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Oracle9iAS Containers for J2EE Support for JavaServer Pages Developer's Guide
Release 2 (9.0.3)
Part Number A97679-01
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6
Additional Considerations

This chapter discusses an assortment of programming and runtime considerations in developing and executing JSP applications. The following categories are covered:

JSP Programming Strategies, Tips, and Traps

This section discusses issues you should consider when programming JSP pages, regardless of the particular target environment. The following assortment of topics are covered:

Note:

In addition to being aware of what is discussed in this section, you should be aware of JSP translation and deployment issues and behavior. See Chapter 7, "JSP Translation and Deployment".

JavaBeans Versus Scriptlets

The section "Separation of Business Logic from Page Presentation: Calling JavaBeans" describes a key advantage of JavaServer Pages technology--Java code containing the business logic and determining the dynamic content can be separated from the HTML code containing the request processing, presentation logic, and static content. This separation allows HTML experts to focus on presentation, while Java experts focus on business logic in JavaBeans that are called from the JSP page.

A typical JSP page will have only brief snippets of Java code, usually for Java functionality for request processing or presentation. The sample page in "JSP Data-Access Sample Using JDBC", although illustrative, is probably not an ideal design. Data access, such as in the runQuery() method in the sample, is usually more appropriate in a JavaBean. However, the formatResult() method in the sample, which formats the output, is more appropriate for the JSP page itself.

Static Includes Versus Dynamic Includes

The include directive, described in "Directives", makes a copy of the included page and copies it into a JSP page (the "including page") during translation. This is known as a static include (or translate-time include) and uses the following syntax: 

<%@ include file="/jsp/userinfopage.jsp" %>

The jsp:include tag, described in "Standard Actions: JSP Tags", dynamically includes output from the included page within the output of the including page, during runtime. This is known as a dynamic include (or runtime include) and uses the following syntax: 

<jsp:include page="/jsp/userinfopage.jsp" flush="true" />

For those familiar with C syntax, a static include is comparable to a #include statement. A dynamic include is similar to a function call. They are both useful, but serve different purposes.

Note:

You can use static includes and dynamic includes only between pages in the same servlet context.

Logistics of Static Includes

A static include increases the size of the generated code for the including JSP page, as though the text of the included page is physically copied into the including page during translation (at the point of the include directive). If a page is included multiple times within an including page, multiple copies are made.

A JSP page that is statically included is not required to be an independent, translatable entity. It simply consists of text that will be copied into the including page. The including page, with the included text copied in, must then be translatable. And, in fact, the including page does not have to be translatable prior to having the included page copied into it. A sequence of statically included pages can be fragments unable to stand on their own.

Logistics of Dynamic Includes

A dynamic include does not significantly increase the size of the generated code for the including page, although method calls, such as to the request dispatcher, will be added. The dynamic include results in runtime processing being switched from the including page to the included page, as opposed to the text of the included page being physically copied into the including page.

A dynamic include does increase processing overhead, with the necessity of the additional call to the request dispatcher.

A page that is dynamically included must be an independent entity, able to be translated and executed on its own. Likewise, the including page must be independent as well, able to be translated and executed without the dynamic include.

Advantages, Disadvantages, and Typical Uses

Static includes affect page size; dynamic includes affect processing overhead. Static includes avoid the overhead of the request dispatcher that a dynamic include necessitates, but may be problematic where large files are involved. (The service method of the generated page implementation class has a 64 KB size limit--see "Workarounds for Large Static Content in JSP Pages".)

Overuse of static includes can also make debugging your JSP pages difficult, making it harder to trace program execution. Avoid subtle interdependencies between your statically included pages.

Static includes are typically used to include small files whose content is used repeatedly in multiple JSP pages. For example:

Dynamic includes are useful for modular programming. You may have a page that sometimes executes on its own but sometimes is used to generate some of the output of other pages. Dynamically included pages can be reused in multiple including pages without increasing the size of the including pages.

Note:

OC4J offers global includes as a convenient way to statically include a file into multiple pages. See "Oracle JSP Global Includes".

When to Consider Creating and Using JSP Tag Libraries

Some situations dictate that the development team consider creating and using custom tags. In particular, consider the following situations:

Replacing Java Syntax

Because one cannot count on JSP developers being experienced in Java programming, they may not be ideal candidates for coding Java logic in the page--logic that dictates presentation and format of the JSP output, for example.

