h：dataTable显示数据库中更多和重复的重复项

Question

我试图在h:dataTable中显示名为BOOKS的数据库表中的数据，该表从名为getBooks()的方法中获取数据。

public ArrayList<Book> getBooks() throws SQLException {
    if (ds == null) {
        throw new SQLException("ds is null; Can't get data source");
    }

    Connection conn = ds.getConnection();

    if (conn == null) {
        throw new SQLException("conn is null; Can't get db connection");
    }
    try {
        PreparedStatement ps = conn.prepareStatement(
                "select TITLE, AUTHOR, PRICE from BOOKS"
        );

        // retrieve customer data from database
        ResultSet result = ps.executeQuery();

        while (result.next()) {
            Book b = new Book();
            b.setTitle(result.getString("TITLE"));
            b.setAuthor(result.getString("AUTHOR"));
            b.setPrice(result.getDouble("PRICE"));
            books.add(b);
        }

    } finally {
        conn.close();
    }
    return books;
}

SQL语句工作正常并且它检索正确的数据，但是当这个JSF页面运行时：

<h:body>
    <center>
        <h1>Welcome to the Java Bookstore!</h1>
        <h:form>
            <h3>Inventory List</h3>
            <h:dataTable value="#{bookDatabase.books}" var="book" border="5" class="bookList">

                <h:column>
                    <f:facet name="header">Title</f:facet>
                    <h:outputText value="#{book.title}" />
                </h:column>
                <h:column>
                    <f:facet name="header">Author</f:facet>
                    <h:outputText value="#{book.author}" />
                </h:column>
                <h:column>
                    <f:facet name="header">Price</f:facet>
                    <h:outputText value="#{book.price}">
                        <f:convertNumber currencySymbol="$" groupingUsed="true"
                                         maxFractionDigits="2" type="currency"/>
                    </h:outputText>
                </h:column>

                <h:column>
                    <h:commandLink value="Add to Cart" 
                                   action="#{shoppingCart.add(book)}" >
                        <f:ajax render="@all"/>
                    </h:commandLink>
                </h:column>

            </h:dataTable>

        </h:form>
        <br/>
        <br/>
        <h3>Shopping Cart</h3>
        <h:form>
            <h:dataTable value="#{shoppingCart.box}" var="cart" border="5" class="shoppingCart">
                <h:column>
                    <f:facet name="header">Title</f:facet>
                    <h:outputText value="#{cart.title}" />
                </h:column>
                <h:column>
                    <f:facet name="header">Author</f:facet>
                    <h:outputText value="#{cart.author}" />
                </h:column>
                <h:column>
                    <f:facet name="header">Price</f:facet>
                    <h:outputText value="#{cart.price}">
                        <f:convertNumber currencySymbol="$" groupingUsed="true"
                                         maxFractionDigits="2" type="currency"/>
                    </h:outputText>
                </h:column>

                <h:column>
                    <f:facet name="header">Quantity</f:facet>
                    <h:outputText value="#{cart.quantity}" />
                </h:column>

                <h:column>
                    <h:commandLink value="Remove By One" action="#{shoppingCart.remove(cart)}" >
                        <f:ajax render="@form" />
                    </h:commandLink>
                </h:column>

            </h:dataTable>
            <br/>
            <h3>
                Total Price:
                <h:outputText value="#{shoppingCart.getTotal()}">
                    <f:convertNumber currencySymbol="$" groupingUsed="true"
                                     maxFractionDigits="2" type="currency"/>
                </h:outputText>
            </h3>


            <h:commandButton value="Checkout"
                             action="#{shoppingCart.checkout}"/>
        </h:form>
    </center>
</h:body>

它为库存清单显示此表，该清单只应包含4个元素，但似乎迭代次数超过需要，即在到达最后一本书后再次在表格中显示4本书。

为什么它会给我这个bug？我有什么不对的吗？