

JSF is a component technology, and architecturally is not related to JSP at all. In fact, using JSP with JSF has drawbacks, included serious performance implications due to the architecture of JSP and how the JSP view handler works.

Note

Sun probably made the default view handler as one that uses JSP, so they would not have to admit that JSP did not meet the needs of users and needed to be replaced.

Component Technology

What I mean by JSF being a component technology, is that HTML is produced by the processing of a component tree by renderers. This design is much more similar to Swing than it is Servlets and JSP. Regardless of the view handler that is used, a tree of components is built.

Note

A component is simply a Java Object that implements the UIComponent interface, nothing more.

These components simply store attributes, have some behaviors and are similar in nature to the Swing Tree component. JSF relies on renderers (classes that extend Renderer) to produce content from the component tree. During the restore view, a JSF view should be restored to its previous state, so that it appears to code that there was never a round trip for the code to the client (this is important as I discuss the JSTL tags and especially c:forEach).

Note

Components do not have to use renderers to produce their content, but it is considered bad design to use the encode methods in a component to render a response.

Note

JSF is typically used to produce HTML, but can be used to generate any output that doesn't even have to be XML related, although the ResponseWriter's API is designed for XML content.

JSF with JSP

JSF is implemented using JSP by default (see my note above why this stinks). What this means is that JSP is used to build the JSF component tree. This is important to note because JSP is not used to generate any of the HTML, unlike a typical JSP page. There are two main stages of JSF on JSP:

  1. Building of the component tree using JSP tags
  2. Rendering of the component tree using the standard JSF lifecycle

Building of the component tree using JSP tags

As I mentioned earlier, JSF is a component technology. When JSP tags are used, instead of writing text (HTML) onto the servlet response, the tags create components and set attribute values on them. Most of this work is done by UIComponentClassicTagBase. The class hierarchy is:

  • java.lang.Object
    • javax.faces.webapp.UIComponentTagBase
      • javax.faces.webapp.UIComponentClassicTagBase
        • javax.faces.webapp.UIComponentELTag
        • javax.faces.webapp.UIComponentTag

The doStartTag method of UIComponentClassicTagBase calls findComponent, which despite its name creates the component (if needed) and calls setProperties. Therefore, the JSF component is created when the start of the JSP tag is processed. The setProperties method is responsible for setting the attributes of the component. Typically there is a one-to-one map of JSP tag attribute to JSF component attribute, so the setProperties simply copies the attributes over, doing any necessary conversion of the string value in the JSP attribute to an object that the component expects for the attribute.

How JSTL fits in

Most of the "work" of a component takes place during rendering. For example, the h:dataTable sets up the var attribute during rendering (and other phases too, but that is not pertinent to this article). This means that EL expressions that rely on variables that only have scope during the rendering phase are not valid during component creation. That is to say, there is no component lifecycle method for when the component is created.

Since JSTL tags are plain JSP tags and do not involve the JSF lifecycle, they do not have access to the environment that components create, like the var variable of an h:dataTable. Take this code for example:

<f:view> <h:dataTable var="_row" value="#{bean.rows}> <h:column> <c:choose> <c:when test="#{_row.editable}"> <h:inputText value="#{_row.value}" /> </c:when> <c:otherwise> <h:outputText value="#{_row.value}" /> </c:otherwise> </c:choose> </h:column> </h:dataTable> </f:view>

Although from the pure XML standpoint this looks valid, it is not. Thinking about what I just said it is a simple problem:

  1. Data table component created from the data table JSP tag
  2. Column component created from its JSP tag
  3. The when tag in the choose attempts to evaluate #{_row.editable}
    • Since _row is not bound (yet) in the EL resolver, null is returned
      (an argument could be made that the EL engine should throw an error or at least give a warning, but that is not how it is designed)
    • Since null is technically false, the outputText is created instead of the inputText
  4. The output text component is created by its tag

So the page author was probably expecting that the choose would be evaluated for every row in the table. But the table has no rows at this point, the component is being created. So, the inputText component is never created as the when tag will not process its contents if the test fails.

