meljun_cortes_jedi slides web programming chapter06 advanced jsps

8/3/2019 MELJUN_CORTES_JEDI Slides Web Programming Chapter06 Advanced JSPs

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Advanced JSPs

Web Programming


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JSP Expression Language

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Was introduced with the JSP 2.0 specification

Provides a simple and clean syntax for writing expressionsthat perform simple logic or access scoped values

Example: If we want to access the property of a JavaBean, we could use

either:

or


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(Example continued)

Using this method, developers are exposed to Java programmingconstructs: to access a bean's property, developers have to use thebean's getter methods.

The developer also needs to be aware of the Java type of the

property.

This method is more programming-neutral: access to the bean'sproperty is handled through tags that are conceptually similar toHTML tags.

However, this method of accessing a bean's property is long andcumbersome.

Also, this method presents its output directly to the client browser;no manipulation can be performed on the retrieved value.


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JSP Expression Language

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(Example continued)

Using the EL, the bean's property can be accessed as:

${user.lastName}

The above construct is programming-neutral: the developer does not need toknow Java types or syntax.

It is also short and to the point: the construct contains only the scoped variableand the property to be accessed, with a minimum of characters added.

Also, it can be used equally for displaying output directly to the user, or to exposethe value for custom tags to process.


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EL Syntax

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An EL construct can be a literal or an expression enclosedin between ${ and }.

Literals

EL Supported literals: Boolean

Long

Float

String

Null Treated identically as they are in Java. For example, boolean values

can either be true or false, String values need to be enclosed ineither double-quotes ("") or single quotes (''), null defines a non-existent value.


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EL Syntax

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Operators

EL also defines standard operators that can be applied on literals.

Include:

Basic arithmetic operators (+, -, *, /, %) Logical operators (&&, ||, !)

Comparison operators (==, !=, =)


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EL Syntax

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Reserved words

EL also defines reserved words that mimic the functionality of someof the operators:

and (&&), eq (==), gt(>), ge(>=), or (||), ne (!=), le (


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EL Syntax

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Examples of EL Expressions

${'JEDI'} - evaluates to the String 'JEDI'

${5 + 37} - evaluates to 42

${ (10 % 5) == 2} - evaluates to true ${empty ''} - evaluates to true


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Accessing Scoped Variables

and Properties

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While being able to evaluate simple literal expressions isuseful, EL shows its utility the most when accessing andprocessing variables and properties in different scopes

Variable access is simple with EL:

Variables are simply referenced by name.

Bean properties, methods, and arrays can all be accessed from a namedvariable using the "." notation.

Nested methods/properties are accessed the same way.


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Accessing Scoped Variables

and Properties

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Example:

${user.lastName}

Accesses a JavaBean that can be referenced by the name user andretrieves the value of its lastName property.

The scope of the bean does not matter.

EL performs the scope lookup, checking within the page, request, session, andapplication scopes (in that order) for a bean with the specified name.

If EL was provided the name of a bean that does not exist within any scope, avalue of null will be returned.


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EL Implicit Objects

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While automatic scope lookup makes it easier fordevelopers to write code, specifying a variable's scopeexplicitly makes our construct easier to maintain by futuredevelopers.

EL provides us with several implicit objects that represent aMap of the objects within the different scopes:

pageScope

requestScope

sessionScope applicationScope


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EL Implicit Objects

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In addition, EL also defines the following implicit objects:

param: Represents as a Map request parameter names and values.For example invoking ${param.loginName} is equivalent to callingrequest.getParameter("loginName"). The values stored here are

single String values. paramValues: A Map that maps parameter names to String arrays

representing the values for the name. Invoking ${paramValues.choices} is similar to callingrequest.getParameterValues("choices").

header: A Map representing the headers available from a givenrequest. Its contents are similar to that retrieved by calling thegetHeader method from a ServletRequest object.


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EL Implicit Objects

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(Other implicit objects continued)

headerValues: A Map representing the headers available from agiven request. Its contents are similar to that retrieved by calling thegetHeaders method from a ServletRequest object.

cookies: Maps the cookies available in a request. Similar to invokingthe getCookies method from an HttpServletRequest object.


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The [] Notation

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Aside from the "." notation, EL also provides the "[]" notationin accessing member variables, methods, and arrays.

