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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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1
General JSP Overview

This chapter reviews standard features and functionality of JavaServer Pages technology, then concludes with a discussion of JSP execution models. For further general information, consult the Sun Microsystems JavaServer Pages Specification, Version 1.2.

JSP 1.2 functionality depends upon servlet 2.3 functionality. You can also refer to the Sun Microsystems Java Servlet Specification, Version 2.3 for information.

For an overview of the JSP implementation in OC4J, see Chapter 2, "Overview of the Oracle JSP Implementation". Also note that Appendix A, "Servlet and JSP Technical Background", provides related background on standard servlet and JSP technology.

The following topics are covered here:

Introduction to JavaServer Pages

JavaServer Pages(TM) is a technology specified by Sun Microsystems as a convenient way of generating dynamic content in pages that are output by a Web application (an application running on a Web server).

This technology, which is closely coupled with Java servlet technology, enables you to include Java code snippets and calls to external Java components within the HTML code (or other markup code, such as XML) of your Web pages. JavaServer Pages (JSP) technology works nicely as a front-end for business logic and dynamic functionality in JavaBeans and Enterprise JavaBeans (EJBs).

JSP code is distinct from other Web scripting code, such as JavaScript, in a Web page. Anything that you can include in a normal HTML page can be included in a JSP page as well.

In a typical scenario for a database application, a JSP page will call a component such as a JavaBean or Enterprise JavaBean, and the bean will directly or indirectly access the database, generally through JDBC (perhaps using SQLJ).

A JSP page is translated into a Java servlet before being executed, and processes HTTP requests and generates responses similarly to any other servlet. JSP technology offers a more convenient way to code the servlet. The translation typically occurs on demand, but sometimes in advance.

Furthermore, JSP pages are fully interoperable with servlets--JSP pages can include output from a servlet or forward to a servlet, and servlets can include output from a JSP page or forward to a JSP page.


Note:

See the OC4J demos for some basic JSP sample applications.


What a JSP Page Looks Like

Here is an example of a simple JSP page. For an explanation of JSP syntax elements used here, see "Overview of JSP Syntax Elements".

<HTML>
<HEAD><TITLE>The Welcome User JSP</TITLE></HEAD>
<BODY>
<% String user=request.getParameter("user"); %>
<H3>Welcome <%= (user==null) ? "" : user %>!</H3>
<P><B> Today is <%= new java.util.Date() %>. Have a nice day! :-)</B></P>
<B>Enter name:</B>
<FORM METHOD=get>
<INPUT TYPE="text" NAME="user" SIZE=15>
<INPUT TYPE="submit" VALUE="Submit name">
</FORM>
</BODY>
</HTML>

In a traditional JSP page, Java elements are set off by tags such as <% and %>, as in the preceding example. (JSP XML syntax is different, as described in "Details of JSP XML Documents".) In this example, Java snippets get the user name from an HTTP request object, print the user name, and get the current date.

This JSP page will produce the following output if the user inputs the name "Amy":

Text description of welcoamy.gif follows.

Text description of the illustration welcoamy.gif

Convenience of JSP Coding Versus Servlet Coding

Combining Java code and Java calls into an HTML page is more convenient than using straight Java code in a servlet. JSP syntax gives you a shortcut for coding dynamic Web pages, typically requiring much less code than Java servlet syntax. Following is an example contrasting servlet code and JSP code.

Servlet Code

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class Hello extends HttpServlet
{
   public void doGet(HttpServletRequest rq, HttpServletResponse rsp)
   {
      rsp.setContentType("text/html");
      try {
         PrintWriter out = rsp.getWriter();
         out.println("<HTML>");
         out.println("<HEAD><TITLE>Welcome</TITLE></HEAD>");
         out.println("<BODY>");
         out.println("<H3>Welcome!</H3>");
         out.println("<P>Today is "+new java.util.Date()+".</P>");
         out.println("</BODY>");
         out.println("</HTML>");
      } catch (IOException ioe)
      {
        // (error processing) 
      }
   }
}

See "The Servlet Interface" for some background information about the standard HttpServlet abstract class, HttpServletRequest interface, and HttpServletResponse interface.

JSP Code

<HTML>
<HEAD><TITLE>Welcome</TITLE></HEAD>
<BODY>
<H3>Welcome!</H3>
<P>Today is <%= new java.util.Date() %>.</P>
</BODY>
</HTML>

Note how much simpler JSP syntax is. Among other things, it saves Java overhead such as package imports and try...catch blocks.


Note:

The list of packages imported into a JSP page by default changed in the OC4J 9.0.3 implementation. The default list was reduced to follow the JSP specification. See "Default Package Imports" for more information. (Starting with 9.0.3, the preceding JSP example requires a configuration setting to import the java.io package.)


Additionally, the JSP translator automatically handles a significant amount of servlet coding overhead for you in the .java file that it outputs, such as directly or indirectly implementing the standard javax.servlet.jsp.HttpJspPage interface (covered in "Standard JSP Interfaces and Methods") and adding code to acquire an HTTP session.

Also note that because the HTML of a JSP page is not embedded within Java print statements, as it is in servlet code, you can use HTML authoring tools to create JSP pages.

Separation of Business Logic from Page Presentation: Calling JavaBeans

JSP technology allows separating the development efforts between the HTML code that determines static page presentation, and the Java code that processes business logic and presents dynamic content. It therefore becomes much easier to split maintenance responsibilities between presentation and layout specialists who may be proficient in HTML but not Java, and code specialists who may be proficient in Java but not HTML.

In a typical JSP page, most Java code and business logic will not be within snippets embedded in the JSP page--instead, it will be in JavaBeans or Enterprise JavaBeans that are invoked from the JSP page.

JSP technology offers the following syntax for defining and creating an instance of a JavaBeans class:

<jsp:useBean id="pageBean" class="mybeans.NameBean" scope="page" />

This example creates an instance, pageBean, of the mybeans.NameBean class. The scope parameter will be explained later in this chapter.

Later in the page, you can use this bean instance, as in the following example:

Hello <%= pageBean.getNewName() %> !

This prints "Hello Julie !", for example, if the name "Julie" is in the newName attribute of pageBean, which might occur through user input.

The separation of business logic from page presentation allows convenient division of responsibilities between the Java expert who is responsible for the business logic and dynamic content (the person who owns and maintains the code for the NameBean class) and the HTML expert who is responsible for the static presentation and layout of the Web page that the application users see (the person who owns and maintains the code in the .jsp file for this JSP page).

Tags used with JavaBeans--useBean to declare the JavaBean instance and getProperty and setProperty to access bean properties--are further discussed in "Standard Actions: JSP Tags".

JSP Pages and Alternative Markup Languages

JavaServer Pages technology is typically used for dynamic HTML output, but the Sun Microsystems JavaServer Pages Specification, Version 1.2 also supports additional types of structured, text-based document output. A JSP translator does not process text outside of JSP elements, so any text that is appropriate for Web pages in general is typically appropriate for a JSP page as well.

A JSP page takes information from an HTTP request and accesses information from a data server (such as through a SQL database query). It combines and processes this information and incorporates it, as appropriate, into an HTTP response with dynamic content. The content can be formatted as HTML, DHTML, XHTML, or XML, for example.

For information about JSP support for XML, refer to Chapter 5, "JSP XML Support" and to the Oracle9iAS Containers for J2EE JSP Tag Libraries and Utilities Reference.

Overview of JSP Syntax Elements

You have seen a simple example of JSP syntax in "What a JSP Page Looks Like". Now here is a top-level list of syntax categories and topics:

This section introduces each category, including basic syntax and a few examples. There is also discussion of bean property conversions, and an introduction to custom tag libraries (used for custom actions). For more information, see the Sun Microsystems JavaServer Pages Specification, Version 1.2.


Note:

This section describes traditional JSP syntax. For information about JSP XML syntax and JSP XML documents, see Chapter 5, "JSP XML Support".


Directives

Directives provide instruction to the JSP container regarding the entire JSP page. This information is used in translating or executing the page. The basic syntax is as follows:

<%@ directive attribute1="value1" attribute2="value2"... %>

The JSP 1.2 specification supports the following directives:


Notes:

  • The default buffer size is 8 KB.

  • It is illegal to set autoFlush="true" when buffer="none".

  • A JSP page using an error page must be buffered. Forwarding to an error page (not outputting it to the browser) clears the buffer.

  • In the Oracle JSP implementation, "java" is the default language setting. It is good programming practice to set it explicitly, however. You can also use a "sqlj" setting for SQLJ JSP pages.

  • For information about using page directive attributes to set the content type and character set for the JSP page and response object, see "Content Type Settings in the page Directive".



Notes:

  • The include directive, referred to as a "static include", is comparable in nature to the jsp:include action discussed later in this chapter, but jsp:include takes effect at request-time instead of translation-time. See "Static Includes Versus Dynamic Includes".

  • The include directive can be used only between files in the same servlet context (application).

  • See "JSP File Naming Conventions" for information about naming conventions for included files.


Scripting Elements

JSP scripting elements include the following categories of Java code snippets that can appear in a JSP page:


Note:

Method variables, as opposed to member variables, are declared within JSP scriptlets as described below. (See "Method Variable Declarations Versus Member Variable Declarations" for a comparison between the two.)



Note:

A JSP expression in a request-time attribute, such as in a jsp:setProperty statement, need not be converted to a string value.



Note:

Use a JSP scriptlet to declare method variables, as opposed to member variables, as in the following example:

<% double f2=0.0; %>

This scriptlet declares a method variable, f2. In the servlet class code generated by the JSP translator, f2 will be declared as a variable within the service method of the servlet.

Member variables are declared in JSP declarations as described above.

For a comparative discussion, see "Method Variable Declarations Versus Member Variable Declarations".


JSP Objects and Scopes

In this document, the term JSP object refers to a Java class instance declared within or accessible to a JSP page. JSP objects can be either:

or:

This section covers the following topics:

Explicit Objects

Explicit objects are typically JavaBean instances that are declared and created in jsp:useBean action statements. The jsp:useBean statement and other action statements are described in "Standard Actions: JSP Tags", but here is an example:

<jsp:useBean id="pageBean" class="mybeans.NameBean" scope="page" />

This statement defines an instance, pageBean, of the NameBean class that is in the mybeans package. The scope parameter is discussed in "Object Scopes".

You can also create objects within Java scriptlets or declarations, just as you would create Java class instances in any Java program.

Implicit Objects

JSP technology makes available to any JSP page a set of implicit objects. These are Java objects that are created automatically by the JSP container and that allow interaction with the underlying servlet environment.

The following implicit objects are available. For information about methods available with these objects, refer to the Sun Microsystems Javadoc for the noted classes and interfaces at the following locations (for servlet 2.2 and servlet 2.3 classes, respectively):

http://java.sun.com/products/servlet/2.2/javadoc/index.html

http://java.sun.com/products/servlet/2.3/javadoc/index.html

Using an Implicit Object

Any of the implicit objects discussed in the preceding section might be useful. The following example uses the request object to retrieve and display the value of the username parameter from the HTTP request:

<H3> Welcome <%= request.getParameter("username") %> ! <H3>

The request object, like the other implicit objects, is available automatically; it is not explicitly instantiated.

Object Scopes

Objects in a JSP page, whether explicit or implicit, are accessible within a particular scope. In the case of explicit objects, such as a JavaBean instance created in a jsp:useBean action, you can explicitly set the scope with the following syntax, as in the example in "Explicit Objects":

scope="scopevalue"

There are four possible scopes:

You can think of these four scopes as being in the following progression, from narrowest scope to broadest scope:

page < request < session < application

If you want to share an object between different pages in an application, such as when forwarding execution from one page to another, or including content from one page in another, you cannot use page scope for the shared object; in this case, there would be a separate object instance associated with each page. The narrowest scope you can use to share an object between pages is request. (For information about including and forwarding pages, see "Standard Actions: JSP Tags" below.)


Note:

The request, session, and application scopes also apply to servlets.


Standard Actions: JSP Tags

JSP action elements result in some sort of action occurring while the JSP page is being executed, such as instantiating a Java object and making it available to the page. Such actions may include the following:

For standard actions, there is a set of tags defined in the JSP specification. Although directives and scripting elements described earlier in this chapter are sufficient to code a JSP page, the standard tags described here provide additional functionality and convenience.

Here is the general tag syntax for JSP standard actions:

<jsp:tag attr1="value1" attr2="value2" ... attrN="valueN">
...body...
</jsp:tag>

or, where there is no body:

<jsp:tag attr1="value1", ..., attrN="valueN" />

The JSP specification includes the following standard action tags, which are introduced and briefly discussed here.


Important:

For property="*", the JSP 1.2 specification does not stipulate the order in which properties are set. If order matters, and if you want to ensure that your JSP page is portable, you should use a separate jsp:setProperty statement for each property.

Also, if you use separate jsp:setProperty statements, then the JSP translator can generate the corresponding setXXX() methods directly. In this case, introspection occurs only during translation. There will be no need to introspect the bean during runtime, which is more costly.



Notes:

  • The jsp:include tag, known as a "dynamic include", is similar in nature to the include directive discussed earlier in this chapter, but takes effect at request-time instead of translation-time. See "Static Includes Versus Dynamic Includes".

  • The jsp:include tag can be used only between pages in the same servlet context (application).