The proper solution is to use JSF functionality and not JSP functionality. To the confusion of JSP developers, there are no c:if, c:choose or c:forEach equivalent components (see the Tomahawk Sandbox limitRendered for c:if and c:choose functionality and the Tomahawk dataList or Trinidad iterator components for c:forEach type of functionality). Without third party components, the rendered attribute can also work, although it requires more work:

<f:view> <h:dataTable var="_row" value="#{bean.rows}> <h:column> <h:inputText value="#{_row.value}" rendered="#{_row.editable}" /> <h:outputText value="#{_row.value}" rendered="#{not _row.editable}" /> </h:column> </h:dataTable> </f:view>

In this case both the input and the output components are created. The rendered is evaluated by the renderer during the rendering phase while the data table is iterating over each of its rows.

When it is okay to use JSP tags with JSF

So now that you know the problem, you may ask why is using any non-JSF, JSP tags allowed? This is because they still work, but must be used correctly. Since JSP tags are evaluated while components are being built, they can control which components to create. For example, I may want to include certain components in a page template if the current user is an administrator. Because the components are not created if the user is not an administrator, there will be less components objects created and less components to process for each JSF lifecycle phase, and thus improving performance.

It is very important to remember that you cannot have components "re-appear" on post back of a JSF form. This is because a JSF component tree should never be altered between having its state saved and when its state is restored. This is very important, so let me say again, a JSF component tree should never be altered between having its state saved and when its state is restored. The reason is that this would violate the JSF specification/design. Take for example a c:forEach loop that when evaluated had five items it its list. Assuming that there was one JSF component tag inside the for each tag, that means five components were created. Now JSF saves the state for five components. Then lets say this for each loop data is coming from a database that can change and now there are only four items. When JSF attempts to restore the component tree, there is data for 5 components, but there are only 4 in the component tree. This causes a restore state exception.

Therefore, always ensure that when a component tree is restored, it is not changed! There is no problem changing a component tree after restoring it or before saving it, only between those times.

Hey, what about Facelets?

I did mention that I would talk about facelets, didn't I?

Facelets is similar in its behavior to JSP although it has much more efficient code and is designed quite differently than JSP. Instead of JSP tags, facelets has TagHandlers. These tag handlers analyze the XML content of a facelet and create components. Just like JSP tags, the tag handlers have no access to the render-time scope of the components. In fact, facelets provides tag handlers to mimic the JSTL functionality. These tag handlers have the same pitfalls as the JSP tags. Therefore, the same considerations and rules apply to tag handlers that apply to JSP tags.

Other Blogs

Here are some other helpful resources on this topic:

7 comments:

Patman said...

Great post! thank you

Roger said...

Hey Andrew. Nice summary :)

[quote]Components do not have to use renderers to produce their content, but it is considered bad design to use the encode methods in a component to render a response.[/quote]

/me just went momentarily deaf

jayc said...

Thank You. Helped Loads.

farmer said...

thx a lot mate. This really helped us out.

Marcel said...

Hi Andrew,



You wrote:

a JSF component tree should never be altered between having its state saved and when its state is restored

But what I don't understand is when it could be changed anyway. When you first request for a page, a component tree is created based on a page descriptor (in my case a xhtml file).The second time (postback) the component tree already exists and will be restored (not be created again).
Even when you have a c:forEach loop in your file and the number of items changed I don't see how it can effect the component tree.



Marcel.

Andrew said...

Even though it is referred to as a "restore view", it should be called "re-create view"; the component tree is re-created for every request and the state is restored back into the components themselves (components have a lifespan of one request, they are not re-used unless you are using a 3rd party component library that does this). Therefore, if the c:forEach changes, the component tree will be affected. I have seen many problems with iterators, for each loops and other such devices when the data they use is request scoped and they do not realize that that data is not there on the post back.

Marcel said...

I am using c:forEach and c:if (condition is different between two sequential requests) and have a lot of problems with duplicate id's (created from index of c:forEach).
Now I know that component tree is not part of the state, things are more clear to me.
Thanks.