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JSP: JavaServer Pages
-- 4 -JSP: JavaServer Pages
Turning Servlets Inside Out
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JavaServer Pages: Java code embedded in HTML More similar to languages such as PHP
▪ out.println(”<p>The value of pi is: ” + Math.pi); // Servlet
▪ <p>The value of pi is: <%= Math.PI %>
// JSP
Code is still executed by the server, not the client!
▪ Use the .jsp extension to indicate that it is not pure HTML
▪ The server translates the file into a servlet, executes it
Useful when there is much more HTML than Java code
JSP source:
HTML code
with JSP code added
Output:
Compilation
Servlet code
Execution
Pure HTML
jonkv@ida
JSP 1: What is it?
57
Initial versions defined “JSP pages”
Three different pseudo‐tags identify Java code
▪ <%= EXPRESSION %> to insert the value of an expression
▪ <% SCRIPTLET %> to insert plain Java code
▪ <%! DECLARATION %> to insert a declaration
Current versions also support ”JSP documents”
Based on pure XML, true tags
▪ Positive: Use XML tools, validate against standard JSP DTD, …
▪ Negative: Must use well‐formed XML (longer tags, …)
▪ <jsp:expression> EXPRESSION </jsp:expression>
▪ <jsp:scriptlet> SCRIPTLET </jsp:scriptlet>
▪ <jsp:declaration>DECLARATION</jsp:declaration>
Current lab instructions use both “page” and “document”,
since you may choose to use either one!
jonkv@ida
JSP 2: Pages and Documents
58
Must declare the contents of the page
Which scripting language is used on the server side?
Which document langauge is generated and seen on the client?
Older JSP ”pages” (.jsp) use JSP‐specific tools
The source file has a standard HTML doctype,
plus a JSP pseudo‐tag declaring JSP options
▪ <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 //EN"
"http://www.w3.org/TR/html4/strict.dtd">
<%@ page contentType="text/html;charset=UTF-8” %>
<html>
…
</html>
The source file lies, claiming to be HTML.
JSP‐specific tools still know that the source file is JSP.
jonkv@ida
JSP 3: Language and Content Type
59
Newer JSP ”documents” (.jspx) use the XML standard
Suppose the doc source file started with an HTML doctype
▪ Standard XML tools apply this to the entire document – wrong!
But the client should still see an HTML DOCTYPE at the top!
▪ Solution: Use code that ”prints” the doctype!
▪ <jsp:output doctype-root-element="html”
doctype-public="-//W3C//DTD XHTML 1.0 Strict//EN"
doctype-system=”http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”
omit-xml-declaration=”false” />
▪ (By default, jsp:output will declare your document to be XML,
which is only true for XHTML.
Use omit‐xml‐declaration=”true” for ordinary HTML.)
Then, the page directive must be different
▪ <%@ page …> is not well‐formed XML, can’t be used here
▪ <jsp:directive.page contentType="text/html;charset=UTF-8" />
<html>…</html>
jonkv@ida
JSP 4: Language and Content Type
Inserting the value of an expression:
In JSP pages, use <%= … %>
▪ …DOCTYPE…
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head><title>A Simple JSP Page</title></head>
<body>
<p>Today is <%= new java.util.Date() %>
<p>The value of pi is: <%= Math.PI %>
</body>
</html> <jsp:expression> new java.util.Date()</jsp:expression>
Automatically converted into a servlet:
▪ public void doGet(HttpServletRequest req, HttpServletResponse res) {
… res.setContentType("text/html;charset=UTF-8"); …
out.print("<html>\n<head><title>…Today is ");
out.print(new java.util.Date());
out.print("\n\n<p>The value…");
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jonkv@ida
JSP 5: Inserting Expressions
61
Use scriptlets to insert complete Java code fragments
Use <% … %> or <jsp:scriptlet> … </jsp:scriptlet>
▪ <p>Will you win the lottery?
<%
double rand = Math.random();
if (rand < 0.1) {
out.println("You win!");
} else {
out.println(“Reload the page to try again!");
}
What’s inside <% … %> is inserted as-is in the resulting servlet!
%>
What’s inside <%= … %> was inserted inside out.print(…).
<p>Random: <%= double r = Math.random(); out.println(r); %>
out.println(”<p> Random: ” + double r = Math.random(); out.println(r););
<p>Pi is equal to <% Math.PI %>
out.println(”<p>Pi is equal to ”); Math.Pi
jonkv@ida
JSP 6: Inserting Code Fragments
Declare methods and fields
Use <%! … %> or <jsp:declaration>DECLARATION</jsp:declaration>
▪ <%!
private int visitsSinceServerStarted = 0;
%>
▪ <%! private Connection getDatabaseConnection() {
…
}
%>
Ends up outside the doGet() method
▪ At the top level of the resulting servlet class
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jonkv@ida
JSP 7: Declaring Methods and Fields
Using JSP to display a database table:
▪ <%
Connection conn = …;
Statement statement = conn.createStatement();
ResultSet result = statement.executeQuery(…); %>
<TABLE …>
<% while (result.next()) { %>
<tr>
<td><%= result.getString("name") %></td>
<td><%= result.getString("phone") %></td>
</tr>
<% } %>
HTML within a Java loop! Works due to the translation:
▪ while (result.next()) {
out.print("<tr>\n");
out.print(" <td>"); out.print(result.getString("name")); out.print("</td>");
…
}
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jonkv@ida
JSP 8: Database Example
A number of variables are set up for your use:
request
response
pageContext
session
‐ an HTTPServletRequest
‐ an HTTPServletResponse
‐ a javax.servlet.jsp.PageContext
‐ a javax.servlet.http.HttpSession
‐ a javax.servlet.ServletContext
application
▪ Example: Retrieve a <context‐param> from web.xml using
<%= application.getInitParameter("Webmaster") %>
out
config
‐ a javax.servlet.jsp.JspWriter
‐ a javax.servlet.ServletConfig
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jonkv@ida
JSP 9: Implicit Variables
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Processing Forms using JSP
First, the form:
▪ <html>
<body>
<form method="POST" action="form.jsp">
<input type="checkbox" name="city" value="LA">LA<br>
<input type="checkbox" name="city" value="Boston">Boston<br>
<input type="checkbox" name="city" value="Calgary">Calgary<br>
<br>
<input type="radio" name="month" value="January">January<br>
<input type="radio" name="month" value="June">June<br>
<input type="radio" name="month" value="August">August<br>
<br>
<input type="submit" value="Submit">
</form>
</body>
</html>
jonkv@ida
JSP 10: Form example (HTML form)
66
Processing Forms using JSP
Then, the JSP page, using request (the HTTPServletRequest):
▪ <p>The cities selected are:</p>
<%
final String[] cities = request.getParameterValues("city");
final String month = request.getParameter("month");
if (cities != null) { %>
<ul>
<% for (int i=0; i < cities.length; i++) { %>
<li><%= cities[i] %>
<% } %>
Some of this code might be
</ul>
extracted into "real" classes!
<% } else { %>
<p>None selected.</p>
<% } %>
…
jonkv@ida
JSP 11: Form example (JSP)
67
Page directives (at the top of the file) are used to:
Import necessary Java classes
▪ <%@ page import=“{package.class | package.* }” %>
▪ <jsp:directive import=“{package.class | package.* }” />
Turn on or off session handling (discussed later)
▪ … session = ”true | false” (default is true)
Change buffer sizes
▪ … buffer = ”none | 8kb | sizekb” (default is 8kb)
Declare whether your page is threadsafe
▪ … isThreadSafe = ”true | false”
▪ Default is true: The page may be called multiple times concurrently!
jonkv@ida
JSP 12: Page Directives
68
Init parameters for JSP pages
Also specified within servlet tag – JSP compiled into servlet!
▪ <servlet>
<servlet‐name>newuser</servlet‐name>
<jsp‐file>directory/newuser.jsp</jsp‐file>
…
<init‐param>
<param‐name>emailConfirmationRequired</param‐name>
<param‐value>true</param‐value>
</init‐param>
<init‐param>…</init‐param>
</servlet>
▪ JSP: <%= config.getInitParameter("emailConfirmationRequired") %>
jonkv@ida
JSP 13: Configuration
-- 5 -JSP Standard Tag Library (JSTL)
JavaServer Pages Standard Tag Library
Custom Tag Libraries
Java code and HTML don't mix very well
Harder to read mixed languages with mixed indentation…
▪ <% for (int i=0; i < cities.length; i++) { %>
<li><%= cities[i] %>
<% } %>
Also, Java is strongly typed, exceptions must be handled, …
▪ A nuisance if you prefer the ”script language” style
Solution: Use tag libraries
▪ Well‐defined interface to define your own ”server‐side tags”
JSTL: JavaServer Pages Standard Tag Library
▪ Standard tags, relevant for most applications
▪ Written in Java, but used as tags
JSTL EL: Expression Language
▪ Calculate simple expressions within JSP, without Java code
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jonkv@ida
JSTL 1: Why?
Consider a simple example…
Square numbers using println to generate HTML
▪ <%
for (int i = 0; i < 10; i++) {
out.println(”<p>Here's a number: ” + i + ”</p>”);
out.println(”<p>Its square is ” + (i*i) + ”</p>”);
}
%>
Square numbers using mixed HTML and Java
▪ <% for (int i = 0; i < 10; i++) { %>
<p>Here's a number: <%= i %></p>
<p>Its square is <%= i*i %></p>
<% } %>
jonkv@ida
JSTL 2: Square Example and <c:forEach> 71
A JSTL version:
Declare the use of the "core" tag library, give it the prefix ”c”
▪ <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
▪ JSPX: <html xmlns:c="http://java.sun.com/jsp/jstl/core" …>
▪ The tag library itself consists of Java code
(class library must be included with the final application)
Use tags from JSTL Core, prefixed by ”c:”
▪ <c:forEach var="i" begin="0" end="9">
<p>Here's a number: ${i}</p>
<p>Its square is ${i*i}</p>
</c:forEach>
<c:forEach> creates a scoped variable
▪ Not the same as ”ordinary” Java variables
▪ Its value can be used in the Expression Language (EL), inside ${…}
▪ The Expression Language is not plain Java either!
jonkv@ida
JSTL 3: Square Example and <c:forEach> 72
73
Conditionals are available…
▪ <c:forEach var="i" begin="0" end="9">
<p>Here's a number: ${i}.</p>
<c:if test="${i mod 2 eq 0}">
<c:if> contains a test written in the
<p>The number is even.</p>
Expression Language. (Not Java –
</c:if>
notice the operators "mod" and "eq",
<p>Its square is ${i*i}.</p>
though "%" and "==" are also available
here.)
</c:forEach>
jonkv@ida
JSTL 4: Conditionals, <c:if>
74
There is no "else”!
But there is a choose/when/otherwise:
▪ <c:forEach var="i" begin="0" end="9">
<p>Here's a number: ${i}</p>
<c:choose>
<c:when test="${i mod 2 eq 0}">
<p>The number is even.</p>
This is similar to switch/case in Java, in
</c:when>
that you can have multiple "when" tags
<c:otherwise>
<p>The number is odd.</p> in a "choose" tag.
</c:otherwise>
</c:choose>
<p>Its square is ${i*i}</p>
</c:forEach>
jonkv@ida
JSTL 5: Conditionals, <c:choose>
Then, the JSP page, using JSTL tags
The EL variable ”param” contains all parameter values
▪ Implicitly declared, always available
▪ Parameters are provided by forms (PUT or POST),
or by URLs of the form ”something.jsp?yourname=Jonas&par1=val1&par2=val2&…”
▪ <p>Your name is: <c:out value="${param.yourname}” /></p>
The ”empty” operator tests for null, empty strings, empty arrays, …
▪ <c:if test="${empty param.yourname}">
<p>You forgot to specify a name.</p>
</c:if>
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jonkv@ida
JSTL 6: Forms, Revisited (JSP+JSTL)
76
Use ”paramValues” for parameters with multiple values
▪ Many checkboxes may be selected
Use <c:forEach> to iterate over arrays, collections, …
▪ <c:choose>
<c:when test="${empty paramValues.city}">
<p>None selected.</p>
</c:when>
paramValues.city is a list of cities
<c:otherwise>
selected in the form (in Java code:
<p>The cities selected are:</p>
request.getParameterValues(”city”
<ul>
)
<c:forEach var="city" items="${paramValues.city}">
<li><c:out value="${city}"/></li>
</c:forEach>
”city” is a new scoped variable in the
</ul>
Expression Language
</c:otherwise>
</c:choose>
jonkv@ida
JSTL 7: Forms, Revisited (JSP+JSTL)
77
JSTL has special tags for SQL access
Specify use of the SQL tag library in each JSP page/doc
▪ <%@ taglib uri="http://java.sun.com/jsp/jstl/sql" prefix="sql"%>
Specify a default DataSource used by these tags
▪ A DataSource was already configured and named in Tomcat’s config
▪ Tell JSTL you want to use this DataSource using web.xml:
<context-param>
<param-name>javax.servlet.jsp.jstl.sql.dataSource</param-name>
<param-value>jdbc/mydb</param-value>
</context-param>
▪ …where jdbc/mydb is replaced with the name you chose before!
jonkv@ida
JSTL/SQL 1: Databases – setup
78
Display a database table using JSTL:
Use <sql:query> ‐‐ opens and closes connections as needed
Returns a Result, whose ”rows” field can be iterated over
▪ <sql:query var="result">SELECT * FROM myTable; </sql:query>
<TABLE …>
<tbody>
<c:forEach var="row" items="${result.rows}">
<tr>
<td><c:out value="${row.name}"/></td>
<td><c:out value="${row.phone}"/></td> </tr>
</c:forEach>
</tbody>
</TABLE>
Use <c:out /> tag to convert ”<” into ”<” etc.
▪ If the user enters name = ”<b>hello”, you will see ”<b>hello”
▪ With just plain ”<td>${row.name}</td>”, the rest of the web page would be bold…
jonkv@ida
JSTL/SQL 2: Queries – <sql:query>
79
Queries are always PreparedStatements
Use ? as placeholder for a parameter
▪ Can be used in most places, but not (for example) in LIMIT clauses
▪ <sql:param> for most parameter values
▪ <sql:dateparam> for dates
▪ <sql:query var="result">
SELECT * FROM myTable WHERE name LIKE ’?’
<sql:param value="${param.namepattern}"/>
</sql:query>
jonkv@ida
JSTL/SQL 3: Query Parameters
80
You may want to limit the number of result rows
If you have a thousand topics, you don’t display all on one page
JSTL always reads all rows resulting from a query…
Solution:
<sql:query var="result" startRow=”0" maxRows=”50”>
…
</sql:query>
<c:if test=”${result.limitedByMaxRows}”>
There are additional results that have not been returned.
</c:if>
jonkv@ida
JSTL/SQL 4: Limiting Results
81
Database updates using JSTL
A form to enter information for a new user
▪ <form method="post" action="addUser.jsp">
<input type="text" name="subject"></input> …
</form>
A JSP+JSTL page to which this information is sent
▪ <!‐‐ First ensure that parameters are correctly provided! ‐‐>
<sql:update var="updateResult"> <!‐‐ Result: # rows changed ‐‐>
INSERT INTO messages VALUES(NULL,?,?,NULL,?,?)
