Question

PersistenceContext 和 Autowire 依赖项注入注释不起作用。我得到NullPointerException。我通过在各自的Abstract类中创建getEntityManager和getDaoInstance方法来解决该问题。但是，我仍然想弄清楚/理解为什么批注最初不起作用。

我查看了StackOverflow上的相关内容，但是运气不好。

PS：：我是Java的新

AbstractDao 类，带有getEntityManager方法（完整代码，https://pastebin.com/6nKh1ujY）

package com.beetlehand.model.dao;

import org.springframework.transaction.annotation.Transactional;

import javax.persistence.EntityManager;
import javax.persistence.Persistence;
import javax.persistence.Query;
import java.util.List;
import java.util.Map;

public abstract class AbstractDao<T, S> implements AbstractDaoInterface<T, S> {

    protected EntityManager entityManager;

    public EntityManager getEntityManager() {
        if(this.entityManager == null) {
            this.entityManager = Persistence
                .createEntityManagerFactory("NewPersistenceUnit")
                .createEntityManager();
        }

        return this.entityManager;
    }
    
    // more code below
    
 }

在Dao类中如何调用 getEntityManager 方法的示例（完整代码，https://pastebin.com/dhjz17sN）

package com.beetlehand.model.dao;

import com.beetlehand.model.AttributeEntity;
import org.apache.commons.lang3.StringUtils;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class AttributeDao extends AbstractDao<AttributeEntity, Integer> {

    public AttributeEntity getById(Integer id) {
        if(id == null) return null;
        return getEntityManager().find(AttributeEntity.class, id);
    }
    
    // more code below
    
 }

AbstractService 和getDaoInstance方法（完整代码，https://pastebin.com/H4HGB43y）

package com.beetlehand.model.service;

import com.beetlehand.model.dao.AbstractDaoInterface;
import org.apache.commons.lang3.text.WordUtils;

import java.lang.reflect.Constructor;
import java.util.HashMap;

public class AbstractService {

    protected HashMap<String, Object> daoInstance = new HashMap<>();

    public AbstractDaoInterface getDaoInstance(String className) {
        try {
            if(!daoInstance.containsKey(className)) {
                className = WordUtils.capitalizeFully(className, new char[]{'_'})
                        .replaceAll("_", "");
                Class<?> clazz = Class.forName("com.beetlehand.model.dao." + className);
                Constructor<?> ctr = clazz.getConstructor();
                daoInstance.put(className, ctr.newInstance());
            }
        }catch(Exception ex) {
            ex.printStackTrace();

            return null;
        }

        return (AbstractDaoInterface) daoInstance.get(className);
    }
}

在服务类中调用 getDaoInstance 方法的示例（完整代码，https://pastebin.com/86yNSmsX）

package com.beetlehand.model.service;

import com.beetlehand.model.*;
import com.beetlehand.model.dao.AbstractDaoInterface;

public class CatalogService extends AbstractService {

    public AttributeEntity addAttribute(AttributeEntity attributeEntity) {
        AbstractDaoInterface attributeDao = this.getDaoInstance("attribute_dao");
        attributeDao.saveRow(attributeEntity);

        return attributeEntity;
    }

    public AttributeValueEntity addAttributeValue(AttributeValueEntity attributeValueEntity) {
       AbstractDaoInterface attributeValueDao = this.getDaoInstance("attribute_value_dao") ;
       attributeValueDao.saveRow(attributeValueEntity);

       return attributeValueEntity;
    }
    
    // more code below
    
 }

通过注释进行依赖注入不起作用

0 个答案: