在Spring MVC中自动生成ID

问题描述

我正在尝试将JSP页面收集的数据保存在数据库中（Postgres）。最初，我尝试在表单中手动插入任何值（包括 id ），并且将数据保存在数据库中没有问题。现在，我尝试自动生成 id 值，但是我对如何正确执行操作有些困惑。

I'm trying to save data collected by a JSP page within a database (Postgres). At first I tried to insert any values manually (including id) in a form and I had no problem saving the data in my db. Now I'm trying to auto generate the id values, but I'm a little bit confused about how to do it properly.

我的模型-产品

public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String description;
    private double price;
    @DateTimeFormat(pattern = "dd/MM/yyyy")
    private Date date;

    public Product() { }

    public Product(Long id, String description, double price, Date date) {
        this.id = id;
        this.description = description;
        this.price = price;
        this.date = date;
    }
    //getters and setters
}

我的控制器-DBConnection

@Controller
public class DBConnection {

    @Autowired
    private ProductDao prDao;

    @RequestMapping(value = "/newproduct", method = RequestMethod.GET)
    public ModelAndView showForm() {
        Product product = new Product();
        return new ModelAndView("newproduct", "product", product);
    }

    @RequestMapping(value = "/newproduct", method = RequestMethod.POST)
    public ModelAndView submitForm(@ModelAttribute("product") Product product) {
        ModelAndView mav = new ModelAndView();

        prDao.addProduct(product.getId(), product.getDescription(), product.getPrice(), product.getDate());
        mav.setViewName("product");
        mav.addObject("product", product);
        return mav;
    }
}

ProductDaoImpl

public class ProductDaoImpl implements ProductDao {
    private DataSource dataSource;
    private JdbcTemplate jdbcTemplate;

    @Override
    public void setDataSource(DataSource ds) {
        this.dataSource = ds;
        this.jdbcTemplate = new JdbcTemplate(dataSource);

    }

    @Override
    public void addProduct(Long id, String description, double price, Date date) {
        jdbcTemplate.update("INSERT INTO products values(?, ?, ?, ?)", id, description, price, date);
    }
}

我在newproduct.jsp中的表单

<form:form method="POST" action="newproduct" modelAttribute="product">
    <table>
        <tr>
            <td><form:label path="description">Description</form:label></td>
            <td><form:input path="description" /></td>
        </tr>
        <tr>
            <td><form:label path="price">Price</form:label></td>
            <td><form:input path="price" /></td>
        </tr>
        <tr>
            <td><form:label path="date">Date (dd/mm/yyyy)</form:label></td>
            <td><form:input path="date" /></td>
        </tr>
        <tr>
            <td><input type="submit" value="Submit" /></td>
        </tr>
    </table>
</form:form>

错误，当我提交表单

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: PreparedStatementCallback; SQL [INSERT INTO products values(?, ?, ?, ?)]; ERROR: null values in column "id" violates not-null constraint
  Detail: The row contains error (null, nnvfe, 10.00, 2010-10-10).; nested exception is org.postgresql.util.PSQLException: ERROR: null values in column "id" violates not-null constraint
  Detail: The row contains error (null, nnvfe, 10.00, 2010-10-10).

我认为问题出在 addProduct 方法： id 值在我尝试获取时尚未创建。

I think the problem is in the addProduct method: the id value isn't yet created when I tried to get it.

如何实现自动生成的ID？ JPA批注是执行此操作的正确方法吗？

How can I implements an auto generated id? Is the JPA annotation the correct way to do that thing?

推荐答案

如果要自动生成 id ，然后不提供，即使它是 null 。因此，从 addProduect 方法中删除 id ：

If you want to automatically generate the id, then do not provide one, even it's null. So, remove the id from addProduect method:

@Override
public void addProduct(String description, double price, Date date) {
    jdbcTemplate.update("INSERT INTO products values(?, ?, ?)", description, price, date);
}

另外，输入您的 id 字段自动递增，如此 。

Also, make your id field auto-increment, as in this question.

或者，更改 addProduct 方法并使用 EntityManager＃persist 方法：

Or, change the addProduct method and use EntityManager#persist method:

@Override
public void addProduct(Product product) {
    entityManager.persist(product);
}

关于错误：

不从客户端获取 id 值， / newproduct 端点将具有 Product 以 null 作为其 id 值：

Since your form does not take the id value from the client, the /newproduct endpoint would have a Product with null as its id value:

@RequestMapping(value = "/newproduct", method = RequestMethod.POST)
public ModelAndView submitForm(@ModelAttribute("product") Product product) { ... }

然后您通过此 null 值作为 addProduct 方法的参数：

Then you pass this null value as the parameter to the addProduct method:

prDao.addProduct(product.getId(), ...)

最后是 addProduct 会尝试将 null 的值保存为主键的值，该键具有 Non-Null 约束，所以您会遇到错误

And finally addProduct would try to save that null value as the value of the Primary Key, which has a Non-Null constraint, so you're got that error.

此外，还使用 Entity 对象作为与客户端进行通信的手段，例如产品，不是一个好习惯。尝试定义一些辅助抽象，例如Forms或DTO，并将其用于表单处理或响应组装。

Also, using Entity objects as the means of communication with client, e.g. Product, is not a good practice. Try to define some auxiliary abstractions like Forms or DTOs and use them for form handling or response assembling.

这篇关于在Spring MVC中自动生成ID的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！