Notes:

  • The difference between the jsp:forward examples here and the jsp:include examples earlier is that the jsp:include examples insert userinfopage.jsp within the output of the current page; the jsp:forward examples stop executing the current page and display userinfopage.jsp instead.

  • The jsp:forward tag can be used only between pages in the same servlet context.

  • The jsp:forward tag results in the original request object being forwarded to the target page. As an alternative, if you do not want the request object forwarded, you can use the sendRedirect(String) method specified in the standard javax.servlet.http.HttpServletResponse interface. This sends a temporary redirect response to the client using the specified redirect-location URL. You can specify a relative URL; the servlet container will convert the relative URL to an absolute URL.


Bean Property Conversions from String Values

As noted earlier, when you use a JavaBean through a jsp:useBean tag in a JSP page, and then use a jsp:setProperty tag to set a bean property, string input can be used to specify the value of a non-string property through conversions that happen behind the scenes. There are two conversion scenarios:

Typical Property Conversions

For a bean property that does not have an associated property editor, Table 1-1 shows how conversion is accomplished when using a string value to set the property.

Table 1-1 Attribute Conversion Methods  
Property Type Conversion

boolean or Boolean

according to valueOf(String) method of Boolean class

byte or Byte

according to valueOf(String) method of Byte class

char or Character

according to charAt(0) method of String class (inputting an index value of 0)

double or Double

according to valueOf(String) method of Double class

int or Integer

according to valueOf(String) method of Integer class

float or Float

according to valueOf(String) method of Float class

long or Long

according to valueOf(String) method of Long class

short or Short

according to valueOf(String) method of Short class

Object

as if String constructor is called, using literal string input

The String instance is returned as an Object instance.

Conversions for Property Types with Property Editors

A bean property can have an associated property editor, which is a class that implements the java.beans.PropertyEditor interface. Such classes can provide support for GUIs used in editing properties. Generally speaking, there are standard property editors for standard Java types, and there can be user-defined property editors for user-defined types. In the OC4J JSP implementation, however, only user-defined property editors are searched for. Default property editors of the sun.beans.editors package are not taken into account.

For information about property editors and how to associate a property editor with a type, you can refer to the Sun Microsystems JavaBeans API Specification, Version 1.01.

You can still use a string value to set a property that has an associated property editor, as specified in the JavaBeans specification. In this situation, the setAsText(String text) method specified in the PropertyEditor interface is used in converting from string input to a value of the appropriate type. (If the setAsText() method throws an IllegalArgumentException, the conversion will fail.)

Custom Tag Libraries

In addition to the standard JSP tags discussed above, the JSP specification lets vendors define their own tag libraries, and lets vendors implement a framework that allows customers to define their own tag libraries as well.

A tag library defines a collection of custom tags and can be thought of as a JSP sub-language. Developers can use tag libraries directly when manually coding a JSP page, but they might also be used automatically by Java development tools. A standard tag library must be portable between different JSP container implementations.

Import a tag library into a JSP page using the taglib directive introduced in "Directives".

Key concepts of standard JavaServer Pages support for JSP tag libraries include the following topics:

For information about these topics, see Chapter 8, "JSP Tag Libraries". For further information, see the Sun Microsystems JavaServer Pages Specification, Version 1.2.

For complete information about the tag libraries provided with OC4J, see the Oracle9iAS Containers for J2EE JSP Tag Libraries and Utilities Reference.

JSP Execution

This section provides a top-level look at how a JSP page is run, including on-demand translation (the first time a JSP page is run), the role of the JSP container and the servlet container, and error processing.


Note:

The term JSP container first appeared in the Sun Microsystems JavaServer Pages Specification, Version 1.1, replacing the term JSP engine that was used in earlier specifications. The two terms are synonymous.


JSP Containers in a Nutshell

A JSP container is an entity that translates, executes, and processes JSP pages and delivers requests to them.

The exact make-up of a JSP container varies from implementation to implementation, but it will consist of a servlet or collection of servlets. The JSP container, therefore, is executed by a servlet container. Servlet containers are summarized in "Servlet Containers".

A JSP container can be incorporated into a Web server if the Web server is written in Java, or the container can be otherwise associated with and used by the Web server.

JSP Execution Models

There are two distinct execution models for JSP pages:

On-Demand Translation Model

It is typical to run JSP pages in an on-demand translation scenario. When a JSP page is requested from a Web server that incorporates the JSP container, a front-end servlet is instantiated and invoked, assuming proper Web server configuration. This servlet can be thought of as the front-end of the JSP container. In OC4J, it is oracle.jsp.runtimev2.JspServlet.

