[FIXED] Java Spring Boot JPA - Throws NullPointer Exception when saving to database

Issue

I'm making a small application using Spring Boot (and hibernate). I've encountered a strange problem while making this app, when I use my DAO object to save an object I've created to the database. It throws a null pointer exception and I haven't managed to figure out why, I've checked the usual suspect (DI) but I am using the annotation @autowired so it should work. I'm nearly tearing my hair out, maybe you'll see what it is that I've missed, thank you?

EDIT: I've followed the instructions posted by . and ., and it works but only if I comment out my deleteCategoryByName method. I've made some changes below, I've renamed the Demo1 class to LibraryApplication instead. I now get the error:

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 '!

My code

The 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);
}

The Domain-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

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());
    };
  }
}

Picture of the table and column.

Solution

1.you should add a @Component / @Service above CategoryService

2.you are not starting spring boot app correctly . you should do this instead of 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);
  }

}

more on this can be found in https://spring.io/guides/gs/accessing-data-jpa/

answering your question directly , the reason your are getting NPE is because categoryDao in CategoryService is null , since you are not starting spring boot correctly.

P.S

you should note that having your app class (AccessingDataJpaApplication ) in a package "higher" in the hierarchy(library package) then the rest of your components will make this work out of the book , otherwise you will have to mention the correct packages in the @SpringBootApplication annotation

Answered By - asafb
Answer Checked By - Terry (JavaFixing Volunteer)