<sql:param value="${param.topicid}"/>
<sql:param value="${param.user}"/>
<sql:param value="${user.messagesWritten + 1}"/>
<sql:param value="${param.message}"/>
</sql:update>
jonkv@ida
JSTL/SQL 5: Updates – <sql:update>
Database transactions using JSTL
Enclose in <sql:transaction>
▪ <sql:transaction>
<sql:query> … </sql:query>
<!– do something based on the query ‐‐>
<sql:update> … </sql:update>
<sql:update> … </sql:update>
<sql:update> … </sql:update>
</sql:transaction>
Java (and JSP / JSTL) signal errors using exceptions
▪ If an exception occurs ”inside” <sql:transaction>,
the transaction is automatically rolled back
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jonkv@ida
JSTL/SQL 6: <sql:transaction>
83
Processing and rewriting relative URLs
May be necessary to add session ID for session tracking
<c:url> generates a variable containing a rewritten URL
Message forum example:
Suppose this webapp is mapped at "/forum"
<c:url var="myURL" value="/adduser" >
<c:param name=“user" value="${param.name}" />
</c:url>
Result: "myURL" variable may become either…
▪ "http://www.host.com/forum/adduser?user=Jonas+Kvarnstr%F6m" or
▪ "http://www.host.com/forum/adduser;jsession=ede1ac355149582fb3
4ba9e6d5a347bc?user=Jonas+Kvarnstr%F6m"
Used later in the page
▪ <p><a href="${myURL}">Add user!</a>
jonkv@ida
JSTL: Building URLs – <c:url>
84
JSTL is just the standard tag library – many others exist
You can write your own tags!
▪ http://java.sun.com/developer/technicalArticles/xml/WebAppDev3/
▪ public class Subtract extends TagSupport {
private int num1, num2;
public void setNum1(int num1) { this.num1 = num1; }
public void setNum2(int num2) { this.num2 = num2; }
public int doStartTag() throws JspException {
try {
pageContext.getOut().print("Welcome to First Grade Math! ");
pageContext.getOut().print("If you subtract" + num2 + " from " + num1 + ", you get: "+ (num1 ‐ num2));
} catch (IOException ioe) { … }
return SKIP_BODY;
}
}
▪ … plus XML config JSP tag <math:sub num1="12" num2="3" />
jonkv@ida
Custom Tag Libraries
-- 6 -The JSTL Expression Language
Expression language used within ${…} is NOT JAVA!
JSP‐specific expressions where some operators are similar
Translated into Java by servlet container
Operators:
== (or eq), != (or ne), < (or lt), > (or gt), <= (or le), >= (or ge)
▪ Operator names without <> can be used more easily in XML!
Addition (+), subtraction (‐), multiplication (*),
division (/ or div), remainder/modulo (% or mod)
&& (or and), || (or or), and ! (or not).
"empty" is used to determine if a value is null or empty
▪ <c:if test=”${empty param.name}”>
<p>Please specify your name.</p>
</c:if>
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jonkv@ida
EL 1: Not Java; operator overview
87
Implicit variables are set up automatically to access:
HTTP request parameters (such as form input)
▪ param.name or param["name"]: Single‐valued parameter "name"
▪ paramValues.name: When multiple values are possible (returns array)
HTTP request headers (user‐agent, accept‐language, …)
▪ Via header and headerValues
Servlet/JSP initialization parameters
▪ initParam (specified in web.xml for configuration; discussed later)
Cookies
▪ cookie (cookies received in request; discussed later)
The PageContext
▪ pageContext gives access to a JSP PageContext object
▪ Gives access to HttpServletRequest, ServletContext, and HttpSession
▪ ServletContext: Global configuration for the entire webapp
jonkv@ida
EL: Some implicit variables
Variables available in EL are not Java variables
<c:out value="${param}" /> ‐‐ OK
<% System.out.println(param) %> ‐‐ "param" unknown!
Reason: You can choose their scope
Choose how they are shared and how long they live
Page: Local to this page (the most restricted)
Request: A single call from the browser to the server
▪ May consist of multiple pages, using e.g. <jsp:include>
Session: Persists during multiple calls from the same user
▪ Will discuss sessions in detail later
Application: Globally available in the web application
▪ Many sessions for this and other users)
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jonkv@ida
EL: Variables are not Java Variables
Supporting scopes requires special care
Use JSTL maps containing variable names and values
▪ One applicationScope map
▪ One sessionScope map per session
▪ One requestScope map per request
▪ One pageScope map per page
▪ <c:set value=”val" var=”myVar” scope=”session” />
▪ Actually results in sessionScope.put(”myVar”, ”val”);
▪ ${sessionScope.myVar} …
▪ <c:remove var="variableName" [scope=…] />
(important for long‐lived scopes such as session/application!)
Or use corresponding JSP/Java objects and methods
▪ pageContext.getAttribute(”…”)
▪ …
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jonkv@ida
EL: Variables are not Java Variables
90
Implicit scope references
If you don’t specify a scope, all scopes are searched
▪ Smallest scope first
▪ <c:set value=”val" var=”myVar” scope=”session” />
▪ ${sessionScope.myVar}
▪ Guaranteed to refer to the variable above
▪ ${myVar}
▪ Checks the page scope for myVar
▪ If not found, checks request scope
▪ If not found, checks sessionScope and returns the same myVar used above
jonkv@ida
EL: Variables are not Java Variables
91
Variables may be composite (objects, arrays, maps)
Access using "." or [] ▪ ".foo" for identifier syntax
▪ "[foo]" when an expression (foo) identifies the ”index”
Example: param is a map from parameter names to values
▪ In form.html:
<FORM action="http://somesite.com/add.jsp" method="get">
<table class="mytable">
<tr><td>Name:<td><INPUT type="text" name="name">
<tr><td><INPUT type="submit" value="Send">
</table>
</FORM>
▪ Fill in "Bob Jones" and press Send
▪ In add.jsp: ${param.name} is "Bob Jones"
▪ In add.jsp: ${param["name"]} is also "Bob Jones"
▪ In add.jsp: ${param["na" + "me"]} is also "Bob Jones"
jonkv@ida
EL: Composite variables
JSTL variables have default values
No null pointer exceptions (but less exact error checking)
▪ Java: If user.address is null, user.address.city NPException
▪ JSTL/EL: If user.address is null or not defined,
user.address.city is null
Integers default to 0
▪ <c:forEach items=”${products}” begin=”${param.start + 10}”>
...
</c:forEach>
▪ If param.start undefined (no "start=…" provided to the script), param.start is considered to be 0
Provide your own defaults in <c:out>!
▪ <c:out value="${user.address.city}" default="N/A" />
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jonkv@ida
EL: Default Values
93
Standard EL functions available in functions tag library
JSP: <%@ taglib
uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
JSPX: <html
xmlns:c="http://java.sun.com/jsp/jstl/core"
xmlns:fn="http://java.sun.com/jsp/jstl/functions" … >
▪
▪
▪
▪
▪
▪
▪
▪
length (of array, Collection, String, …)
toUpperCase, toLowerCase
substring, substringAfter, substringBefore
trim replace indexOf, startsWith, endsWith, contains, containsIgnoreCase
split, join escapeXml
jonkv@ida
EL: Standard Functions
94
A JSP page, test.jsp, contains the following:
▪ ${param.a} > ${param.b}: ${param.a > param.b}
${param.a} > 13: ${param.a > 13}
We call it: test.jsp?a=2&b=13
▪ 2 > 13: true
2 > 13: false
Why?
All query parameters are stored as strings!
▪ Converted to numbers when required
▪ The string “2” is greater than the string “13”
The string “2” is converted to the number 2 which is not greater than 13
jonkv@ida
EL: Variable Types…
