Question

Book.java

package pl.spring.guru.spring5webapp.model;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

@Entity
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    private String title;
    private String isbn;
    @OneToOne
    private Publisher publisher;

   @ManyToMany
   @JoinTable(name = "author_book", joinColumns = @JoinColumn(name = 
   "book_id"), inverseJoinColumns = @JoinColumn(name = "author_id"))

   private Set<Author> authors = new HashSet<>();

   public Book() {
   }

   public Book(String title, String isbn, Publisher publisher){
       this.title=title;
       this.isbn=isbn;
       this.publisher=publisher;
   }

   public String getTitle() {
       return title;
   }

   public void setTitle(String title) {
       this.title = title;
   }

   public String getIsbn() {
      return isbn;
  }

   public void setIsbn(String isbn) {
      this.isbn = isbn;
  }

  public Publisher getPublisher() {
      return publisher;
  }

  public void setPublisher(Publisher publisher) {
     this.publisher = publisher;
  }

  public Set<Author> getAuthors() {
     return authors;
  }

  public void setAuthors(Set<Author> authors) {
     this.authors = authors;
  }
}

BookController.java

package pl.spring.guru.spring5webapp.controllers;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import pl.spring.guru.spring5webapp.repositories.BookRepository;

@Controller
public class BookController {

    private BookRepository bookRepository;

    public BookController(BookRepository bookRepository) {
        this.bookRepository = bookRepository;
    }

    @RequestMapping(value = "/books")
    public String getBooks(Model model){
        model.addAttribute("books",bookRepository.findAll());

    return "books";
  }
}

books.html-百里香模板

<!DOCTYPE html>
<html lang="en" xmlns:th=”http://www.thymeleaf.org”>
<head>
    <meta charset="UTF-8">
    <title>Spring Framework Guru</title>
</head>
<body>
<h1>Books list</h1>
<table>
   <tr>
       <th>ID</th>
       <th>Title</th>
       <th>Author</th>
   </tr>
   <tr th:each="book : ${books}">
        <td th:text="${book.id}">123</td>
       <td th:text="${book.title}">Spring in Action</td>
       <td th:text="${book.publisher.name}">Wrox</td>
   </tr>
</table>

</body>
</html>

当我尝试转到localhost / books时，我会得到

异常评估SpringEL表达式：“ book.id”（模板：“ books” -第16行，第13行）

我刚刚学习Spring，并且开始使用spring框架大师教程。我也不明白，为什么需要在book.java中添加一个空的构造函数。没有它，我有下一个问题：

没有默认的实体构造函数： pl.spring.guru.spring5webapp.model.Book;嵌套异常为 org.hibernate.InstantiationException：没有默认的构造函数实体：： pl.spring.guru.spring5webapp.model.Book

Answer 1

您缺少ID的获取者。否则，模板引擎将无法访问它。

关于默认构造函数-Spring要求创建实体实例。如果没有可用的默认构造函数，Spring应该如何知道要使用哪些参数。

除了本教程之外，您还应该查看Spring Framework参考文档。阅读很多东西，但是特别是对DI基础进行了很好的解释。

评估SpringEL表达式的异常，Spring Guru课程

1 个答案: