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> ## Core Tag Library

> Table 6-2 summarizes the core tags, which include those related to variables and flow control, as well as a generic way to access URL-based resources whose content can then be included or processed within the JSP page.

> Table 6-2 Core Tags
> Area | Function | Tags | Prefix
> --- | --- | --- | ---
> Core | Variable support | `remove` `set` | `c`
> Flow control | `choose` `when` `otherwise` `forEach` `forTokens` `if`
> URL management | `import` `param` `redirect` `param` `url` `param`
> Miscellaneous | `catch` `out`

> ### Variable Support Tags

> The `set` tag sets the value of an EL variable or the property of an EL variable in any of the JSP scopes (page, request, session, or application). If the variable does not already exist, it is created.

> The JSP EL variable or property can be set either from the attribute `value`:

>     <c:set var="foo" scope="session" value="..."/>

> or from the body of the tag:

>     <c:set var="foo">
>       ...
>     </c:set>

> For example, the following sets an EL variable named `bookID` with the value of the request parameter named `Remove`:

>     <c:set var="bookId" value="${param.Remove}"/>

> To remove an EL variable, you use the `remove` tag. When the bookstore JSP page `bookreceipt.jsp` is invoked, the shopping session is finished, so the `cart` session attribute is removed as follows:

>     <c:remove var="cart" scope="session"/>

> The `value` attribute of the `set` tag can also take a deferred value expression (See Immediate and Deferred Evaluation Syntax, page 110) so that JavaServer Faces component tags can access the value at the appropriate stage of the page life cycle.

> JavaServer Faces technology (see Chapter 9) supports a multi-phase life cycle, which includes separate phases for rendering components, validating data, updating model values, and performing other tasks. What this means is that any JavaServer Faces component tags that reference the value set by the `set` tag must have access to this value at different phases of the life cycle, not just during the rendering phase. Consider the following code:

>     <c:set var="bookId" scope="page" value="#{BooksBean.books}"/>
>     ...
>     <h:inputText id="bookId" value="#{bookId}"/>
>     ...

> The `value` attribute of the c:set tag uses a deferred value expression, which means that the `bookId` variable it references is available not only during the rendering phase of the JavaServer Faces life cycle but also during the later stages of the life cycle. Therefore, whatever value the user enters into the `bookId` component tag is updated to the external data object during the appropriate stage of the life cycle.

> If the expression referenced by the `value` attribute used immediate evaluation syntax then the `bookId` variable would be available only when the component is rendered during the render response phase. This would prevent the value the user enters into the component from being converted, validated, or updated to the external data object during the later phases of the life cycle.

> ### Flow Control Tags

> To execute flow control logic, a page author must generally resort to using scriptlets. For example, the following scriptlet is used to iterate through a shopping cart:

>     <%
>       Iterator i = cart.getItems().iterator();
>       while (i.hasNext()) {
>         ShoppingCartItem item =
>           (ShoppingCartItem)i.next();
>         ...
>     %>
>         <tr>
>         <td align="right" bgcolor="#ffffff">
>         ${item.quantity}
>         </td>
>         ...
>     <%
>       }
>     %>

> Flow control tags eliminate the need for scriptlets. The next two sections have examples that demonstrate the conditional and iterator tags.

> #### Conditional Tags

> The `if` tag allows the conditional execution of its body according to the value of the `test` attribute. The following example from `bookcatalog.jsp` tests whether the request parameter `Add` is empty. If the test evaluates to `true`, the page queries the database for the book record identified by the request parameter and adds the book to the shopping cart:

>     <c:if test="${!empty param.Add}">
>       <c:set var="bid" value="${param.Add}"/>
>       <jsp:useBean id="bid"  type="java.lang.String" />
>        <sql:query var="books"
>         dataSource="${applicationScope.bookDS}">
>         select * from PUBLIC.books where id = ?
>         <sql:param value="${bid}" />
>       </sql:query>
>       <c:forEach var="bookRow" begin="0" items="${books.rows}">
>         <jsp:useBean id="bookRow" type="java.util.Map" />
>         <jsp:useBean id="addedBook"
>           class="database.Book" scope="page" />
>       ...
>       <% cart.add(bid, addedBook); %>
>     ...
>     </c:if>

> The `choose` tag performs conditional block execution by the embedded `when` subtags. It renders the body of the first `when` tag whose test condition evaluates to `true`. If none of the test conditions of nested `when` tags evaluates to `true`, then the body of an `otherwise` tag is evaluated, if present.

> For example, the following sample code shows how to render text based on a customer's membership category.

>     <c:choose>
>       <c:when test="${customer.category == 'trial'}" >
>         ...
>       </c:when>
>       <c:when test="${customer.category == 'member'}" >
>         ...
>       </c:when>
>         <c:when test="${customer.category == 'preferred'}" >
>         ...
>       </c:when>
>       <c:otherwise>
>         ...
>       </c:otherwise>
>     </c:choose>

> The `choose`, `when`, and `otherwise` tags can be used to construct an `if`-`then`-`else` statement as follows:

>     <c:choose>
>       <c:when test="${count == 0}" >
>         No records matched your selection.
>       </c:when>
>       <c:otherwise>
>         ${count} records matched your selection.
>       </c:otherwise>
>     </c:choose>

> #### Iterator Tags

> The `forEach` tag allows you to iterate over a collection of objects. You specify the collection via the `items` attribute, and the current item is available through a variable named by the `var` attribute.

