Oracle® Application Server Containers for J2EE Support for JavaServer Pages Developer's Guide
10g Release 2 (10.1.2) B14014-02 |
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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.
JSP functionality depends upon servlet functionality. You can also refer to the Sun Microsystems Java Servlet Specification for information.
For an overview of the JSP implementation in Oracle Application Server Containers for J2EE (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 chapter contains the following sections:
Note: The Sample Applications chapter available in previous releases has been removed. Applications that were listed there are available in the OC4J demos, available from the following location on the Oracle Technology Network (requiring an OTN membership, which is free of charge):
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JavaServer Pages 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.
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.
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 output as shown in Figure 1-1 if the user inputs the name "Amy".
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.
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.
<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. Therefore, beginning with Oracle9iAS Release 2 (9.0.3), the preceding JSP example requires a configuration setting to import thejava.io package.
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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.
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 might 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".
JavaServer Pages technology is typically used for dynamic HTML output, but the JSP specification 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 database 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 Oracle Application Server Containers for J2EE JSP Tag Libraries and Utilities Reference.
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:
Directives: These convey information regarding the JSP page as a whole.
Scripting elements: These are Java coding elements such as declarations, expressions, scriptlets, and comments.
Objects and scopes: JSP objects can be created either explicitly or implicitly and are accessible within a given scope, such as from anywhere in the JSP page or the session.
Actions: These create objects or affect the output stream in the JSP response (or both).
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.
Note: This section describes traditional JSP syntax. For information about JSP XML syntax and JSP XML documents, see Chapter 5, "JSP XML Support". |
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 specification supports the following directives:
page
include
taglib
Use this directive to specify any of a number of page-dependent attributes, such as scripting language, content type, character encoding, class to extend, packages to import, an error page to use, the JSP page output buffer size, and whether to automatically flush the buffer when it is full. For example:
<%@ page language="java" import="packages.mypackage" errorPage="boof.jsp" %>
Alternatively, to enable auto-flush and set the JSP page output buffer size to 20 KB:
<%@ page autoFlush="true" buffer="20kb" %>
This example unbuffers the page:
<%@ page buffer="none" %>
Notes:
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Use this directive to specify a resource that contains text or code to be inserted into the JSP page when it is translated. For example:
<%@ include file="/jsp/userinfopage.jsp" %>
Specify either a page-relative or context-relative path to the resource. See "Requesting a JSP Page" for discussion of page-relative and context-relative paths.
Notes:
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Use this directive to specify a library of custom JSP tags that will be used in the JSP page. Vendors can extend JSP functionality with their own sets of tags. This directive includes a pointer to a tag library descriptor file and a prefix to distinguish use of tags from that library. For example:
<%@ taglib uri="/oracustomtags" prefix="oracust" %>
Later in the page, use the oracust
prefix whenever you want to use one of the tags in the library. Presume this library includes a tag dbaseAccess
:
<oracust:dbaseAccess ... > ... </oracust:dbaseAccess>
JSP tag libraries and tag library descriptor files are introduced later in this chapter, in "Custom Tag Libraries", and discussed in detail in Chapter 8, "JSP Tag Libraries".
JSP scripting elements include the following categories of Java code snippets that can appear in a JSP page:
Declarations
Expressions
Scriptlets
Comments
These are statements declaring methods or member variables that will be used in the JSP page.
A JSP declaration uses standard Java syntax within the <%!...%>
declaration tags to declare a member variable or method. This will result in a corresponding declaration in the generated servlet code. For example:
<%! double f1=0.0; %>
This example declares a member variable, f1
. In the servlet class code generated by the JSP translator, f1
will be declared at the class top level.
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. |
These are Java expressions that are evaluated, converted into string values as appropriate, and displayed where they are encountered on the page.
A JSP expression does not end in a semicolon, and is contained within <%=...%>
tags. For example:
<P><B> Today is <%= new java.util.Date() %>. Have a nice day! </B></P>
Note: A JSP expression in a request-time attribute, such as in ajsp:setProperty statement, need not be converted to a string value.
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These are portions of Java code intermixed within the markup language of the page.
