Hibernate（9）懒加载问题解决方案

小咪咪 2024-02-17 21:47 18阅读 0赞

### 0 懒加载概念 ###

①懒加载（load on demand）是一种独特而又强大的数据获取方法，是指程序**推迟访问数据库**，这样做可以保证有时候不必要的访问数据库，因为访问一次数据库是比较耗时的。  
②当我们查询一个对象的时候，默认情况下，返回的只是该对象的**普通属性**，当用户去使用**对象属性**时，才会向数据库发出再一次的查询，这种现象称为lazy加载现象。

### 1在Student.hbm.xml中**配置**关闭懒加载方式： ###

### 2 Hibernate.initialize(代理对象) ###

显示的初始化代理对象`Hibernate.initialize((new Student()).getStudcourses());`

### 3 OpenInView—通过过滤器Filter实现 ###

**原理：**  
![这里写图片描述][SouthEast]  
缺点是：session关闭会延时  
**Filter文件**，不需要在web.xml中配置filter

public class MyFilter extends HttpServlet implements Filter {
        
    
        @Override
        public void doFilter(ServletRequest arg0, ServletResponse arg1,
                FilterChain arg2) throws IOException, ServletException {
            Session session = null;
            Transaction ts = null;
            try {
                session = HibernateUtil.getCurrentSession();
                ts = session.beginTransaction();
                // 整个请求完成，才会执行doFilter方法
                arg2.doFilter(arg0, arg1);
                ts.commit();
            } catch (Exception e) {
                if(ts!=null) {
                    ts.rollback();
                }
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                HibernateUtil.closeCurrentSession();
            }
        }
    
        @Override
        public void init(FilterConfig arg0) throws ServletException {
            // TODO Auto-generated method stub
    
        }
    }

**HibernateUtil工具类文件**  
以**OpenInView**为后缀的配合上边的Filter使用

public final class HibernateUtil {
        private static SessionFactory sessionFactory = null;
        // 线程局部模式
        private static ThreadLocal<Session> threadLoacal = new ThreadLocal<Session>();
        private HibernateUtil() {}
        static {
            sessionFactory = new Configuration().configure().buildSessionFactory();
        }
        // 获取全新的session
        public static Session openSession() {
            return sessionFactory.openSession();
        }
        // 获取和线程关联的session
        public static Session getCurrentSession() {
            Session session = threadLoacal.get();
            // 判断是否得到
            if(session==null) {
                session = sessionFactory.openSession();
                // 将session放入threadLocal
                threadLoacal.set(session);
            }
            return session;
        }
        public static void closeCurrentSession() {
            Session session = getCurrentSession();
            if(session!=null && session.isOpen()) {
                session.close();
                threadLoacal.set(null);
            }
        }
    
        // 根据id返回对象的方法
        public static Object findById(Class clazz, java.io.Serializable id) {
            Session session = null;
            Transaction ts = null;
            Object obj = null;
            try {
                session = openSession();
                ts = session.beginTransaction();
    
                obj = session.load(clazz, id);
    
                ts.commit();
            } catch (Exception e) {
                if(ts!=null) {
                    ts.rollback();
                }
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                if(session!=null && session.isOpen()) {
                    session.close();
                }
            }
            return obj;
        }
    
        // 根据id返回对象的方法
        public static Object findByIdOpenInView(Class clazz, java.io.Serializable id) {
            Session session = getCurrentSession();
            Object obj = session.load(clazz, id);
            return obj;
        }
        // 返回至多一个对象
        public static Object uniqueQuery(String hql, String[] paras) {
            Session session = null;
            Object obj = null;
            try {
                session = openSession();
    
                Query query = session.createQuery(hql);
    
                if(paras!=null && paras.length>0) {
                    for(int i=0; i<paras.length; i++) {
                        query.setString(i, paras[i]);
                    }
                }
                obj = query.uniqueResult();
    
