hinbernate懒加载异常org.hibernate.LazyInitializationException: could not initialize proxy - no Session

- 日理万妓 2022-09-26 02:43 172阅读 0赞

DataGridJSONModel json = new DataGridJSONModel();       
            PageHolder<IP> holder = ipdao.findPage(page, rows);
            List<IP> list = holder.getList();
            json.setRows(list);
            json.setTotal(holder.getTotal());       
            return json;

在将返回对象json转换为JSON语句时报错

org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: could not initialize proxy - no Session (through reference chain: com.liyunet.common.json.DataGridJSONModel["rows"]->java.util.ArrayList[0]->com.liyunet.domain.IP["attachments"]->com.liyunet.domain.IPAttachments_$$_jvst7f7_1c["id"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: could not initialize proxy - no Session (through reference chain: com.liyunet.common.json.DataGridJSONModel["rows"]->java.util.ArrayList[0]->com.liyunet.domain.IP["attachments"]->com.liyunet.domain.IPAttachments_$$_jvst7f7_1c["id"])
        org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:276)
        org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:100)
        ... ...

可以看出类IP中attachments属性初始化代理失败，原因是没有session(session已关闭)。下面是在实体类IP中的定义代码

@OneToOne(fetch = FetchType.LAZY)//懒加载
        @JoinColumn(name = "attachements_id")
        private IPAttachments attachments;

所以，实际上返回的对象json中rows中对象IP的属性attachments是个代理，所以要初始化。  
有三种方法解决此问题：

##### 1. 遍历传递给rows的list，使用Hinbernate.initialize(attachments);初始化attachments字段。 #####

list.forEach(ip -> {
        IPAttachments attachments = ip.getAttachments();
        Hibernate.initialize(attachments);
    });

##### 2. 使用hql语句立即加载attachments字段。 #####

修改查询list的hql语句为：

String hql = "from IP ip inner join fetch ip.attachments order by ip.points desc";

##### 3.使用VO，即将对象IP中需要的几个字段查询出来保存到一个实体类中，POJO，这样session关闭之前就会加载，便不会懒加载异常。 #####

class VO{
        private String id;
        ...
    }

另外，还有过滤器法，这个还没有搞懂，暂时搁置。