A scriptlet, or code fragment, can consist of anything from a partial line to multiple lines of Java code. You can use them within the HTML code of a JSP page to set up conditional branches or a loop, for example.
A JSP scriptlet is contained within <%...%>
scriptlet tags, using normal Java syntax.
Example 1:
<% if (pageBean.getNewName().equals("")) { %> I don't know you. <% } else { %> Hello <%= pageBean.getNewName() %>. <% } %>
Three one-line JSP scriptlets are intermixed with two lines of HTML code, one of which includes a JSP expression (which does not require a semicolon). Note that JSP syntax allows HTML code to be the code that is conditionally executed within the if
and else
branches (inside the Java brackets set out in the scriptlets).
The preceding example assumes the use of a JavaBean instance, pageBean
.
Example 2:
<% if (pageBean.getNewName().equals("")) { %> I don't know you. <% empmgr.unknownemployee(); } else { %> Hello <%= pageBean.getNewName() %>. <% empmgr.knownemployee(); } %>
This example adds more Java code to the scriptlets. It assumes the use of a JavaBean instance, pageBean
, and assumes that some object, empmgr
, was previously instantiated and has methods to execute appropriate functionality for a known employee or an unknown employee.
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, Member variables are declared in JSP declarations as described above. For a comparative discussion, see "Method Variable Declarations Versus Member Variable Declarations". |
These are developer comments embedded within the JSP code, similar to comments embedded within any Java code.
Comments are contained within <%--...--%>
syntax. For example:
<%-- Execute the following branch if no user name is entered. --%>
Unlike HTML comments, JSP comments are not visible when users view the page source from their browsers.
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:
Explicit: Explicit objects are declared and created within the code of your JSP page, accessible to that page and other pages according to the scope
setting you choose.
or:
Implicit: Implicit objects are created by the underlying JSP mechanism and accessible to Java scriptlets or expressions in JSP pages according to the inherent scope
setting of the particular object type.
These topics are discussed in the following sections:
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.
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 implicit objects listed immediately below 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 location:
http://java.sun.com/products/servlet/2.3/javadoc/index.html
This is an instance of the JSP page implementation class and is created when the page is translated. The page implementation class implements the interface javax.servlet.jsp.HttpJspPage
. Note that page
is synonymous with this
within a JSP page.
This represents an HTTP request and is an instance of a class that implements the javax.servlet.http.HttpServletRequest
interface, which extends the javax.servlet.ServletRequest
interface.
This represents an HTTP response and is an instance of a class that implements the javax.servlet.http.HttpServletResponse
interface, which extends the javax.servlet.ServletResponse
interface.
The response
and request
objects for a particular request are associated with each other.
This represents the page context of a JSP page, which is provided for storage and access of all page
scope objects of a JSP page instance. A pageContext
object is an instance of the javax.servlet.jsp.PageContext
class.
The pageContext
object has page
scope, making it accessible only to the JSP page instance with which it is associated.
This represents an HTTP session and is an instance of a class that implements the javax.servlet.http.HttpSession
interface.
This represents the servlet context for the Web application and is an instance of a class that implements the javax.servlet.ServletContext
interface.
The application
object is accessible from any JSP page instance running as part of any instance of the application within a single JVM. (The programmer should be aware of the server architecture regarding use of JVMs.)
This is an object that is used to write content to the output stream of a JSP page instance. It is an instance of the javax.servlet.jsp.JspWriter
class, which extends the java.io.Writer
class.
The out
object is associated with the response
object for a particular request.
This represents the servlet configuration for a JSP page and is an instance of a class that implements the javax.servlet.ServletConfig
interface. Generally speaking, servlet containers use ServletConfig
instances to provide information to servlets during initialization. Part of this information is the appropriate ServletContext
instance.
exception
(JSP error pages only)
This implicit object applies only to JSP error pages, which are pages to which processing is forwarded when an exception is thrown from another JSP page. They must have the page
directive isErrorPage
attribute set to true
.
The implicit exception
object is a java.lang.Exception
instance that represents the uncaught exception that was thrown from another JSP page and that resulted in the current error page being invoked.
The exception
object is accessible only from the JSP error page instance to which processing was forwarded when the exception was encountered. For an example of JSP error processing and use of the exception
object, see "Runtime Error Processing".