In many ways, the two notations are similar:

${user.lastName}

${user[lastName]}


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JSTL

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Java Standard Tag Library

Frees us from the use of scriptlets within our JSP pages

Custom tags

One of the strong points of the JSP specification is that it allows for adegree of customization and extensibility through the use of customtags.

Provide for a way to expose reusable functionality within a JSP pageusing a very simple interface while hiding the internal implementation.

There are a number of such tag libraries available, and it is unavoidable for customtags developed by programmers to have some sort of overlap in the kind offunctionality that they provide.

Java has recognized this and, in cooperation with the programming community,has provided a standard tag library that addresses the areas of overlap, called theJava Standard Tag Library or JSTL.


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Including the JSTL in our

Application

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To include JSTL functionality in our application:

Simply unpack the zip file available as a download from Java's site.

Copy the standard.jar and jstl.jar files located in the /lib directory intothe /lib directory of our application.

There are several sets of tag libraries packaged underJSTL. Only the core library is covered in this discussion.


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Core Library

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Provide functionality that is useful for any web project

The Core set can be further sub-categorized into:

General purpose tags

Iteration Conditionals

URL manipulation

For every JSP page that will make use of the functionality in

the Core tag library, the following directive must be added tothe page:

This exposes the tags inside the core library using the c prefix.


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General Purpose Tags

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Perform common and simple tasks, though one of them hassince become irrelevant with the release of the JSP 2.0specification

The tags comprising the general purpose set are:

out

set

remove

catch


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Takes in an EL expression, evaluates its result, and thenoutput the result directly to the Writer object correspondingto the page's output.

Functions much the same way as the standard action, with the exception that a JavaBean is notneeded to access the functionality.

Has become rather obsolete with the release of the JSP 2.0specification:

EL expressions can be evaluated direct into the output stream in anypart of the JSP page without using any tags with JSP 2.0.


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Performs much the same functionality as the action in that it is able to set values withina target JavaBean.

Also able to set a variable within a specified scope that canbe used later by the JSP or elsewhere in the application.


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Has the following attributes:

value: The value that will be set into the target bean. Can be an ELexpression.

var: The name of the variable that will be set into a specified scope.

scope: Defines the scope of the variable defined by the var attribute.The value can be one of the following: page, request, session, andapplication.

target: The name of the JavaBean whose property will be set.

property: The name of the property within the JavaBean that willreceive the value.


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As mentioned, there are two primary uses for this tag:

To set the value within a target JavaBean

Tag makes use of only the value, target, and property attributes

which is equivalent to

To set a variable into a defined scope for later use

Tag makes use of only the value, var, and scope attributes, though the scopeattribute is optional. When the scope attribute is omitted, it defaults into usingpage scope.


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In the spirit of limiting the JSP for presentation purposes, weshould limit the use of this tag.


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Provides a way to remove variables from a defined scope.

Has two attributes:

scope the scope of the variable to be removed.

var the name of the variable to be removed from the definedscope.

Like the tag though, use of this tag should belimited.


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Provides error handling capability in specific areas in a JSP.

It is simple to use:

Place the JSP content that could potentially throw errors inside thebody of the tag.

Has only one attribute:

var: Defines the name that will be used to expose the thrownexception. The variable thus created will have page scope; that is, itis accessible even after the catch block has ended.


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Iteration

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Provide alternatives to embedding do-while, while, and/or forloops inside our JSP page as scriptlets.

Two tags:

forEach

forToken


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By far the more commonly used iteration tag.

Allows iteration over:

primitive arrays

instances of: java.util.Collection

java.util.Iterator

java.util.Map

java.util.Enumeration

Does iteration by going through each element in the targetand exposing the current value of the iteration to the JSPcode contained inside the tag's body.


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Has the following attributes:

var: Defines the name of the variable used to expose the currentvalue of the iteration to the tag body. Its type is dependent on thecollection being iterated over.

items: Defines the collection to be iterated over. This can bespecified as an EL expression.


