Java技术
2005: 03 04 05 06 07 08
09 10 11 12
2006: 01 02
Asp.net
2005: 07 08 09 10 11 12
2006: 01 02
| JSP Element | Syntax | Interpretation | Notes |
|---|---|---|---|
| JSP Expression | <%= expression %> | Expression is evaluated and placed in output. | XML equivalent is<jsp:expression>. Predefined variables are request, response, out, session, application, config, and pageContext (available in scriptlets also).
|
| JSP Scriptlet | <% code %> | Code is inserted in service method.
| XML equivalent is<jsp:scriptlet>.
|
| JSP Declaration | <%! code %> | Code is inserted in body of servlet class, outside of service method.
| XML equivalent is<jsp:declaration>.
|
JSP page Directive
| <%@ page att="val" %> | Directions to the servlet engine about general setup. | XML equivalent is<jsp:directive.page att="val"\>. Legal attributes, with default values in bold, are:
|
JSP include Directive
| <%@ include file="url" %> | A file on the local system to be included when the JSP page is translated into a servlet. | XML equivalent is<jsp:directive.include.The URL must be a relative one. Use the jsp:include action to include a file at request time instead of translation time.
|
| JSP Comment | <%-- comment --%> | Comment; ignored when JSP page is translated into servlet. | If you want a comment in the resultant HTML, use regular HTML comment syntax of <-- comment -->.
|
The jsp:include Action
| <jsp:include page="relative URL" flush="true"/> | Includes a file at the time the page is requested. | If you want to include the file at the time the page is translated, use the page directive with the include attribute instead. Warning: on some servers, the included file must be an HTML file or JSP file, as determined by the server (usually based on the file extension).
|
The jsp:useBean Action
| <jsp:useBean att=val*/> or<jsp:useBean att=val*>
| Find or build a Java Bean. | Possible attributes are:
|
The jsp:setProperty Action
| <jsp:setProperty att=val*/> | Set bean properties, either explicitly or by designating that value comes from a request parameter. | Legal attributes are
|
The jsp:getProperty Action
| <jsp:getProperty name="propertyName" value="val"/> | Retrieve and output bean properties. | |
The jsp:forward Action
| <jsp:forward page="relative URL"/> | Forwards request to another page. | |
The jsp:plugin Action
| <jsp:plugin attribute="value"*> ... </jsp:plugin> | Generates OBJECT or EMBED tags, as appropriate to the browser type, asking that an applet be run using the Java Plugin.
|
The one minor exception to the "template text is passed straight through" rule is that, if you want to have "<%" in the output, you need to put "<\%" in the template text.
<%= expression %> that are evaluated and inserted into the output,
<% code %> that are inserted into the servlet´s service method, and
<%! code %> that are inserted into the body of the servlet class, outside of any existing methods. request, the HttpServletRequest;
response, the HttpServletResponse;
session, the HttpSession associated with the request (if any); and
out, the PrintWriter (a buffered version of type JspWriter) used to send output to the client. out variable. <%
String queryData = request.getQueryString();
out.println("Attached GET data: " + queryData);
%>
Note that code inside a scriptlet gets inserted exactly as written, and any static HTML (template text) before or after a scriptlet gets converted to print statements. This means that scriptlets need not contain complete Java statements, and blocks left open can affect the static HTML outside of the scriptlets. For example, the following JSP fragment, containing mixed template text and scriptlets <% if (Math.random() < 0.5) { %>
Have a <B>nice</B> day!
<% } else { %>
Have a <B>lousy</B> day!
<% } %>
will get converted to something like: if (Math.random() < 0.5) {
out.println("Have a <B>nice</B> day!");
} else {
out.println("Have a <B>lousy</B> day!");
}
If you want to use the characters "%>" inside a scriptlet, enter "%\>" instead. Finally, note that the XML equivalent of <% Code %> is <jsp:scriptlet>
Code
</jsp:scriptlet>
service method processing the request). It has the following form: <%! Java Code %>
Since declarations do not generate any output, they are normally used in conjunction with JSP expressions or scriptlets. For example, here is a JSP fragment that prints out the number of times the current page has been requested since the server booted (or the servlet class was changed and reloaded): <%! private int accessCount = 0; %>
Accesses to page since server reboot:
<%= ++accessCount %>
As with scriptlets, if you want to use the characters "%>", enter "%\>" instead. Finally, note that the XML equivalent of <%! Code %> is <jsp:declaration>
Code
</jsp:declaration>
page, which lets you do things like import classes, customize the servlet superclass, and the like; and include, which lets you insert a file into the servlet class at the time the JSP file is translated into a servlet. The specification also mentions the taglib directive, which is not supported in JSP version 1.0, but is intended to let JSP authors define their own tags. It is expected that this will be the main new contribution of JSP 1.1.
page Directivepage directive lets you define one or more of the following case-sensitive attributes:
import="package.class" or import="package.class1,...,package.classN". This lets you specify what packages should be imported. For example:<%@ page import="java.util.*" %>import attribute is the only one that is allowed to appear multiple times.
contentType="MIME-Type" or contentType="MIME-Type; charset=Character-Set"text/html. For example, the directive <%@ page contentType="text/plain" %><% response.setContentType("text/plain"); %>
isThreadSafe="true|false". A value of true (the default) indicates normal servlet processing, where multiple requests can be processed simultaneously with a single servlet instance, under the assumption that the author synchronized access to instance variables. A value of false indicates that the servlet should implement SingleThreadModel, with requests either delivered serially or with simultaneous requests being given separate servlet instances.
session="true|false". A value of true (the default) indicates that the predefined variable session (of type HttpSession) should be bound to the existing session if one exists, otherwise a new session should be created and bound to it. A value of false indicates that no sessions will be used, and attempts to access the variable session will result in errors at the time the JSP page is translated into a servlet.
buffer="sizekb|none". This specifies the buffer size for the JspWriter out. The default is server-specific, but must be at least 8kb.
autoflush="true|false". A value of true, the default, indicates that the buffer should be flushed when it is full. A value of false, rarely used, indicates that an exception should be thrown when the buffer overflows. A value of false is illegal when also using buffer="none".
extends="package.class". This indicates the superclass of servlet that will be generated. Use this with extreme caution, since the server may be using a custom superclass already.
info="message". This defines a string that can be retrieved via the getServletInfo method.
errorPage="url". This specifies a JSP page that should process any Throwables thrown but not caught in the current page.
isErrorPage="true|false". This indicates whether or not the current page can act as the error page for another JSP page. The default is false.
language="java". At some point, this is intended to specify the underlying language being used. For now, don´t bother with this since java is both the default and the only legal choice. include DirectiveFor example, many sites include a small navigation bar on each page. Due to problems with HTML frames, this is usually implemented by way of a small table across the top of the page or down the left-hand side, with the HTML repeated for each page in the site. The include directive is a natural way of doing this, saving the developers from the maintenance nightmare of actually copying the HTML into each separate file. Here´s some representative code: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Servlet Tutorial: JavaServer Pages (JSP) 1.0</TITLE>
<META NAME="author" CONTENT="webmaster@somesite.com">
<META NAME="keywords" CONTENT="...">
<META NAME="description" CONTENT="...">
<LINK REL=STYLESHEET
HREF="Site-Styles.css"
TYPE="text/css">
</HEAD>
<BODY>
<%@ include file="/navbar.html" %>
<!-- Part specific to this page ... -->
</BODY>
</HTML>
Note that since the include directive inserts the files at the time the page is translated, if the navigation bar changes, you need to re-translate all the JSP pages that refer to it. This is a good compromise in a situation like this, since the navigation bar probably changes infrequently, and you want the inclusion process to be as efficient as possible. If, however, the included files changed more often, you could use the jsp:include action instead. This includes the file at the time the JSP page is requested, and is discussed in the tutorial section on JSP actions.
request, response, out, session, application, config, pageContext, and page. Details for each are given below.
HttpServletRequest associated with the request, and lets you look at the request parameters (via getParameter), the request type (GET, POST, HEAD, etc.), and the incoming HTTP headers (cookies, Referer, etc.). Strictly speaking, request is allowed to be a subclass of ServletRequest other than HttpServletRequest, if the protocol in the request is something other than HTTP. This is almost never done in practice.
HttpServletResponse associated with the response to the client. Note that, since the output stream (see out below) is buffered, it is legal to set HTTP status codes and response headers, even though this is not permitted in regular servlets once any output has been sent to the client.
PrintWriter used to send output to the client. However, in order to make the response object (see the previous section) useful, this is a buffered version of PrintWriter called JspWriter. Note that you can adjust the buffer size, or even turn buffering off, through use of the buffer attribute of the page directive. This was discussed in Section 5. Also note that out is used almost exclusively in scriptlets, since JSP expressions automatically get placed in the output stream, and thus rarely need to refer to out explicitly.
