JSP - Syntax(语法)

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https://www.tutorialspoint.com/jsp/jsp_syntax.htm

This tutorial will give basic idea on simple syntax (ie. elements) involved with JSP development:

The Scriptlet:

A scriptlet can contain any number of JAVA language statements, variable or method declarations, or expressions that are valid in the page scripting language.

Following is the syntax of Scriptlet:

<% code fragment %>

You can write XML equivalent of the above syntax as follows:

<jsp:scriptlet>   code fragment</jsp:scriptlet>

Any text, HTML tags, or JSP elements you write must be outside the scriptlet. Following is the simple and first example for JSP:

<html><head><title>Hello World</title></head><body>Hello World!<br/><%out.println("Your IP address is " + request.getRemoteAddr());%></body></html>

NOTE: Assuming that Apache Tomcat is installed in C:\apache-tomcat-7.0.2 and your environment is setup as per environment setup tutorial.

Let us keep above code in JSP file hello.jsp and put this file in C:\apache-tomcat-7.0.2\webapps\ROOT directory and try to browse it by giving URL http://localhost:8080/hello.jsp. This would generate following result:

JSP Declarations:

A declaration declares one or more variables or methods that you can use in Java code later in the JSP file. You must declare the variable or method before you use it in the JSP file.

Following is the syntax of JSP Declarations:

<%! declaration; [ declaration; ]+ ... %>

You can write XML equivalent of the above syntax as follows:

<jsp:declaration>   code fragment</jsp:declaration>

Following is the simple example for JSP Declarations:

<%! int i = 0; %> <%! int a, b, c; %> <%! Circle a = new Circle(2.0); %>

JSP Expression:

A JSP expression element contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file.

Because the value of an expression is converted to a String, you can use an expression within a line of text, whether or not it is tagged with HTML, in a JSP file.

The expression element can contain any expression that is valid according to the Java Language Specification but you cannot use a semicolon to end an expression.

Following is the syntax of JSP Expression:

<%= expression %>

You can write XML equivalent of the above syntax as follows:

<jsp:expression>   expression</jsp:expression>

Following is the simple example for JSP Expression:

<html> <head><title>A Comment Test</title></head> <body><p>   Today's date: <%= (new java.util.Date()).toLocaleString()%></p></body> </html>

This would generate following result:

Today's date: 11-Sep-2010 21:24:25

JSP Comments:

JSP comment marks text or statements that the JSP container should ignore. A JSP comment is useful when you want to hide or "comment out" part of your JSP page.

Following is the syntax of JSP comments:

<%-- This is JSP comment --%>

Following is the simple example for JSP Comments:

<html> <head><title>A Comment Test</title></head> <body> <h2>A Test of Comments</h2> <%-- This comment will not be visible in the page source --%> </body> </html>

This would generate following result:

A Test of Comments

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:

SyntaxPurpose<%-- comment --%>A JSP comment. Ignored by the JSP engine.An HTML comment. Ignored by the browser.<\%Represents static <% literal.%\>Represents static %> literal.\'A single quote in an attribute that uses single quotes.\"A double quote in an attribute that uses double quotes.

JSP Directives:

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

<%@ directive attribute="value" %>

There are three types of directive tag:

DirectiveDescription<%@ page ... %>Defines page-dependent attributes, such as scripting language, error page, and buffering requirements.<%@ include ... %>Includes a file during the translation phase.<%@ taglib ... %>Declares a tag library, containing custom actions, used in the page

We would explain JSP directive in separate chapter JSP - Directives

JSP 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.

There is only one syntax for the Action element, as it conforms to the XML standard:

<jsp:action_name attribute="value" />

Action elements are basically predefined functions and there are following JSP actions available:

SyntaxPurposejsp:includeIncludes a file at the time the page is requestedjsp:useBeanFinds or instantiates a JavaBeanjsp:setPropertySets the property of a JavaBeanjsp:getPropertyInserts the property of a JavaBean into the outputjsp:forwardForwards the requester to a new pagejsp:pluginGenerates browser-specific code that makes an OBJECT or EMBED tag for the Java pluginjsp:elementDefines XML elements dynamically.jsp:attributeDefines dynamically defined XML element's attribute.jsp:bodyDefines dynamically defined XML element's body.jsp:textUse to write template text in JSP pages and documents.

