java - 评估 SpringEL 表达式的异常，Spring Guru 类(class)-6ren

java - 评估 SpringEL 表达式的异常，Spring Guru 类(class)

转载作者：行者123 更新时间：2023-12-02 01:54:55

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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 - thymeleaf 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>

当我尝试访问 localhost/books 时，我得到一个

Exception evaluating SpringEL expression: "book.id" (template: "books" - line 16, col 13)

我刚刚学习spring，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。没有它我就会遇到下一个问题:

No default constructor for entity: : pl.spring.guru.spring5webapp.model.Book; nested exception is org.hibernate.InstantiationException: No default constructor for entity: : pl.spring.guru.spring5webapp.model.Book

最佳答案

您缺少 id 的 getter。如果没有它，模板引擎就无法访问它。

至于默认构造函数 - Spring 需要它来创建实体的实例。如果没有可用的默认构造函数，Spring 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 框架引用文档。需要阅读的内容有很多，但尤其是 DI 基础知识，解释得非常好。

关于java - 评估 SpringEL 表达式的异常，Spring Guru 类(class)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/52455245/

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