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.
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:
scope="page"
(default scope): The object is accessible only from within the JSP page where it was created. A page-scope object is stored in the implicit pageContext
object. The page
scope ends when the page stops executing.
Note that when the user refreshes the page while executing a JSP page, new instances will be created of all page-scope objects.
scope="request"
: The object is accessible from any JSP page servicing the same HTTP request that is serviced by the JSP page that created the object. A request-scope object is stored in the implicit request
object. The request
scope ends at the conclusion of the HTTP request.
scope="session"
: The object is accessible from any JSP page that is sharing the same HTTP session as the JSP page that created the object. A session-scope object is stored in the implicit session
object. The session
scope ends when the HTTP session times out or is invalidated.
scope="application"
: The object is accessible from any JSP page that is used in the same Web application as the JSP page that created the object, within any single Java virtual machine. The concept is similar to that of a Java static variable. An application-scope object is stored in the implicit application
servlet context object. The application
scope ends when the application itself terminates, or when the JSP container or servlet container shuts down.
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: Therequest , session , and application scopes also apply to servlets.
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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 can include the following:
Creating a JavaBean instance and accessing its properties
Forwarding execution to another HTML page, JSP page, or servlet
Including an external resource in the JSP page
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>
Alternatively, if 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 immediately below:
jsp:usebean
jsp:setProperty
jsp:getProperty
jsp:param
jsp:include
jsp:forward
jsp:plugin
The jsp:useBean
tag accesses or creates an instance of a Java type, typically a JavaBean class, and associates the instance with a specified name, or ID. The instance is then available by that ID as a scripting variable of specified scope. Scripting variables are introduced in "Custom Tag Libraries". Scopes are discussed in "JSP Objects and Scopes".
The key attributes are class
, type
, id
, and scope
. (There is also a less frequently used beanName
attribute, discussed below.)
Use the id
attribute to specify the instance name. The JSP container will first search for an object by the specified ID, of the specified type, in the specified scope. If it does not exist, the container will attempt to create it.
Intended use of the class
attribute is to specify a class that can be instantiated, if necessary, by the JSP container. The class cannot be abstract and must have a no-argument constructor. Intended use of the type
attribute is to specify a type that cannot be instantiated by the JSP container—either an interface, an abstract class, or a class without a no-argument constructor. You would use type
in a situation where the instance will already exist, or where an instance of an instantiable class will be assigned to the type. There are three typical scenarios:
Use type
and id
to specify an instance that already exists in the target scope.
Use class
and id
to specify the name of an instance of the class—either an instance that already exists in the target scope or an instance to be newly created by the JSP container.
Use class
, type
, and id
to specify a class to instantiate and a type to assign the instance to. In this case, the class must be legally assignable to the type.
Use the scope
attribute to specify the scope of the instance—either page
for the instance to be associated with the page context object, request
for it to be associated with the HTTP request object, session
for it to be associated with the HTTP session object, or application
for it to be associated with the servlet context.
As an alternative to using the class
attribute, you can use the beanName
attribute. In this case, you have the option of specifying a serializable resource instead of a class name. When you use the beanName
attribute, the JSP container creates the instance by using the instantiate()
method of the java.beans.Beans
class.
The following example uses a request-scope instance reqobj
of type MyIntfc
. Because MyIntfc
is an interface and cannot be instantiated directly, reqobj
would have to already exist.
<jsp:useBean id="reqobj" type="mypkg.MyIntfc" scope="request" />
This next example uses a page-scope instance pageobj
of class PageBean
, first creating it if necessary:
<jsp:useBean id="pageobj" class="mybeans.PageBean" scope="page" />
The following example creates an instance of class SessionBean
and assigns the instance to the variable sessobj
of type MyIntfc
:
<jsp:useBean id="sessobj" class="mybeans.SessionBean" type="mypkg.MyIntfc scope="session" />
The jsp:setProperty
tag sets one or more bean properties. The bean must have been previously specified in a jsp:useBean
tag. You can directly specify a value for a specified property, or take the value for a specified property from an associated HTTP request parameter, or iterate through a series of properties and values from the HTTP request parameters.