> A large number of collection types are supported by `forEach`, including all implementations of `java.util.Collection` and `java.util.Map`. If the `items` attribute is of type `java.util.Map`, then the current item will be of type `java.util.Map.Entry`, which has the following properties:

> - `key`: The key under which the item is stored in the underlying `Map`
> - `value`: The value that corresponds to the key

> Arrays of objects as well as arrays of primitive types (for example, `int`) are also supported. For arrays of primitive types, the current item for the iteration is automatically wrapped with its standard wrapper class (for example, `Integer` for `int`, `Float` for `float`, and so on).

> Implementations of `java.util.Iterator` and `java.util.Enumeration` are supported, but they must be used with caution. `Iterator` and `Enumeration` objects are not resettable, so they should not be used within more than one iteration tag. Finally, `java.lang.String` objects can be iterated over if the string contains a list of comma-separated values (for example: Monday,Tuesday,Wednesday,Thursday,Friday).

> Here's the shopping cart iteration from the preceding section, now with the `forEach` tag:

>     <c:forEach var="item" items="${sessionScope.cart.items}">
>       ...
>       <tr>
>         <td align="right" bgcolor="#ffffff">
>         ${item.quantity}
>       </td>
>       ...
>     </c:forEach>

> The `forTokens` tag is used to iterate over a collection of tokens separated by a delimiter.

> Similarly to the value attribute of the `c:set` tag (see Variable Support Tags), the `items` attribute of `forEach` and `forTokens` can also take a deferred value expression so that JavaServer Faces tags can be included within these tags.

> As described in Variable Support Tags, JavaServer Faces technology (see Chapter 9) supports a multi-phase life cycle. Therefore, any JavaServer Faces component tags that are included in the `forEach` tag or the `forTokens` tag must have access to the variable referenced by the `items` attribute at different phases of the life cycle, not just during the rendering phase. Consider the following code:

>     <c:forEach var="book" items="#{BooksBean.books}">
>       ...
>       <h:inputText id="quantity" value="#{book.quantity}"/>
>       ...
>     </c:forEach>

> The `items` attribute uses a deferred value expression, which means that the `book` variable it references is available not only during the rendering phase of the JavaServer Faces life cycle but also during the later stages of the life cycle. Therefore, whatever values the user enters into the `quantity` component tags are updated to the external data object during the appropriate stage of the life cycle.

> If the expression referenced by the `items` attribute used immediate evaluation syntax then the `book` variable would be available only when the component is rendered during the render response phase. This would prevent the values the user enters into the components from being converted, validated, or updated to the external data object during the later phases of the life cycle. The JavaServer Faces version of Duke's Bookstore includes a `forEach` tag on its `bookcatalog.jsp` page.

> ### URL Tags

> The `jsp:include` element provides for the inclusion of static and dynamic resources in the same context as the current page. However, `jsp:include` cannot access resources that reside outside the web application, and it causes unnecessary buffering when the resource included is used by another element.

> In the following example, the `transform` element uses the content of the included resource as the input of its transformation. The `jsp:include` element reads the content of the response and writes it to the body content of the enclosing transform element, which then rereads exactly the same content. It would be more efficient if the `transform` element could access the input source directly and thereby avoid the buffering involved in the body content of the transform tag.

>     <acme:transform>
>       <jsp:include page="/exec/employeesList"/>
>     <acme:transform/>

> The `import` tag is therefore the simple, generic way to access URL-based resources, whose content can then be included and or processed within the JSP page. For example, in XML Tag Library, `import` is used to read in the XML document containing book information and assign the content to the scoped variable `xml`:

>     <c:import url="/books.xml" var="xml" />
>     <x:parse doc="${xml}" var="booklist"
>       scope="application" />

> The `param` tag, analogous to the `jsp:param` tag (see jsp:param Element, page 141), can be used with `import` to specify request parameters.

> In Session Tracking (page 88) we discuss how an application must rewrite URLs to enable session tracking whenever the client turns off cookies. You can use the `url` tag to rewrite URLs returned from a JSP page. The tag includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged. Note that this feature requires that the URL be relative. The `url` tag takes `param` subtags to include parameters in the returned URL. For example, `bookcatalog.jsp` rewrites the URL used to add a book to the shopping cart as follows:

>     <c:url var="url" value="/catalog" >
>       <c:param name="Add" value="${bookId}" />
>     </c:url>
>     <p><strong><a href="${url}">

> `The redirect` tag sends an HTTP redirect to the client. The `redirect` tag takes `param` subtags for including parameters in the returned URL.

> ### Miscellaneous Tags

> The `catch` tag provides a complement to the JSP error page mechanism. It allows page authors to recover gracefully from error conditions that they can control. Actions that are of central importance to a page should not be encapsulated in a `catch`; in this way their exceptions will propagate instead to an error page. Actions with secondary importance to the page should be wrapped in a `catch` so that they never cause the error page mechanism to be invoked.

> The exception thrown is stored in the variable identified by `var`, which always has page scope. If no exception occurred, the scoped variable identified by `var` is removed if it existed. If `var` is missing, the exception is simply caught and not saved.

> The `out` tag evaluates an expression and outputs the result of the evaluation to the current `JspWriter` object. The syntax and attributes are as follows:

>     <c:out value="value" [escapeXml="{true|false}"]
>       [default="defaultValue"] />

> If the result of the evaluation is a `java.io.Reader` object, then data is first read from the `Reader` object and then written into the current `JspWriter` object. The special processing associated with `Reader` objects improves performance when a large amount of data must be read and then written to the response.

> If `escapeXml` is true, the character conversions listed in Table 6-3 are applied.

> Table 6-3 Character Conversions
> Character | Character Entity Code
> --- | ---
> `<` | `&lt;`
> `>` | `&gt;`
> `&` | `&amp;`
> ' | `&#039;`
> `"` | `&#034;`