This is a situation where JSP tag libraries might be helpful. If many of your JSP pages will require such logic in generating their output, a tag library to replace Java logic would be a great convenience for JSP developers.

An example of this is the JML tag library provided with OC4J. This library, documented in the Oracle9iAS Containers for J2EE JSP Tag Libraries and Utilities Reference, includes tags that support logic equivalent to Java loops and conditionals.

Providing Convenient JSP Programming Access to API Features

Instead of having Web application programmers rely on Java APIs from servlets or JSP scriptlets to use product functionality or extensions, you can provide a tag library. A tag library can make the programmer's task much more convenient, with appropriate API calls being handled automatically by the tag handlers.

For example, tags as well as JavaBeans are provided with OC4J for e-mail and file access functionality. There is also a tag library as well as a Java API provided with the OC4J Web Object Cache. Similarly, while Oracle9iAS Personalization provides a Java API, OC4J also provides a tag library that you can use instead if you want to program a personalization application.

Manipulating or Redirecting JSP Output

Another common situation for custom tags is if special runtime processing of the response output is required. Perhaps the desired functionality requires an extra processing step, or redirection of the output to somewhere other than the browser.

An example is to create a custom tag that you can place around a body of text whose output will be redirected into a log file instead of to a browser, such as in the following example (where cust is the prefix for the tag library, and log is one of the tags of the library): 

<cust:log>
   Today is <%= new java.util.Date() %>
   Text to log.
   More text to log.
   Still more text to log.
</cust:log>

See "Tag Handlers" for information about processing of tag bodies.

Use of a Central Checker Page

For general management or monitoring of your JSP application, it may be useful to use a central "checker" page that you include from each page in your application. A central checker page could accomplish tasks such as the following during execution of each page:

There could be many more uses as well.

As an example, consider a session checker class, MySessionChecker, that implements the HttpSessionBindingListener interface. (See "Standard Session Resource Management: HttpSessionBindingListener".) 

public class MySessionChecker implements HttpSessionBindingListener
{
   ...
   valueBound(HttpSessionBindingEvent event)
   {...}
   
   valueUnbound(HttpSessionBindingEvent event)
   {...}
   ...
}

You can create a checker page, suppose centralcheck.jsp, that includes something like the following: 

<jsp:useBean id="sessioncheck" class="MySessionChecker" scope="session" />

In any page that includes centralcheck.jsp, the servlet container will call the valueUnbound() method implemented in the MySessionChecker class as soon as sessioncheck goes out of scope (at the end of the session). Presumably this is to manage session resources. You could include centralcheck.jsp at the end of each JSP page in your application.

Note:

OC4J offers "global includes" as a convenient way to statically include a file into multiple pages. See "Oracle JSP Global Includes".

Workarounds for Large Static Content in JSP Pages

JSP pages with large amounts of static content (essentially, large amounts of HTML code without content that changes at runtime) may result in slow translation and execution.

There are two primary workarounds for this (either of which will speed translation):

Important:

A static include directive would not work. It would result in the included file being included at translation-time, with its code being effectively copied back into the including page. This would not solve the problem.

Note:

Putting static HTML into a resource file may result in a larger memory footprint than the jsp:include workaround mentioned above, because the page implementation class must load the resource file whenever the class is loaded.

Another possible, though unlikely, problem with JSP pages that have large static content is that most (if not all) JVMs impose a 64 KB size limit on the code within any single method. Although javac would be able to compile it, the JVM would be unable to execute it. Depending on the implementation of the JSP translator, this may become an issue for a JSP page, because generated Java code from essentially the entire JSP page source file goes into the service method of the page implementation class. (Java code is generated to output the static HTML to the browser, and Java code from any scriptlets is copied directly.)

Similarly, it is possible for the Java scriptlets in a JSP page to be large enough to create a size limit problem in the service method. If there is enough Java code in a page to create a problem, however, then the code should be moved into JavaBeans.

Method Variable Declarations Versus Member Variable Declarations

In "Scripting Elements", it is noted that JSP <%! ... %> declarations are used to declare member variables, while method variables must be declared in <% ... %> scriptlets.

