java-在Spring MVC中自动生成ID

我正在尝试将JSP页面收集的数据保存在数据库(Postgres)中.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,我试图自动生成id值,但对于如何正确执行它有些困惑.

我的模特-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;
    }
}

产品展示

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中的表格

<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>

提交表单时出错

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方法中：当我尝试获取它时,尚未创建id值.

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？

解决方法:

Now I’m trying to auto generate the id values, but I’m a little bit
confused about how to do it properly

如果要自动生成ID,请不要提供ID,即使它为null.因此,从addProduect方法中删除ID：

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

另外,使您的id字段自动递增,如question所示.

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

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

关于错误：

ERROR: null values in column “id” violates not-null constraint

由于您的表单未从客户端获取id值,因此/ newproduct端点将具有一个产品,其id值为null：

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

然后,将此空值作为参数传递给addProduct方法：

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

最后,addProduct将尝试将该空值保存为具有Non-Null约束的主键的值,因此会出现该错误.

同样,使用实体对象作为与客户端通信的方式,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.