JspServlet locates the JSP page, translates and compiles it if necessary (if the translated class does not exist or has an earlier timestamp than the JSP page source), and triggers its execution.

Note that the Web server must be properly configured to map the *.jsp file name extension (in a URL) to JspServlet. This is handled automatically during OC4J installation, as discussed in "JSP Container Setup".

Pre-Translation Model

As an alternative to the typical on-demand scenario, developers may want to pre-translate their JSP pages before deploying them. This can offer the following advantages, for example:

For more information, see "JSP Pre-Translation" and "Deployment of Binary Files Only".

Oracle supplies the ojspc command-line utility for pre-translating JSP pages. This utility has options that allow you to set an appropriate base directory for the output files, depending on how you want to deploy the application. The ojspc utility is documented in "The ojspc Pre-Translation Utility".

JSP Pages and On-Demand Translation

Presuming the typical on-demand translation scenario, a JSP page is usually executed as follows:

  1. The user requests the JSP page through a URL ending with a .jsp file name.

  2. Upon noting the .jsp file name extension in the URL, the servlet container of the Web server invokes the JSP container.

  3. The JSP container locates the JSP page and translates it if this is the first time it has been requested. Translation includes producing servlet code in a .java file and then compiling the .java file to produce a servlet .class file.

    The servlet class generated by the JSP translator extends a class (provided by the JSP container) that implements the javax.servlet.jsp.HttpJspPage interface (described in "Standard JSP Interfaces and Methods"). The servlet class is referred to as the page implementation class. This document will refer to instances of page implementation classes as JSP page instances.

    Translating a JSP page into a servlet automatically incorporates standard servlet programming overhead into the generated servlet code, such as implementing the HttpJspPage interface and generating code for its service method.

  4. The JSP container triggers instantiation and execution of the page implementation class.

The JSP page instance will then process the HTTP request, generate an HTTP response, and pass the response back to the client.


Note:

The preceding steps are loosely described for purposes of this discussion. As mentioned earlier, each vendor decides how to implement its JSP container, but it will consist of a servlet or collection of servlets. For example, there may be a front-end servlet that locates the JSP page, a translation servlet that handles translation and compilation, and a wrapper servlet class that is extended by each page implementation class (because a translated page is not a pure servlet and cannot be run directly by the servlet container). A servlet container is required to run each of these components.


Requesting a JSP Page

A JSP page can be requested either directly--through a URL--or indirectly--through another Web page or servlet.

Directly Requesting a JSP Page

As with a servlet or HTML page, the end-user can request a JSP page directly by URL. For example, suppose you have a HelloWorld JSP page that is located under a myapp directory, as follows, where myapp is mapped to the myapproot root context in the Web server:

myapp/dir1/HelloWorld.jsp

You can request it with a URL such as the following:

http://host[:port]/myapproot/dir1/HelloWorld.jsp

The first time the end-user requests HelloWorld.jsp, the JSP container triggers both translation and execution of the page. With subsequent requests, the JSP container triggers page execution only; the translation step is no longer necessary.


Note:

This is just a general example. By default in OC4J in Oracle9iAS release 2, the context path must start with "/j2ee" if you want processing to be routed to OC4J through the Oracle HTTP Server and mod_oc4j, as in a typical production environment. Oracle HTTP Server and mod_oc4j are introduced in "Role of the Oracle HTTP Server and mod_oc4j". General servlet and JSP invocation are discussed in the Oracle9iAS Containers for J2EE Servlet Developer's Guide.


Indirectly Requesting a JSP Page

JSP pages, like servlets, can also be executed indirectly--linked from a regular HTML page or referenced from another JSP page or from a servlet.

When invoking one JSP page from a JSP statement in another JSP page, the path can be either relative to the application root--known as context-relative or application-relative--or relative to the invoking page--known as page-relative. An application-relative path starts with "/"; a page-relative path does not.

Be aware that, typically, neither of these paths is the same path as used in a URL or HTML link. Continuing the example in the preceding section, the path in an HTML link is the same as in the direct URL request, as follows:

<a href="/myapp/dir1/HelloWorld.jsp" /a>

The application-relative path in a JSP statement is:

<jsp:include page="/dir1/HelloWorld.jsp" flush="true" />

The page-relative path to invoke HelloWorld.jsp from a JSP page in the same directory is:

<jsp:forward page="HelloWorld.jsp" />

("Standard Actions: JSP Tags" discusses the jsp:include and jsp:forward statements.)


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