JAVA SERVER PAGES

 

 

Java Server Pages (JSP) is a standard Java extension that is defined on top of the servlet Extensions. The goal of JSPs is the simplified creation and management of dynamic Web pages.

 

JSPs allow you to combine the HTML of a Web page with pieces of Java code in the same document. The Java code is surrounded by special tags that tell the JSP container that it should use the code to generate a servlet, or part of one.

 

The benefit of JSPs is that you can maintain a single document that represents both the page and the Java code that enables it. The downside is that the maintainer of the JSP page must be skilled in both HTML and Java.

 

The structure of a JSP page is a cross between a servlet and an HTML page. The HSP tags begin and end with angle brackets, just like HTML tags, but the tags also include percent signs, so all JSP tags are denoted by

 

<% JSP code here %>

 

 

1.                          JSP lets you separate the dynamic part of your pages from the static HTML.

·        You simply write the regular HTML in the normal manner, using whatever Web-page-building tools you normally use.

·        You then enclose the code for the dynamic parts in special tags, most of which start with "<%" and end with "%>".

·        For example, here is a section of a JSP page that results in something like "Thanks for ordering Core Web Programming" for a URL of http://host/OrderConfirmation.jsp?title=Core+Web+Programming:

o                          Thanks for ordering 
<I><%= request.getParameter("title") %></I>

 

·        You normally give your file a .jsp extension, and typically install it in any place you could place a normal Web page.

·        Although what you write often looks more like a regular HTML file than a servlet, behind the scenes, the JSP page just gets converted to a normal servlet, with the static HTML simply being printed to the output stream associated with the servlet's service method.

·        This is normally done the first time the page is requested, and developers can simply request the page themselves when first installing it if they want to be sure that the first real user doesn't get a momentary delay when the JSP page is translated to a servlet and the servlet is compiled and loaded.

·        Note also that many Web servers let you define aliases that so that a URL that appears to reference an HTML file really points to a servlet or JSP page.

·        Aside from the regular HTML, there are three main types of JSP constructs that you embed in a page: scripting elements, directives, and actions.

1.     Scripting elements let you specify Java code that will become part of the resultant servlet

2.     directives let you control the overall structure of the servlet, and actions let you specify existing components that should be used

3.     and control the behavior of the JSP engine

 

2.                     What are the Advantages of JSP?

 

·        vs. Active Server Pages (ASP). ASP is a similar technology from Microsoft. The advantages of JSP are twofold. First, the dynamic part is written in Java, not Visual Basic or other MS-specific language, so it is more powerful and easier to use. Second, it is portable to other operating systems and non-Microsoft Web servers.

 

·        vs. Pure Servlets. JSP doesn't give you anything that you couldn't in principle do with a servlet. But it is more convenient to write (and to modify!) regular HTML than to have a zillion println statements that generate the HTML. Plus, by separating the look from the content you can put different people on different tasks: your Web page design experts can build the HTML, leaving places for your servlet programmers to insert the dynamic content.

 

·        vs. Server-Side Includes (SSI). SSI is a widely-supported technology for including externally-defined pieces into a static Web page. JSP is better because it lets you use servlets instead of a separate program to generate that dynamic part. Besides, SSI is really only intended for simple inclusions, not for "real" programs that use form data, make database connections, and the like.

 

·        vs. JavaScript. JavaScript can generate HTML dynamically on the client. This is a useful capability, but only handles situations where the dynamic information is based on the client's environment. With the exception of cookies, HTTP and form submission data is not available to JavaScript. And, since it runs on the client, JavaScript can't access server-side resources like databases, catalogs, pricing information, and the like.

 

·        vs. Static HTML. Regular HTML, of course, cannot contain dynamic information. JSP is so easy and convenient that it is quite feasible to augment HTML pages that only benefit marginally by the insertion of small amounts of dynamic data. Previously, the cost of using dynamic data would preclude its use in all but the most valuable instances.

 

3.                     Template Text: Static HTML

·        In many cases, a large percent of your JSP page just consists of static HTML, known as template text. In all respects except one, this HTML looks just like normal HTML, follows all the same syntax rules, and is simply "passed through" to the client by the servlet created to handle the page. Not only does the HTML look normal, it can be created by whatever tools you already are using for building Web pages. For example, I used Allaire's HomeSite for most of the JSP pages in this tutorial.

·        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.

 

4.                     JSP Scripting Elements

·        JSP scripting elements let you insert Java code into the servlet that will be generated from the current JSP page. There are three forms:

1.     Expressions of the form <%= expression %> that are evaluated and inserted into the output,

2.     Scriptlets of the form <% code %> that are inserted into the servlet's service method, and

3.      Declarations of the form <%! code %> that are inserted into the body of the servlet class, outside of any existing methods.

