Java Spring Boot JPA-保存到数据库时抛出Nullpointer异常

Question

我正在使用Spring Boot（和Hibernate）进行小型应用程序。在制作此应用程序时，我遇到了一个奇怪的问题，当我使用DAO对象保存我创建给数据库的对象时。它引发了一个无效的指针例外，我还没有设法弄清楚为什么，我检查了通常的嫌疑人（di），但是我使用的注释@Autowired，因此应该有效。我几乎把头发撕掉了，也许您会看到我错过的是什么，谢谢？

编辑：我遵循了发布的说明。和。它可以正常工作，但是只有我评论我的DeleteCategoryByName方法。我已经进行了一些更改，而是将Demo1类重命名为Library Application。现在，我会收到错误：

ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'categoryService': Unsatisfied dependency expressed through field 'categoryDao'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'categoryDao': Invocation of init method failed; nested exception is java.lang.IllegalStateException: Using named parameters for method public abstract java.lang.Void consid.programmeringstest.library.dao.CategoryDao.deleteCategoryByName(java.lang.String) but parameter 'Optional[categoryName]' not found in annotated query 'DELETE FROM category WHERE name = :category_name '!

我的代码

dao-class

package library.dao;

import library.domain.Category;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Modifying;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.query.Param;


public interface CategoryDao extends JpaRepository<Category, Long> {
    default Category findByIdOrThrow(Long id) {
        return findById(id).orElseThrow();
    }
    
    @Modifying
    @Query("DELETE FROM category WHERE name = :category_name ")
    public void deleteCategoryByName(@Param("categoryName") String categoryName);
}

域级别的doain-class

package library.domain;

import java.io.Serializable;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Category implements Serializable {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String categoryName;
    
    public Category() {
    }
    
    public Category(String name) {
        this.categoryName = name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getCategoryName() {
        return categoryName;
    }

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

the Service-class class

package library.service;

import library.dao.CategoryDao;
import library.domain.Category;
import org.springframework.beans.factory.annotation.Autowired;


public class CategoryService {
    
    @Autowired
    private CategoryDao categoryDao;
    
    public Category createCategory() {
        Category result = new Category();
        return categoryDao.save(result);
    }
    
    public Category createCategory(String name) {
        Category result = new Category(name);
        return categoryDao.save(result);
    }
    
    public void deleteCategoryByName(String name) {
        categoryDao.deleteCategoryByName(name);
    }
    
}

package library;

import library.dao.CategoryDao;
import library.domain.Category;
import library.service.CategoryService;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
@EnableAutoConfiguration
public class LibraryApplication {
    public static void main(String[] args) {
        SpringApplication.run(LibraryApplication.class, args);
    }
    
  @Bean
  public CommandLineRunner demo(CategoryDao categoryDao) {
    return (args) -> {
        categoryDao.save(new Category("Test"));
        Category category = categoryDao.findByIdOrThrow(1);
        System.out.println(category.getCategoryName());
    };
  }
}

图片表和列。

Answer 1

1.您应该添加@component/@service上面的categoryservice

2.您未正确启动Spring Boot应用程序。您应该这样做，而不是demo1：

package library;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class AccessingDataJpaApplication {

  public static void main(String[] args) {
    SpringApplication.run(AccessingDataJpaApplication.class, args);
  }

}

有关此的更多信息，请参见 https://spring.io/guides/gs/accessing-data-jpa/

直接直接回答您的问题，您获得NPE的原因是categorydao in in categoryservice为null，因为您未正确启动弹簧启动。

ps

您应该注意，在层次结构中“更高”的软件包（library软件包）中，将应用程序类（访问datajpaapplication）在软件包中在这本书中，否则您将不得不在@springbootapplication中提及正确的软件包注释

Answer 2

首先，您需要添加的是@springbootapplication到主类。第二 - springapplication.run（demo1.class，args）;在main（可执行）方法的第一行。而且，不要忘记将@service添加到您的应用程序的服务类中。

Answer 3

您的DAO类的参数名称categoryName。它应该是category_name。并且还使用saveandflush（）而不是save（）

categorydao.java

package library.dao;

import library.domain.Category;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Modifying;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.query.Param;


public interface CategoryDao extends JpaRepository<Category, Long> {
    default Category findByIdOrThrow(Long id) {
        return findById(id).orElseThrow();
    }
    
    @Modifying
    @Query("DELETE FROM category WHERE name = :category_name ")
    public void deleteCategoryByName(@Param("category_name") String categoryName);
}