Spring中的Json错误

Question

我尝试了这个：

@RequestMapping(method = RequestMethod.GET, value = "/getmainsubjects")
@ResponseBody
public JSONArray getMainSubjects( @RequestParam("id") int id) {

List <Mainsubjects> mains = database.getMainSubjects(id, Localization.getLanguage());
JSONArray json = JSONArray.fromObject(mains);
return json;

}

调用 getmainsubjects.html?id=1 时出现错误：

net.sf.json.JSONException：org.hibernate.LazyInitializationException：无法延迟初始化角色集合：fi.utu.tuha.domain.Mainsubjects.aiForms，没有会话或会话已关闭

如何修复？

Answer 1

问题是，
你的模型对象 Mainsubjects 有一些关联（由 OneToMany、ManyToOne 等构建）、列表（PersistentBags）、集合或类似的东西（集合），它们是延迟初始化的。这意味着，在初始化结果集之后，Mainsubjects 并不指向实际的集合对象，而是指向代理。在渲染、访问此集合时，hibernate 尝试使用代理从数据库获取值。但此时还没有开放的会话。因此你会得到这个例外。

您可以将获取策略设置为 EAGER（如果您使用注释），如下所示：
@OneToMany(fetch=FetchType.EAGER)

在此方法中，您必须注意，您不能允许多个 PersistedBag 急切地初始化。

或者您可以使用 OpenSessionInView 模式，这是一个 servlet 过滤器，在控制器处理您的请求之前打开一个新会话，并在 Web 应用程序响应之前关闭：

   public class DBSessionFilter implements Filter {
        private static final Logger log = Logger.getLogger(DBSessionFilter.class);

        private SessionFactory sf;

        @Override
        public void destroy() {
            // TODO Auto-generated method stub

        }

        @Override
        public void doFilter(ServletRequest request, ServletResponse response,
                FilterChain chain) throws IOException, ServletException {
            try {
                log.debug("Starting a database transaction");
                sf.getCurrentSession().beginTransaction();

                // Call the next filter (continue request processing)
                chain.doFilter(request, response);

                // Commit and cleanup
                log.debug("Committing the database transaction");
                sf.getCurrentSession().getTransaction().commit();

            } catch (StaleObjectStateException staleEx) {
                log.error("This interceptor does not implement optimistic concurrency control!");
                log.error("Your application will not work until you add compensation actions!");
                // Rollback, close everything, possibly compensate for any permanent changes
                // during the conversation, and finally restart business conversation. Maybe
                // give the user of the application a chance to merge some of his work with
                // fresh data... what you do here depends on your applications design.
                throw staleEx;
            } catch (Throwable ex) {
                // Rollback only
                ex.printStackTrace();
                try {
                    if (sf.getCurrentSession().getTransaction().isActive()) {
                        log.debug("Trying to rollback database transaction after exception");
                        sf.getCurrentSession().getTransaction().rollback();
                    }
                } catch (Throwable rbEx) {
                    log.error("Could not rollback transaction after exception!", rbEx);
                }

                // Let others handle it... maybe another interceptor for exceptions?
                throw new ServletException(ex);
            }

        }

        @Override
        public void init(FilterConfig arg0) throws ServletException {
            log.debug("Initializing filter...");
            log.debug("Obtaining SessionFactory from static HibernateUtil singleton");
            sf = HibernateUtils.getSessionFactory();

        }