Be careful to use the appropriate mechanism for each of your declarations, depending on how you want to use the variables:

Consider the following example, decltest.jsp: 

<HTML>
<BODY>
<% double f2=0.0; %>
<%! double f1=0.0; %>
Variable declaration test.
</BODY>
</HTML>

This results in something like the following code in the page implementation class: 

package ...;
import ...;

public class decltest extends oracle.jsp.runtime.HttpJsp {
   ...
   // ** Begin Declarations
   double f1=0.0;                  // *** f1 declaration is generated here ***
   // ** End Declarations
   public void _jspService
                (HttpServletRequest request, HttpServletResponse response) 
                throws IOException, ServletException {
      ...
      try {
         out.println( "<HTML>");
         out.println( "<BODY>");
         double f2=0.0;      // *** f2 declaration is generated here ***
         out.println( "");
         out.println( "");
         out.println( "Variable declaration test.");
         out.println( "</BODY>");
         out.println( "</HTML>");
         out.flush();
      }
      catch( Exception e) {
         try {
            if (out != null) out.clear();
         }
         catch( Exception clearException) {
         }
      finally {
         if (out != null) out.close();
      }
   }
}

Note:

This code is provided for conceptual purposes only. Most of the class is deleted for simplicity, and the actual code of a page implementation class generated by the JSP translator would differ somewhat.

Page Directive Characteristics

This section discusses the following page directive characteristics:

Page Directives Are Static

A page directive is static; it is interpreted during translation. You cannot specify dynamic settings to be interpreted at runtime. Consider the following examples.

Example 1

The following page directive is valid. 

<%@ page contentType="text/html; charset=EUCJIS" %>
Example 2

The following page directive is not valid and will result in an error. (EUCJIS is hard-coded here, but the example also holds true for any character set determined dynamically at runtime.) 

<% String s="EUCJIS"; %>
<%@ page contentType="text/html; charset=<%=s%>" %>

For some page directive settings there are workarounds. Reconsidering the second example, there is a setContentType() method that allows dynamic setting of the content type, as described in "Dynamic Content Type Settings".

Duplicate Settings of Page Directive Attributes Are Disallowed

The JSP 1.2 specification states that a JSP container must verify that directive attributes, with the exception of the page directive import attribute, are not set more than once each within a single JSP translation unit (a JSP page plus anything it includes through include directives). In JSP 1.2, this effectively applies to page directives only, but in future JSP versions there might be additional relevant directives.

For backward compatibility to the JSP 1.1 standard, where duplicate settings of directive attributes are allowed, OC4J provides the forgive_dup_dir_attr configuration parameter. See "JSP Configuration Parameters". You might have previously coded a page with multiple included segments that all set the page directive language attribute to "java", for example.

For clarity, be aware of the following points.

Page Directive import Settings Are Cumulative

The page directive import attribute is exempt from JSP 1.2 limitations on duplicate directive attributes. Java import settings in page directives within a JSP page or translation unit (a JSP page plus anything included through include directives) are cumulative.

Within any single JSP page or translation unit, the following two examples are equivalent: 

<%@ page language="java" %>
<%@ page import="sqlj.runtime.ref.DefaultContext, java.sql.*" %>

or: 

<%@ page language="java" %>
<%@ page import="sqlj.runtime.ref.DefaultContext" %>
<%@ page import="java.sql.*" %>

After the first page directive import setting, the import setting in the second page directive adds to the set of classes or packages to be imported, as opposed to replacing the classes or packages to be imported.

JSP Preservation of White Space and Use with Binary Data

JSP containers generally preserve source code white space, including carriage returns and linefeeds, in what is output to the browser. Insertion of such white space may not be what the developer intended, and typically makes JSP technology a poor choice for generating binary data.

White Space Examples

The following two JSP pages produce different HTML output, due to the use of carriage returns in the source code.

