query Tag Result Interface

The Result interface is used to retrieve information from objects returned from a query tag.

public interface Result
    public String[] getColumnNames();
    public int getRowCount()
    public Map[] getRows();
    public Object[][] getRowsByIndex();
    public boolean isLimitedByMaxRows();

For complete information about this interface, see the API documentation for the JSTL packages.

The var attribute set by a query tag is of type Result. The getRows method returns an array of maps that can be supplied to the items attribute of a forEach tag. The JSTL expression language converts the syntax ${result.rows} to a call to result.getRows. The expression ${books.rows} in the following example returns an array of maps.

When you provide an array of maps to the forEach tag, the var attribute set by the tag is of type Map. To retrieve information from a row, use the get("colname") method to get a column value. The JSP expression language converts the syntax ${map.colname} to a call to map.get("colname"). For example, the expression ${book.title} returns the value of the title entry of a book map.

The Duke’s Bookstore page tut-install/javaeetutorial5/examples/web/bookstore4/web/books/bookdetails.jsp retrieves the column values from the book map as follows.

<c:forEach var="book" begin="0" items="${books.rows}">
    <h2>${book.title}</h2>
    &nbsp;<fmt:message key="By"/> <em>${book.firstname}
    ${book.surname}</em>&nbsp;&nbsp;
    (${book.year})<br> &nbsp; <br>
    <h4><fmt:message key="Critics"/></h4>
    <blockquote>${book.description}</blockquote>
    <h4><fmt:message key="ItemPrice"/>:
     <fmt:formatNumber value="${book.price}" type="currency"/>
     </h4>
</c:forEach>

The following excerpt from tut-install/javaeetutorial5/examples/web/bookstore4/web/books/bookcatalog.jsp uses the Row interface to retrieve values from the columns of a book row using scripting language expressions. First, the book row that matches a request parameter (bid) is retrieved from the database. Because the bid and bookRow objects are later used by tags that use scripting language expressions to set attribute values and by a scriptlet that adds a book to the shopping cart, both objects are declared as scripting variables using the jsp:useBean tag. The page creates a bean that describes the book, and scripting language expressions are used to set the book properties from book row column values. Then the book is added to the shopping cart.

You might want to compare this version of bookcatalog.jsp to the versions in Chapter 5, JavaServer Pages Technology and Chapter 8, Custom Tags in JSP Pages that use a book database JavaBeans component.

<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="${books.rowsByIndex}">
     <jsp:useBean id="bid"  type="java.lang.String" />
    <jsp:useBean id="bookRow" type="java.lang.Object[]" />
    <jsp:useBean id="addedBook" class="database.Book"
        scope="page" >
        <jsp:setProperty name="addedBook" property="bookId"
            value="${bookRow[0]}" />
        <jsp:setProperty name="addedBook" property="surname"
            value="${bookRow[1]}" />
        <jsp:setProperty name="addedBook" property="firstName"
            value="${bookRow[2]}" />
        <jsp:setProperty name="addedBook" property="title"
            value="${bookRow[3]}" />
        <jsp:setProperty name="addedBook" property="price"
            value="${bookRow[4])}" />
        <jsp:setProperty name="addedBook" property="year"
            value="${bookRow[6]}" />
        <jsp:setProperty name="addedBook"
             property="description"
             value="${bookRow[7]}" />
        <jsp:setProperty name="addedBook" property="inventory"
            value="${bookRow[8]}" />
    </jsp:useBean>
    <% cart.add(bid, addedBook); %>
    ...
</c:forEach>