Spring与jsp表达式的产生的问题详解编程语言

<form:form commandName="book" action="/book_update" method="post"> 
    <fieldset> 
        <legend>Edit a book</legend> 
        <form:hidden path="id"/> 
        <p> 
            <label for="category">Category: </label> 
             <form:select id="category" path="category.id" items="${categories}" 
                itemLabel="name" itemValue="id"/> 
        </p> 
        <p> 
            <label for="title">Title: </label> 
            <form:input id="title" path="title"/> 
        </p> 
        <p> 
            <label for="author">Author: </label> 
            <form:input id="author" path="author"/> 
        </p> 
        <p> 
            <label for="isbn">ISBN: </label> 
            <form:input id="isbn" path="isbn"/> 
        </p> 
         
        <p id="buttons"> 
            <input id="reset" type="reset" tabindex="4"> 
            <input id="submit" type="submit" tabindex="5"  
                value="Update Book"> 
        </p> 
    </fieldset> 
</form:form>