The following example sets the user
property of the pageBean
instance (defined in the preceding jsp:useBean
example) to a value of "Smith":
<jsp:setProperty name="pageBean" property="user" value="Smith" />
The following example sets the user
property of the pageBean
instance according to the value set for a parameter called username
in the HTTP request:
<jsp:setProperty name="pageBean" property="user" param="username" />
If the bean property and request parameter have the same name (user
), you can simply set the property as follows:
<jsp:setProperty name="pageBean" property="user" />
The following example results in iteration over the HTTP request parameters, matching bean property names with request parameter names and setting bean property values according to the corresponding request parameter values:
<jsp:setProperty name="pageBean" property="*" />
When you use the jsp:setProperty
tag, string input can be used to specify the value of a non-string property through conversions that happen behind the scenes. See "Bean Property Conversions from String Values".
Important: Note the following forproperty="*" :
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The jsp:getProperty
tag reads a bean property value, converts it to a Java string, and places the string value into the implicit out
object so that it can be displayed as output. The bean must have been previously specified in a jsp:useBean
tag. For the string conversion, primitive types are converted directly and object types are converted using the toString()
method specified in the java.lang.Object
class.
The following example puts the value of the user
property of the pageBean
bean into the out
object:
<jsp:getProperty name="pageBean" property="user" />
You can use jsp:param
tags in conjunction with jsp:include
, jsp:forward
, and jsp:plugin
tags (described below).
Used with jsp:forward
and jsp:include
tags, a jsp:param
tag optionally provides name/value pairs for parameter values in the HTTP request
object. New parameters and values specified with this action are added to the request
object, with new values taking precedence over old.
The following example sets the request
object parameter username
to a value of Smith
:
<jsp:param name="username" value="Smith" />
The jsp:include
tag inserts additional static or dynamic resources into the page at request-time as the page is displayed. Specify the resource with a relative URL (either page-relative or application-relative). For example:
<jsp:include page="/templates/userinfopage.jsp" flush="true" />
A "true
" setting of the flush
attribute results in the buffer being flushed to the browser when a jsp:include
action is executed. The JSP specification and the OC4J JSP container support either a "true
" or "false
" setting, with "false
" being the default. (The JSP 1.1 specification supported only a "true
" setting, with flush
being a required attribute.)
You can also have an action body with jsp:param
tags, as shown in the following example:
<jsp:include page="/templates/userinfopage.jsp" flush="true" > <jsp:param name="username" value="Smith" /> <jsp:param name="userempno" value="9876" /> </jsp:include>
Note that the following syntax would work as an alternative to the preceding example:
<jsp:include page="/templates/userinfopage.jsp?username=Smith&userempno=9876" flush="true" />
Notes:
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The jsp:forward
tag effectively terminates execution of the current page, discards its output, and dispatches a new page—either an HTML page, a JSP page, or a servlet.
The JSP page must be buffered to use a jsp:forward
tag; you cannot set buffer="none"
in a page
directive. The action will clear the buffer and not output contents to the browser.
As with jsp:include
, you can also have an action body with jsp:param
tags, as shown in the second of the following examples:
<jsp:forward page="/templates/userinfopage.jsp" />
or:
<jsp:forward page="/templates/userinfopage.jsp" > <jsp:param name="username" value="Smith" /> <jsp:param name="userempno" value="9876" /> </jsp:forward>
Notes:
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The jsp:plugin
tag results in the execution of a specified applet or JavaBean in the client browser, preceded by a download of Java plugin software if necessary.
Specify configuration information, such as the applet to run and the code base, using jsp:plugin
attributes. The JSP container might provide a default URL for the download, but you can also specify attribute nspluginurl="
url
"
(for a Netscape browser) or iepluginurl="
url
"
(for an Internet Explorer browser).
Use nested jsp:param
tags between the jsp:params
start-tag and end-tag to specify parameters to the applet or JavaBean. (Note that the jsp:params
start-tag and end-tag are not necessary when using jsp:param
in a jsp:include
or jsp:forward
action.)
Use a jsp:fallback
start -tag and end-tag to delimit alternative text to execute if the plugin cannot run.
The following example, from the Sun Microsystems JavaServer Pages Specification, Version 1.2, shows the use of an applet plugin:
<jsp:plugin type=applet code="Molecule.class" codebase="/html" > <jsp:params> <jsp:param name="molecule" value="molecules/benzene.mol" /> </jsp:params> <jsp:fallback> <p> Unable to start the plugin. </p> </jsp:fallback> </jsp:plugin>
Many additional parameters—such as ARCHIVE
, HEIGHT
, NAME
, TITLE
, and WIDTH
—are allowed in the jsp:plugin
tag as well. Use of these parameters is according to the general HTML specification.
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, covered in the following sections:
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 |
Byte or byte |
According to |
Character or char |
According to |
Double or double |
According to |
Integer or int |
According to |
Float or float |
According to |
Long or long |
According to |
Short or short |
According to |
Object |
As if The |
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.
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 method setAsText(String text)
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.
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:
Tag library descriptor files
A tag library descriptor (TLD) file is an XML document that contains information about a tag library and about individual tags of the library. The file name of a TLD has the .tld
extension.
Tag handlers
A tag handler specifies the action of a custom tag and is an instance of a Java class that implements either the Tag
, IterationTag
, or BodyTag
interface in the standard javax.servlet.jsp.tagext
package. Which interface to implement depends on whether the tag has a body and whether the tag handler requires access to the body content.
Scripting variables
Custom tag actions can create server-side objects available for use by the tag itself or by other scripting elements such as scriptlets. This is accomplished by creating or updating scripting variables.
Details regarding scripting variables that a custom tag defines are specified in the TLD file or in a subclass of the TagExtraInfo
abstract class (in package javax.servlet.jsp.tagext
). This document refers to a subclass of TagExtraInfo
as a tag-extra-info class. The JSP container uses instances of these classes during translation.
Tag-library-validators
A tag-library-validator class has logic to validate any JSP page that uses the tag library, according to specified constraints.
Event listeners
You can use servlet 2.3 event listeners with a tag library. This functionality is offered as a convenient alternative to declaring listeners in the application web.xml
file.
Use of web.xml
for tag libraries
The Sun Microsystems Java Servlet Specification describes a standard deployment descriptor for servlets: the web.xml
file. JSP applications can use this file in specifying the location of a JSP tag library descriptor file.
For JSP tag libraries, the web.xml
file can include a taglib
element and two subelements: taglib-uri
and taglib-location
.
For information about these topics, see Chapter 8, "JSP Tag Libraries". For further information, see the Sun Microsystems JavaServer Pages Specification.
For complete information about the tag libraries provided with OC4J, see the Oracle Application Server Containers for J2EE JSP Tag Libraries and Utilities Reference.
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. |
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.
There are two distinct execution models for JSP pages:
In most implementations and situations, the JSP container translates pages on demand before triggering their execution; that is, at the time they are requested by the user.
In some scenarios, however, the developer might want to translate the pages in advance and deploy them as working servlets. Command-line tools are available to translate the pages, load them, and publish them to make them available for execution. You can have the translation occur either on the client or in the server. When the user requests the JSP page, it is executed directly, with no translation necessary.
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".
As an alternative to the typical on-demand scenario, developers might want to pretranslate their JSP pages before deploying them. This can offer the following advantages, for example:
It can save time for the users when they first request a JSP page, because translation at execution time is not necessary.
It is useful if you want to deploy binary files only, perhaps because the software is proprietary or you have security concerns and you do not want to expose the code.
For more information, see "JSP Pretranslation" and "Deployment of Binary Files Only".
Oracle supplies the ojspc
command-line utility for pretranslating 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 Pretranslation Utility".
Presuming the typical on-demand translation scenario, a JSP page is usually executed as follows:
The user requests the JSP page through a URL ending with a .jsp
file name.
Upon noting the .jsp
file name extension in the URL, the servlet container of the Web server invokes the JSP container.
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.
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 might 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 actually a pure servlet and cannot be run directly by the servlet container). A servlet container is required to run each of these components. |
A JSP page can be requested either directly through a URL or indirectly through another Web page or servlet.
As with a servlet or HTML page, the 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
context path 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 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: General servlet and JSP invocation are discussed in the Oracle Application Server Containers for J2EE Servlet Developer's Guide. |
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.)