java - Servlet，JSP，JPA-找不到属性异常

我正在写我的第一个更大的应用程序，但有一个问题，下面的代码：

初始化数据库

public void requestInitialized(ServletRequestEvent arg0)  {
    EntityManager em = DBConfig.createEntityManager();
    BooksDAO booksDAO = new BooksDAO(em);
    CategoryDAO categoriesDAO = new CategoryDAO(em);
    ServletRequest req = arg0.getServletRequest();
    req.setAttribute("booksDao", booksDAO);
    req.setAttribute("categoriesDao", categoriesDAO);
}

BooksDAO.java

EntityManager em = DBConfig.createEntityManager();

public BooksDAO(EntityManager em) {
    this.em = em;
}
public List<Books> getBooksByCategory(String category) {
    Query q = this.em.createQuery("SELECT b FROM Books b WHERE b.category = :category ", Books.class).setParameter("category", category);
    List<Books> booksByCategory = q.getResultList();
    return booksByCategory;
}

booksCategoryServlet.java

@WebServlet("/booksCategory")
public class booksCategoryServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String category = request.getParameter("category");
        if (category != null) {
            BooksDAO dao = (BooksDAO) request.getAttribute("booksDao");
            List<Books> booksByCategory = dao.getBooksByCategory(category);
            request.setAttribute("booksByCategory", booksByCategory);
            request.getRequestDispatcher("/booksCategory.jsp").forward(request, response);
        } else
            response.sendRedirect(request.getContextPath() + "/");
    }

bookCategory.jsp

<c:forEach var="book" items="${booksDao.booksByCategory}">
    <tr>
        <td>${book.title}</td>
        <td>${book.author}</td>
        <td>${book.description}</td>
        <td>${book.category}</td>
        <td>${book.year}</td>
        <td><a href="bookDetails?id=${book.id}">show details</a></td>
    </tr>
</c:forEach>

index.jsp

<c:forEach var="category" items="${categoriesDao.categories}">
    <a href="booksCategory?category=${category}"><li>${category}</li></a>
</c:forEach>

在索引页面中，我列出了类别，当我要转到选择的类别并显示该类别的书籍时，出现此异常：


  org.apache.jasper.el.JspPropertyNotFoundException：/booksCategory.jsp(40,4）'$ {booksDao.booksByCategory}'属性'booksByCategory'在DAO.BooksDAO类型上找不到


有人可以告诉我我做错了什么吗？

最佳答案

您正在调用一种方法，以为您正在调用实际对象。
我将在List<Books>中创建一个DAOBooks对象，并将其与请求一起发送到JSP。



解

DAO书籍

EntityManager em = DBConfig.createEntityManager();
List<Books> booksByCategory = new ArrayList<>(); // Or whatever list type you need.

public BooksDAO(EntityManager em) {
    this.em = em;
}
public void setBooksByCategory(String category) {
    Query q = this.em.createQuery("SELECT b FROM Books b WHERE b.category = :category ", Books.class).setParameter("category", category);
    booksByCategory = q.getResultList();
}

public List<Books> getBooksByCategory(){
    return booksByCategory;
}

在你的JSP

<c:forEach var="book" items="${booksByCategory}">

直接引用List<Books>对象，因为它是您通过请求发送的对象。



编辑

尝试使用不同的setter和getter方法。它可以帮助您使代码更具可读性，并立即解决这些类型的问题。

我在帖子中举了一个例子，但这不一定是正确的例子，您必须根据应用程序逻辑找到实现它们的方法。