4.1    JSP Expressions

 

 

A JSP expression is used to insert Java values directly into the output. It has the following form: 

<%= Java Expression %>




 

The Java expression is evaluated, converted to a string, and inserted in the page. This evaluation is performed at run-time (when the page is requested), and thus has full access to information about the request. For example, the following shows the date/time that the page was requested: 

Current time: <%= new java.util.Date() %>

To simplify these expressions, there are a number of predefined variables that you can use. These implicit objects are discussed in more detail later, but for the purpose of expressions, the most important ones are:

Here's an example: 

Your hostname: <%= request.getRemoteHost() %>

Finally, note that XML authors can use an alternative syntax for JSP expressions: 

<jsp:expression>
Java Expression
</jsp:expression>

Remember that XML elements, unlike HTML ones, are case sensitive. So be sure to use lowercase.

4.2 JSP Scriptlets

 

 

If you want to do something more complex than insert a simple expression, JSP scriptlets let you insert arbitrary code into the servlet method that will be built to generate the page. Scriptlets have the following form:

 

<% Java Code %>
 

Scriptlets have access to the same automatically defined variables as expressions. So, for example, if you want output to appear in the resultant page, you would use the 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>
 
 
 

4.3 JSP Declarations

 

A JSP declaration lets you define methods or fields that get inserted into the main body of the servlet class (outside of the 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>
 
 
 
 

5. JSP Directives

A JSP directive affects the overall structure of the servlet class. It usually has the following form:

 

 

<%@ directive attribute="value" %>
 
 

However, you can also combine multiple attribute settings for a single directive, as follows:

 

 

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

There are two main types of directive: 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.

 

 

5.1 The JSP page Directive

 

The page directive lets you define one or more of the following case-sensitive attributes:

The XML syntax for defining directives is

 

<jsp:directive.directiveType attribute=value />
 
 

For example, the XML equivalent of

 

 

 

<%@ page import="java.util.*" %>

is 

<jsp:directive.page import="java.util.*" />
 
 

5.2 The JSP include Directive

 

This directive lets you include files at the time the JSP page is translated into a servlet. The directive looks like this:

 

<%@ include file="relative url" %>
 

The URL specified is normally interpreted relative to the JSP page that refers to it, but, as with relative URLs in general, you can tell the system to interpret the URL relative to the home directory of the Web server by starting the URL with a forward slash. The contents of the included file are parsed as regular JSP text, and thus can include static HTML, scripting elements, directives, and actions.

For 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.

6. Example Using Scripting Elements and Directives

Here is a simple example showing the use of JSP expressions, scriptlets, declarations, and directives.

 

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Using JavaServer Pages</TITLE>
 
<META NAME="author" CONTENT="Marty Hall -- hall@apl.jhu.edu">
<META NAME="keywords"
      CONTENT="JSP,JavaServer Pages,servlets">
<META NAME="description"
      CONTENT="A quick example of the four main JSP tags.">
<LINK REL=STYLESHEET
      HREF="My-Style-Sheet.css"
      TYPE="text/css">
</HEAD>
 
<BODY BGCOLOR="#FDF5E6" TEXT="#000000" LINK="#0000EE"
      VLINK="#551A8B" ALINK="#FF0000">
 
<CENTER>
<TABLE BORDER=5 BGCOLOR="#EF8429">
  <TR><TH CLASS="TITLE">
      Using JavaServer Pages</TABLE>
</CENTER>
<P>
 
Some dynamic content created using various JSP mechanisms:
 
 
<UL>
  <LI><B>Expression.</B><BR>
      Your hostname: <%= request.getRemoteHost() %>.
  <LI><B>Scriptlet.</B><BR>
      <% out.println("Attached GET data: " +
                     request.getQueryString()); %>
  <LI><B>Declaration (plus expression).</B><BR>
      <%! private int accessCount = 0; %>
      Accesses to page since server reboot: <%= ++accessCount %>
  <LI><B>Directive (plus expression).</B><BR>
      <%@ page import = "java.util.*" %>
      Current date: <%= new Date() %>
</UL>
 
 
</BODY>
</HTML>

Here's a typical result:

7. Predefined Variables

To simplify code in JSP expressions and scriptlets, you are supplied with eight automatically defined variables, sometimes called implicit objects. The available variables are request, response, out, session, application, config, pageContext, and page. Details for each are given below.

7.1 request

This is the 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.

7.2 rsponse

This is the 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.

7.3 out

This is the 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. 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.

7.4 session

This is the 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 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.

7.5 application

This is the ServletContext as obtained via getServletConfig().getContext().

7.6 config

This is the ServletConfig object for this page.

7.7 pageContext

JSP introduced a new class called 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.

7.8 page

This is simply a synonym for 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.

8. Actions

JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include:

These actions are described in more detail below. Remember that, as with XML in general, the element and attribute names are case sensitive.

8.1 The jsp:include Action

This action lets you insert files into the page being generated. The syntax looks like this:

 

<jsp:include page="relative URL" flush="true" />
 

Unlike the include directive, which inserts the file at the time the JSP page is translated into a servlet, this action inserts the file at the time the page is requested. This pays a small penalty in efficiency, and precludes the included page from containing general JSP code (it cannot set HTTP headers, for example), but it gains significantly in flexibility. For example, here is a JSP page that inserts four different snippets into a "What's New?" Web page. Each time the headlines change, authors only need to update the four files, but can leave the main JSP page unchanged.

WhatsNew.jsp

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>What's New</TITLE>
<LINK REL=STYLESHEET
      HREF="My-Style-Sheet.css"
      TYPE="text/css">
</HEAD>
 
<BODY BGCOLOR="#FDF5E6" TEXT="#000000" LINK="#0000EE"
      VLINK="#551A8B" ALINK="#FF0000">
 
<CENTER>
<TABLE BORDER=5 BGCOLOR="#EF8429">
  <TR><TH CLASS="TITLE">
      What's New at JspNews.com</TABLE>
</CENTER>
<P>
 
Here is a summary of our four most recent news stories:
<OL>
  <LI><jsp:include page="news/Item1.html" flush="true"/>
  <LI><jsp:include page="news/Item2.html" flush="true"/>
  <LI><jsp:include page="news/Item3.html" flush="true"/>
  <LI><jsp:include page="news/Item4.html" flush="true"/>
</OL>
</BODY>
</HTML>

Here's a typical result:

 

8.2 The jsp:useBean Action

 

This action lets you load in a JavaBean to be used in the JSP page. This is a a very useful capability because it lets you exploit the reusability of Java classes without sacrificing the convenience that JSP adds over servlets alone. The simplest syntax for specifying that a bean should be used is:

 

<jsp:useBean id="name" class="package.class" />
 

This usually means "instantiate an object of the class specified by class, 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.

BeanTest.jsp

<!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="test" class="hall.SimpleBean" />
<jsp:setProperty name="test" 
                 property="message" 
                 value="Hello WWW" />
             
<H1>Message: <I>
<jsp:getProperty name="test" property="message" />
</I></H1>
             
</BODY>
</HTML>

SimpleBean.java

Here's the source code for the bean used in the BeanTest JSP page.. 

package hall;
 
public class SimpleBean {
  private String message = "No message specified";
 
  public String getMessage() {
    return(message);
  }
 
  public void setMessage(String message) {
    this.message = message;
  }
}

Here's a typical result:

8.3 More jsp:useBean Details

The simplest way to use a bean is to use


   <jsp:useBean id="name" class="package.class" />


to load the bean, then use 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>


to indicate that the Body portion should be executed only when the bean is first instantiated, not when an existing bean is found and used. As discussed below, beans can be shared, so not all 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.

8.4 The jsp:setProperty Action

You use jsp: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.

Here's an example that uses a bean to create a table of prime numbers. If there is a parameter named numDigits in the request data, it is passed into the bean's numDigits property. Likewise for numPrimes.

JspPrimes.jsp

<!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:

8.5 The jsp:getProperty Action

This element retrieves the value of a bean property, converts it to a string, and inserts it into the output. The two required attributes are name, the name of a bean previously referenced via jsp:useBean, and property, the property whose value should be inserted.

 

 

<jsp:useBean id="itemBean" ... />
...
<UL>
  <LI>Number of items: 
      <jsp:getProperty name="itemBean" property="numItems" />
  <LI>Cost of each:
      <jsp:getProperty name="itemBean" property="unitCost" />
</UL>
 
 

8.6 The jsp:forward Action

This action lets you forward the request to another page. It has a single attribute, page, 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 %>" />
 
 

8.7 The jsp:plugin Action

This action lets you insert the browser-specific OBJECT or EMBED element needed to specify that the browser run an applet using the Java plugin.

9. Comments and Character Quoting Conventions

There are a small number of special constructs you can use in various cases to insert comments or characters that would otherwise be treated specially. Here's a summary:

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.