java - Spring中的Json错误

Question

我试过这个:

@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: failed to lazily initialize a collection of role: fi.utu.tuha.domain.Mainsubjects.aiForms, no session or session was closed

如何修复？

Answer 1

问题是， 您的模型对象 Mainsubjects 有一些关联(由 OneToMany、ManyToOne 等构建)、列表(PersistentBags)、集合或像这样的东西(集合)，它们是延迟初始化的。这意味着，在结果集初始化之后，Mainsubjects 并不指向实际的集合对象，而是代理。在呈现、访问此集合时，hibernate 尝试使用代理从数据库中获取值。但此时没有 session 打开。出于这个原因，你得到这个异常(exception)。

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

在这个方法中你必须知道，你不能允许多个 PersistentBag 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 Controller 处理您的请求之前打开一个新 session ，并在您的 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();

        }