HttpSession object associated with the request. Recall that sessions are created automatically, so this variable is bound even if there was no incoming session reference. The one exception is if you use the session attribute of the page directive (see Section 5) to turn sessions off, in which case attempts to reference the session variable cause errors at the time the JSP page is translated into a servlet.
ServletContext as obtained via getServletConfig().getContext().
ServletConfig object for this page.
PageContext to encapsulate use of server-specific features like higher performance JspWriters. The idea is that, if you access them through this class rather than directly, your code will still run on "regular" servlet/JSP engines.
this, and is not very useful in Java. It was created as a placeholder for the time when the scripting language could be something other than Java.
jsp:include - Include a file at the time the page is requested. See Section 8.1.
jsp:useBean - Find or instantiate a JavaBean. See Section 8.2 for an overview, and Section 8.3 for details.
jsp:setProperty - Set the property of a JavaBean. See Section 8.4.
jsp:getProperty - Insert the property of a JavaBean into the output. See Section 8.5.
jsp:forward - Forward the requester to a new page. See Section 8.6.
jsp:plugin - Generate browser-specific code that makes an OBJECT or EMBED tag for the Java plugin. See Section 8.7. jsp:include Actionjsp:useBean Actionclass, and bind it to a variable with the name specified by id." However, as we´ll see shortly, you can specify a scope attribute that makes the bean associated with more than just the current page. In that case, it is useful to obtain references to existing beans, and the jsp:useBean action specifies that a new object is instantiated only if there is no existing one with the same id and scope. Now, once you have a bean, you can modify its properties via jsp:setProperty, or by using a scriptlet and calling a method explicitly on the object with the variable name specified earlier via the id attribute. Recall that with beans, when you say "this bean has a property of typeX called foo", you really mean "this class has a method called getFoo that returns something of type X, and another method called setFoo that takes an X as an argument." The jsp:setProperty action is discussed in more detail in the next section, but for now note that you can either supply an explicit value, give a param attribute to say that the value is derived from the named request parameter, or just list the property to indicate that the value should be derived from the request parameter with the same name as the property. You read existing properties in a JSP expression or scriptlet by calling the appropriate getXxx method, or more commonly, by using the jsp:getProperty action.
Note that the class specified for the bean must be in the server´s regular class path, not the part reserved for classes that get automatically reloaded when they change. For example, in the Java Web Server, it and all the classes it uses should go in the classes directory or be in a jar file in the lib directory, not be in the servlets directory.
Here is a very simple example that loads a bean and sets/gets a simple String parameter.
jsp:useBean Details <jsp:useBean id="name" class="package.class" /> jsp:setProperty and jsp:getProperty to modify and retrieve bean properties. However, there are two other options. First, you can use the container format, namely <jsp:useBean ...>
Body
</jsp:useBean>jsp:useBean statements result in a new bean being instantiated. Second, in addition to id and class, there are three other attributes that you can use: scope, type, and beanName. These attributes are summarized in the following table.
| Atribute | Usage |
|---|---|
id
| Gives a name to the variable that will reference the bean. A previous bean object is used instead of instantiating a new one if one can be found with the same id and scope.
|
class
| Designates the full package name of the bean. |
scope
| Indicates the context in which the bean should be made available. There are four possible values: page, request, session, and application. The default, page, indicates that the bean is only available on the current page (stored in the PageContext of the current page). A value of request indicates that the bean is only available for the current client request (stored in the ServletRequest object). A value of session indicates that the object is available to all pages during the life of the current HttpSession. Finally, a value of application indicates that it is available to all pages that share the same ServletContext. The reason that the scope matters is that a jsp:useBean entry will only result in a new object being instantiated if there is no previous object with the same id and scope. Otherwise the previously existing object is used, and any jsp:setParameter elements or other entries between the jsp:useBean start and end tags will be ignored.
|
type
| Specifies the type of the variable that will refer to the object. This must match the classname or be a superclass or an interface that the class implements. Remember that the name of the variable is designated via the id attribute.
|
beanName
| Gives the name of the bean, as you would supply it to the instantiate method of Beans. It is permissible to supply a type and a beanName, and omit the class attribute. |
jsp:setProperty Actionjsp:setProperty to give values to properties of beans that have been referenced earlier. You can do this in two contexts. First, you can use jsp:setProperty after, but outside of, a jsp:useBean element, as below: <jsp:useBean id="myName" ... />
...