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(Attributes continued)

Optional attributes:

varStatus: Defines the name of a variable that can be accessed by the loop bodyto gain information on the current loop status.

begin: An int value that defines the index that will be used as the starting point ofthe iteration. If this value is not supplied, the iteration index starts at 0, thebeginning of the collection. Can be a runtime expression.

end: An int value that defines the index that determines the stopping point of theiteration. If no value is specified, the iteration will continue until the last element isreached.

step: An int value that defines the step size to be used while going through the

iteration.


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A common use of this tag is to iterate over the results ofprocessing performed elsewhere in the application.

Sample scenario

We have an application module that retrieves from the database user details that

fall under a specific search category. Naturally, we want to perform the databaseaccess logic from a servlet and pass on the data to a JSP for presentation.


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...// load user parameters into a MapMap parameters = loadUserParameters();UserDataService service = new UserDataService();// perform database search, and store the results inside a Collection.Collection results = service.searchUsers(parameters);

// store the results for future accessrequest.setAttribute("searchResults", results);// forward the request to a JSP for displayrequest.getRequestDispatcher("/results.jsp").forward(request, response);...

JSP using JSTL


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JSP using JSTL

The following users met your search criteria :

${user.name} - ${user.address}

JSP without JSTL

The following users met your search criteria :

-


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The other iteration tag provided by JSTL.

Tag takes in a String and parses them into tokens based ona given delimiter.

All of the attributes of the forEach tag are shared by this tag,with an addition:

delims: Defines the delimiter to be used when parsing the targetString into tokens

The items attribute now has a new purpose for this tag it

defines the String that will be tokenized.


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Sample JSP

...

${item}


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Output of Sample JSP


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Conditionals

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Mimic the funtionality provided by standard if, else-if, ...statements that can be found in standard Java.

Usage of this tags over standard Java statements lead tocleaner code.

Two main sets of conditionals:

the tag that mimics a simple Java if statement.

the tag along with the related tags and, that altogether mimic the functionality of a switchstatement.


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Allows execution of the contents of its body if the value ofthe evaluated expression in its test attribute is equal to true.If the expression is evaluated to be false, the body is neverexecuted.

Example:

You are not currently logged in! Please correct before continuing any further.

h h


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, ,

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These three tags work together to provide switchfunctionality within our application.

Basically, one or more when tags are placed in the body ofa choose tag.

The choose tag evaluates each of the test attributes of the whentags and executes the body of the when tag which evaluates to true.

If there are no when tags that successfully match, choose calls onthe otherwise tag, if included.

h h


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, ,

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Example:

Outstanding!Good!Failed!

Congratulations ...


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URL Manipulation

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JSTL provides a few custom tags that provides basic URLmanipulation capabilities.

These tags build and improve upon actions alreadyavailable to JSPs:

import

param

url


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Works in the same way as the JSP include action, onlybetter.

The JSP include action allowed only pages inside the webapplication to be referenced and included within the current page's

content. The tag allows developers to specify absolute URLs

pointing to external resources.


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The attribute to use for this tag is:

url: The value of the URL to import, which can either be:

A relative URL, pointing to a resource within the web application

An absolute URL, capable of pointing to external resources

Contains a number of other attributes. However, they areused for programmatic modification of the contents of theincluded resource, something that could be better doneoutside of the JSP.


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A versatile tag used in conjunction with a number of othertags within the URL manipulation group.

Cannot be used on its own, but only as a child tag of othertags available in JSTL.

Used to provide a way to include request parameters andvalues within a URL.


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The attributes used for this tag are:

name: Used to indicate the name of the request parameter.

value: Used to indicate the value of the request parameter.


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Example:

If we want to include a dynamic resource that needed values forrequest parameters, it should look like the snippet presented above.

However, the problem with the above example is that it does nottake into account URL encoding rules.

One rule for encoding URLs is that spaces must be denoted using specialcharacters.


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Example:

The tag is able to abstract those encoding rules for us.Instead of worrying about how to rewrite the URL such that it iscompliant with encoding rules, we could simply say:


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Used to provide a way to automatically reencode a URL withthe information necessary to support sessions.

Relevant attribute:

value: The URL to be processed.

For clarification, this tag does NOT always reencode theURLs. It only does so when it detects that the client browserdoes not support cookies.

Like the import tag, this tag can have zero to many

tags for parameter inclusion.


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Web Programming

Example:

Click here to enter the application.

meljun_cortes_jedi slides web programming chapter06 advanced jsps

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