            } catch (Exception e) {
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                if(session!=null && session.isOpen()) {
                    session.close();
                }
            }
    
            return obj;
        }
        /// 分页
        public static List executeQueryByPage(String hql, String[] paras, int pageSize, int pageNow) {
            Session session = null;
            List list = null;
            try {
                session = openSession();
    
                Query query = session.createQuery(hql);
    
                if(paras!=null && paras.length>0) {
                    for(int i=0; i<paras.length; i++) {
                        query.setString(i, paras[i]);
                    }
                }
    
                /// 
                query.setFirstResult((pageNow-1)*pageSize).setMaxResults(pageSize);
    
                list = query.list();
    
            } catch (Exception e) {
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                if(session!=null && session.isOpen()) {
                    session.close();
                }
            }
    
            return list;
        }
    
        /// 分页
        public static List executeQueryByPageOpenInView(String hql, String[] paras, int pageSize, int pageNow) {
            Session session = getCurrentSession();
    
            Query query = session.createQuery(hql);
    
            if(paras!=null && paras.length>0) {
                for(int i=0; i<paras.length; i++) {
                    query.setString(i, paras[i]);
                }
            }
    
            /// 
            query.setFirstResult((pageNow-1)*pageSize).setMaxResults(pageSize);
    
            List list = query.list();
    
            return list;
        }
    
        /// 查询接口
        public static List executeQuery(String hql, String[] paras) {
            Session session = null;
            List list = null;
            try {
                session = openSession();
    
                Query query = session.createQuery(hql);
    
                if(paras!=null && paras.length>0) {
                    for(int i=0; i<paras.length; i++) {
                        query.setString(i, paras[i]);
                    }
                }
                list = query.list();
    
            } catch (Exception e) {
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                if(session!=null && session.isOpen()) {
                    session.close();
                }
            }
    
            return list;
        }
    
        /// 查询接口
        public static List executeQueryOpenInView(String hql, String[] paras) {
            Session session = getCurrentSession();
    
            Query query = session.createQuery(hql);
    
            if(paras!=null && paras.length>0) {
                for(int i=0; i<paras.length; i++) {
                    query.setString(i, paras[i]);
                }
            }
            List list = query.list();
    
            return list;
        }
        /// 修改和删除 批量sql
        public static void executeUpdate(String hql, String[] paras) {
            Session session = null;
            Transaction ts = null;
            try {
                session = openSession();
                ts = session.beginTransaction();
                Query query = session.createQuery(hql);
    
                if(paras!=null && paras.length>0) {
                    for(int i=0; i<paras.length; i++) {
                        query.setString(i, paras[i]);
                    }
                }
    
                query.executeUpdate();
    
                ts.commit();
            } catch (Exception e) {
                if(ts!=null) {
                    ts.rollback();
                }
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                if(session!=null && session.isOpen()) {
                    session.close();
                }
            }
        }
    
        /// 修改和删除 批量sql
        public static void executeUpdateOpenInView(String hql, String[] paras) {
            Session session = getCurrentSession();
    
            Query query = session.createQuery(hql);
    
            if(paras!=null && paras.length>0) {
                for(int i=0; i<paras.length; i++) {
                    query.setString(i, paras[i]);
                }
            }
    
            query.executeUpdate();
        }
    
        // 添加
        public static void save(Object obj) {
            Session session = null;
            Transaction ts = null;
            try {
                session = openSession();
                ts = session.beginTransaction();
                session.save(obj);
                ts.commit();
            } catch (Exception e) {
                if(ts!=null) {
                    ts.rollback();
                }
                e.printStackTrace();
                throw new RuntimeException(e.getMessage());
            } finally {
                if(session!=null && session.isOpen()) {
                    session.close();
                }
            }
        }
    
        // 添加
        public static void saveOpenInView(Object obj) {
            Session session = getCurrentSession();
            session.save(obj);
        }
    }

[SouthEast]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/7dba5d143a374dfabbbb30fc48db577a.png