<jsp:setProperty name="myName"
property="someProperty" ... />
In this case, the jsp:setProperty is executed regardless of whether a new bean was instantiated or an existing bean was found. A second context in which jsp:setProperty can appear is inside the body of a jsp:useBean element, as below: <jsp:useBean id="myName" ... >
...
<jsp:setProperty name="myName"
property="someProperty" ... />
</jsp:useBean>
Here, the jsp:setProperty is executed only if a new object was instantiated, not if an existing one was found.
There are four possible attributes of jsp:setProperty:
| Attribute | Usage |
|---|---|
name
| This required attribute designates the bean whose property will be set. The jsp:useBean element must appear before the jsp:setProperty element.
|
property
| This required attribute indicates the property you want to set. However, there is one special case: a value of "*" means that all request parameters whose names match bean property names will be passed to the appropriate setter methods.
|
value
| This optional attribute specifies the value for the property. String values are automatically converted to numbers, boolean, Boolean, byte, Byte, char, and Character via the standard valueOf method in the target or wrapper class. For example, a value of "true" for a boolean or Boolean property will be converted via Boolean.valueOf, and a value of "42" for an int or Integer property will be converted via Integer.valueOf. You can´t use both value and param, but it is permissible to use neither. See the discussion of param below.
|
param
| This optional attribute designates the request parameter from which the property should be derived. If the current request has no such parameter, nothing is done: the system does not pass null to the setter method of the property. Thus, you can let the bean itself supply default values, overriding them only when the request parameters say to do so. For example, the following snippet says "set the numberOfItems property to whatever the value of the numItems request parameter is, if there is such a request parameter. Otherwise don´t do anything." <jsp:setProperty name="orderBean"
property="numberOfItems"
param="numItems" />If you omit both value and param, it is the same as if you supplied a param name that matches the property name. You can take this idea of automatically using the request property whose name matches the property one step further by supplying a property name of "*" and omitting both value and param. In this case, the server iterates through available properties and request parameters, matching up ones with identical names. |
numDigits in the request data, it is passed into the bean´s numDigits property. Likewise for numPrimes.
NumberedPrimes bean referenced by the jsp:useBean element. Browse the source code directory for other Java classes used by NumberedPrimes. The best way to try it out on-line is to start with the HTML page that acts as a front end to it. <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Reusing JavaBeans in JSP</TITLE>
<LINK REL=STYLESHEET
HREF="My-Style-Sheet.css"
TYPE="text/css">
</HEAD>
<BODY>
<CENTER>
<TABLE BORDER=5>
<TR><TH CLASS="TITLE">
Reusing JavaBeans in JSP</TABLE>
</CENTER>
<P>
<jsp:useBean id="primeTable" class="hall.NumberedPrimes" />
<jsp:setProperty name="primeTable" property="numDigits" />
<jsp:setProperty name="primeTable" property="numPrimes" />
Some <jsp:getProperty name="primeTable" property="numDigits" />
digit primes:
<jsp:getProperty name="primeTable" property="numberedList" />
</BODY>
</HTML>
Here´s a typical result:
jsp:getProperty Actionjsp:forward Actionpage, which should consist of a relative URL. This could be a static value, or could be computed at request time, as in the two examples below. <jsp:forward page="/utils/errorReporter.jsp" />
<jsp:forward page="<%= someJavaExpression %>" />
jsp:plugin ActionOBJECT or EMBED element needed to specify that the browser run an applet using the Java plugin.
| Syntax | Purpose |
|---|---|
| <%-- comment --%> | A JSP comment. Ignored by JSP-to-scriptlet translator. Any embedded JSP scripting elements, directives, or actions are ignored. |
| <!-- comment --> | An HTML comment. Passed through to resultant HTML. Any embedded JSP scripting elements, directives, or actions are executed normally. |
| <\% | Used in template text (static HTML) where you really want "<%".
|
| %\> | Used in scripting elements where you really want "%>".
|
| \´ | A single quote in an attribute that uses single quotes. Remember, however, that you can use either single or double quotes, and the other type of quote will then be a regular character. |
| \" | A double quote in an attribute that uses double quotes. Remember, however, that you can use either single or double quotes, and the other type of quote will then be a regular character. |
| %\> | %> in an attribute.
|
| <\% | <% in an attribute. |