Example 1--No Carriage Returns

The following JSP page does not have carriage returns after the Date() and getParameter() calls. (The third and fourth lines, starting with the Date() call, actually form a single wraparound line of code.)

nowhitsp.jsp: 

<HTML>
<BODY>
<%= new java.util.Date() %> <% String user=request.getParameter("user"); %> <%= 
(user==null) ? "" : user %>
<B>Enter name:</B>
<FORM METHOD=get>
<INPUT TYPE="text" NAME="user" SIZE=15>
<INPUT TYPE="submit" VALUE="Submit name">
</FORM>
</BODY>
</HTML>

This code results in the following HTML output to the browser. (Note that there are no blank lines after the date.) 

<HTML>
<BODY>
Tue May 30 20:07:04 PDT 2000  
<B>Enter name:</B>
<FORM METHOD=get>
<INPUT TYPE="text" NAME="user" SIZE=15>
<INPUT TYPE="submit" VALUE="Submit name">
</FORM>
</BODY>
</HTML>
Example 2--Carriage Returns

The following JSP page does include carriage returns after the Date() and getParameter() calls.

whitesp.jsp: 

<HTML>
<BODY>
<%= new java.util.Date() %>
<% String user=request.getParameter("user"); %>
<%= (user==null) ? "" : user %>
<B>Enter name:</B>
<FORM METHOD=get>
<INPUT TYPE="text" NAME="user" SIZE=15>
<INPUT TYPE="submit" VALUE="Submit name">
</FORM>
</BODY>
</HTML>

This code results in the following HTML output to the browser. 

<HTML>
<BODY>
Tue May 30 20:19:20 PDT 2000


<B>Enter name:</B>
<FORM METHOD=get>
<INPUT TYPE="text" NAME="user" SIZE=15>
<INPUT TYPE="submit" VALUE="Submit name">
</FORM>
</BODY>
</HTML>

Note the two blank lines between the date and the "Enter name:" line. In this particular case the difference is not significant, because both examples produce the same appearance in the browser, as shown below. However, this discussion nevertheless demonstrates the general point about preservation of white space.

Text description of whitesp.gif follows.

Text description of the illustration whitesp.gif

Reasons to Avoid Binary Data in JSP Pages

For the following reasons, JSP pages are a poor choice for generating binary data. Generally, you should use servlets instead.

Trying to generate binary data in JSP pages largely misses the point of JSP technology anyway, which is intended to simplify the programming of dynamic textual content.

JSP Runtime Considerations and Optimization

This section describes some of the JSP runtime functionality, particularly regarding dynamic page retranslation and class reloading, and points out some considerations for optimizing execution. The following topics are covered:

Dynamic Page Retranslation and Class Reloading

By default, particularly for use in development environments where code is in flux, the JSP container has the following behavior during page execution.

In a typical production environment, where source code will not change, comparing timestamps is unnecessary. In this case, you can avoid all timestamp comparisons and any possible retranslation and reloading by setting the JSP main_mode flag to justrun. This will optimize program execution.

If you want to reload modified class files but not retranslate modified JSP pages, you can set main_mode to reload.

For more information about the main_mode flag, see "JSP Configuration Parameters".

Notes:

  • This discussion is not relevant for pre-translation scenarios.

  • Because of the usage of in-memory values for class file last-modified times, removing a page implementation class file from the file system will not by itself cause retranslation of the associated JSP page source.

  • The page implementation class file will be regenerated when the memory cache is lost. This happens whenever a request is directed to this page after the server is restarted or after another page in this application has been retranslated.

  • In OC4J, if a statically included page is updated (that is, a page included through an include directive), the page that includes it will be automatically retranslated the next time it is invoked. (This is not true in JServ.)

Optimization Considerations

This section describes additional settings you can consider to optimize JSP performance.

Unbuffering a JSP Page

By default, a JSP page uses an area of memory known as a page buffer. This buffer (8 KB by default) is required if the page uses dynamic globalization support content type settings, forwards, or error pages. If it does not use any of these features, you can disable the buffer in a page directive: 

<%@ page buffer="none" %>

This will improve the performance of the page by reducing memory usage and saving the output step of copying the buffer.

Not Using an HTTP Session

If a JSP page does not require an HTTP session (essentially, does not require storage or retrieval of session attributes), then you can direct that no session be used. Specify this with a page directive such as the following: 

<%@ page session="false" %>

This will improve the performance of the page by eliminating the overhead of session creation or retrieval.

Note that although servlets by default do not use a session, JSP pages by default do use a session. For background information, see "Servlet Sessions".

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