使用Spring+Thymeleaf提交表单向控制器传递null对象问题求助

问题说明 #

Spring搭配Thymeleaf开发时，表单提交后控制器拿到的Product对象属性全为null，完全参照教程实现仍无法正常运行，相关实现代码如下：

1. Model实体类 #

import java.math.BigDecimal;

public class Product {
    public String name;
    public String description;
    public BigDecimal price;

    public Product(String name, String description, BigDecimal price) {
        this.name = name;
        this.description = description;
        this.price = price;
    }

    public Product() {

    }

    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    }

    public BigDecimal getPrice() {
        return price;
    }
}

2. DAO数据访问层 #

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import com.example.sklepp.service.Product;
import org.springframework.stereotype.Repository;

@Repository
public class ProductDao {

    private List<Product> products = Arrays.asList(
            new Product("Tea", "green", new BigDecimal("25.00")),
            new Product("Coffee", "black", new BigDecimal("30.00")));
           

    public List<Product> all() {
        return products;
    }

    public void addProduct(Product product) {
        this.products.add(product);
    }

    public Product byName(String name) {
        for (Product product : products) {
            if (name.equals(product.getName())) {
                return product;
            }
        }
        return null;
    }
}

3. Controller控制器代码 #

import com.example.sklepp.dao.ProductDao;
import com.example.sklepp.service.Product;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
@RequestMapping("admin")
public class AdminController {

    private final ProductDao productDao;

    public AdminController(ProductDao productDao) {
        this.productDao = productDao;
    }

    @GetMapping("/")
    public String admin(Model model) {
        model.addAttribute("newProduct", new Product());
        return "admin/admin";
    }


    @PostMapping("/add-product")
    public String addProduct(@ModelAttribute Product product) {
        System.out.println(product.getName());
        return "redirect:/sklep";
    }
}

4. Thymeleaf模板代码 #

<html xmlns:th="http://www.thymeleaf.org" lang="en">
<head>
    <meta charset="UTF-8">
    <title>Admin service</title>
</head>
<body>
<h1>ADMIN</h1>

<p>+++++++++++++++++++++</p>

<form th:action="@{/admin/add-product}" th:object="${newProduct}" method="post">
    <p>Adding new product: </p>
    <p>Name: <input type="text" th:field="*{name}"></p>
    <p>Price: <input type="text" th:field="*{price}"></p>
    <p>Description: <input type="text" th:field="*{description}"></p>
    <p><input type="submit" value="Add product"></p>
</form>

</body>
</html>

修复方案 #

1. 实体类补全setter方法：Spring MVC做表单参数绑定时需要通过setter方法给对象属性赋值，当前Product类仅实现了getter方法，参数无法写入所以返回全null值，需补充对应setter：

public void setName(String name) {
    this.name = name;
}

public void setDescription(String description) {
    this.description = description;
}

public void setPrice(BigDecimal price) {
    this.price = price;
}

也可以引入Lombok依赖，给Product类加@Data注解自动生成通用方法，简化代码。
2. DAO层列表初始化逻辑修改：Arrays.asList()返回的是固定长度的列表，后续调用addProduct方法新增商品时会抛出UnsupportedOperationException异常，建议改为ArrayList初始化：

private List<Product> products = new ArrayList<>(Arrays.asList(
        new Product("Tea", "green", new BigDecimal("25.00")),
        new Product("Coffee", "black", new BigDecimal("30.00"))));

内容的提问来源于stack exchange，提问作者Daniel