We would explain JSP actions in separate chapter JSP - Actions

JSP Implicit Objects:

JSP supports nine automatically defined variables, which are also called implicit objects. These variables are:

ObjectsDescriptionrequestThis is the HttpServletRequest object associated with the request.responseThis is the HttpServletResponse object associated with the response to the client.outThis is the PrintWriter object used to send output to the client.sessionThis is the HttpSession object associated with the request.applicationThis is the ServletContext object associated with application context.configThis is the ServletConfig object associated with the page.pageContextThis encapsulates use of server-specific features like higher performance JspWriters.pageThis is simply a synonym for this, and is used to call the methods defined by the translated servlet class.ExceptionThe Exception object allows the exception data to be accessed by designated JSP.

We would explain JSP Implicit Objects in separate chapter JSP - Implicit Objects.

Control-Flow Statements:

JSP provides full power of Java to be embedded in your web application. You can use all the APIs and building blocks of Java in your JSP programming including decision making statements, loops etc.

Decision-Making Statements:

The if...else block starts out like an ordinary Scriptlet, but the Scriptlet is closed at each line with HTML text included between Scriptlet tags.

<%! int day = 3; %> <html> <head><title>IF...ELSE Example</title></head> <body><% if (day == 1 | day == 7) { %>      <p> Today is weekend</p><% } else { %>      <p> Today is not weekend</p><% } %></body> </html>

This would produce following result:

Today is not weekend

Now look at the following switch...case block which has been written a bit differentlty using out.println() and inside Scriptletas:

<%! int day = 3; %> <html> <head><title>SWITCH...CASE Example</title></head> <body><% switch(day) {case 0:   out.println("It\'s Sunday.");   break;case 1:   out.println("It\'s Monday.");   break;case 2:   out.println("It\'s Tuesday.");   break;case 3:   out.println("It\'s Wednesday.");   break;case 4:   out.println("It\'s Thursday.");   break;case 5:   out.println("It\'s Friday.");   break;default:   out.println("It's Saturday.");}%></body> </html>

This would produce following result:

It's Wednesday.

Loop Statements:

You can also use three basic types of looping blocks in Java: for, while,and do…while blocks in your JSP programming.

Let us look at the following for loop example:

<%! int fontSize; %> <html> <head><title>FOR LOOP Example</title></head> <body><%for ( fontSize = 1; fontSize <= 3; fontSize++){ %>   <font color="green" size="<%= fontSize %>">    JSP Tutorial   </font><br /><%}%></body> </html>

This would produce following result:

       JSP Tutorial   
        JSP Tutorial   
        JSP Tutorial

Above example can be written using while loop as follows:

<%! int fontSize; %> <html> <head><title>WHILE LOOP Example</title></head> <body><%while ( fontSize <= 3){ %>   <font color="green" size="<%= fontSize %>">    JSP Tutorial   </font><br /><%fontSize++;%><%}%></body> </html>

This would also produce following result:

       JSP Tutorial           JSP Tutorial           JSP Tutorial

JSP Operators:

JSP supports all the logical and arithmetic operators supported by Java. Following table give a list of all the operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom.

Within an expression, higher precedence operators will be evaluated first.

Category Operator Associativity Postfix () [] . (dot operator)Left to right Unary ++ - - ! ~Right to left Multiplicative * / % Left to right Additive + - Left to right Shift >> >>> << Left to right Relational > >= < <= Left to right Equality == != Left to right Bitwise AND & Left to right Bitwise XOR ^ Left to right Bitwise OR | Left to right Logical AND && Left to right Logical OR || Left to right Conditional ?: Right to left Assignment = += -= *= /= %= >>= <<= &= ^= |= Right to left Comma , Left to right

JSP Literals:

The JSP expression language defines the following literals:

Boolean: true and false
Integer: as in Java
Floating point: as in Java
String: with single and double quotes; " is escaped as \", ' is escaped as \', and \ is escaped as \\.
Null: null

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