hibernate延迟加载org,Hibernate延迟加载

public static void main(String[] args) {
 
  DeptEntity dept = getDept(“402882e762ae888d0162ae888e420000”);

延迟加载在Hibernate中是默认延迟加载;

SQL Error: 17008, SQLState: null
每次程序运行一段时间以后,就报此错
08-08-17 09:55:29 WARN JDBCExceptionReporter:71 – SQL Error: 17008,
SQLState: null
08-08-17 09:55:29 ERROR JDBCExceptionReporter:72 – 关闭的连接
08-08-17 09:55:29 WARN JDBCExceptionReporter:71 – SQL Error: 17008,
SQLState: null
08-08-17 09:55:29 ERROR JDBCExceptionReporter:72 – 关闭的连接
org.hibernate.exception.GenericJDBCException: could not execute query
Caused by: java.sql.SQLException: 关闭的连接

  //dept.getEmp()得到子表的记录集合
  System.out.println(dept.getEmp());

测试代码一:

我的的struts2 +hibernate3

}

HibernateTest.java

dao是
public Userinfo checkUserinfo(String name, String pwd) {

private static DeptEntity getDept(String did){
  Session session = sessionFactory.openSession();
  DeptEntity dept = (DeptEntity)session.get(DeptEntity.class, did);
  session.close();
  return dept;
}

代码:

 Session session = HibernateSessionFactory.getSession(); Transaction ta = session.beginTransaction(); Userinfo userinfo =  session.createCriteria(Userinfo.class) .add(Expression.eq("userinfoName", name)).add( Expression.eq("userinfoPassword", pwd)).uniqueResult(); ta.commit(); HibernateSessionFactory.closeSession(); return userinfo;}

 

/**
 *
 */
package com.b510.examples;

[b]问题补充:[/b]
package com.shop.hibernate.util;

运行结果:

import java.util.Set;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;

Exception in thread “main” org.hibernate.LazyInitializationException:
failed to lazily initialize a collection of role:
com.javakc.hibernate.onetomany.entity.DeptEntity.emp, could not
initialize proxy – no Session
at
org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:566)
at
org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:186)
at
org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:545)
at
org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:124)
at
org.hibernate.collection.internal.PersistentSet.toString(PersistentSet.java:326)
at java.lang.String.valueOf(String.java:2827)
at java.io.PrintStream.println(PrintStream.java:771)
at
com.javakc.hibernate.onetomany.action.TestAction.main(TestAction.java:74)

import org.hibernate.Session;

/**

 

/**
 *
 * @author XHW
 *
 * @date 2011-7-18
 *
 */
public class HibernateTest {
 public static void main(String[] args) {
  new HibernateTest().update();
 }
 public void update(){
  Session
session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
  session.beginTransaction();
  Category
category=(Category)session.get(Category.class, 1);
  System.out.println(“id:”+category.getId()+” 
,name:”+category.getName()+”,
description:”+category.getDescription());
  
  Set<Product>
products=category.getProducts();
  
  session.getTransaction().commit();  
 }
 
 
}

  • Configures and provides access to Hibernate sessions, tied to the
  • current thread of execution. Follows the Thread Local Session
  • pattern, see {@link }.
    */
    public class HibernateSessionFactory {

    /**

    • Location of hibernate.cfg.xml file.
    • Location should be on the classpath as Hibernate uses
    • #resourceAsStream style lookup for its configuration file.
    • The default classpath location of the hibernate config file is
    • in the default package. Use #setConfigFile() to update
    • the location of the configuration file for the current
      session.
      */private static String CONFIG_FILE_LOCATION =
      “/hibernate.cfg.xml”;private static final ThreadLocal
      threadLocal = new ThreadLocal();private static Configuration
      configuration = new Configuration();private static
      org.hibernate.SessionFactory sessionFactory;private static
      String configFile = CONFIG_FILE_LOCATION;

    static {
    try {
    configuration.configure(configFile);
    sessionFactory = configuration.buildSessionFactory();
    } catch (Exception e) {
    System.err
    .println(“%%%% Error Creating SessionFactory %%%%”);
    e.printStackTrace();
    }
    }
    private HibernateSessionFactory() {
    }

    /**

    • Returns the ThreadLocal Session instance. Lazy initialize
    • the SessionFactory if needed.*
    • @return Session
    • @throws HibernateException
      */
      public static Session getSession() throws HibernateException {
      Session session = threadLocal.get();

      if (session == null || !session.isOpen {
      if (sessionFactory == null) {
      rebuildSessionFactory();
      }
      session = (sessionFactory != null) ?
      sessionFactory.openSession()
      : null;
      threadLocal.set;
      }

      return session;
      }

    /**

    • Rebuild hibernate session factory**/public static void
      rebuildSessionFactory() {try {
      configuration.configure(configFile); sessionFactory =
      configuration.buildSessionFactory();} catch (Exception e) {
      System.err .println(“%%%% Error Creating SessionFactory %%%%”);
      e.printStackTrace();}}

    /**

    • Close the single hibernate session instance.*
    • @throws HibernateException
      */
      public static void closeSession() throws HibernateException {
      Session session = threadLocal.get();
      threadLocal.set;

      if (session != null) {
      session.close();
      }
      }

    /**

    • return session factory**/public static
      org.hibernate.SessionFactory getSessionFactory() {return
      sessionFactory;}

    /**

    • return session factory*
    • session factory will be rebuilded in the next call*/public
      static void setConfigFile(String configFile)
      {HibernateSessionFactory.configFile = configFile;sessionFactory
      = null;}

    /**

    • return hibernate configuration**/public static Configuration
      getConfiguration() {return configuration;}

集合延迟加载初始化失败,不能初始化一个代理。就是集合在非一对一对象关系中,为了节省资源是默认延迟加载,而get方法又是非延迟加载,所以在执行完一次数据库查询后就执行session.close();关闭了session,而集合是延迟加载,在使用集合时再加载,此时session已经关闭,所以得不到代理。解决方法:可以在主表的hbm配置文件中,在<set>标签里设置lazy=”false”,集合就不延迟加载了,因此在执行get方法时,集合也获取到了,就不会出现延迟加载问题了。

运行结果:

}

log4j:WARN No appenders could be found for logger
(org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
    select
        category0_.id as id1_0_,
        category0_.name as name1_0_,
        category0_.description as descript3_1_0_
    from
        users.category category0_
    where
        category0_.id=?
id:1  ,name:java,
description:java好啊

这是自动生成的

这里我们看到我们关心的是id,name和description属性,

[b]问题补充:[/b]

Caused by: java.sql.SQLException: 关闭的连接
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134)
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:179)
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:269)
at
oracle.jdbc.driver.OracleConnection.privatePrepareStatement(OracleConnection.java:895)
at
oracle.jdbc.driver.OracleConnection.prepareStatement(OracleConnection.java:802)
at
org.hibernate.jdbc.AbstractBatcher.getPreparedStatement(AbstractBatcher.java:396)
at
org.hibernate.jdbc.AbstractBatcher.getPreparedStatement(AbstractBatcher.java:334)
at
org.hibernate.jdbc.AbstractBatcher.prepareQueryStatement(AbstractBatcher.java:88)
at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1162)
at org.hibernate.loader.Loader.doQuery(Loader.java:390)
at
org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:218)
at org.hibernate.loader.Loader.doList(Loader.java:1593)
… 122 more

虽然有:  Set<Product>
products=category.getProducts();
代码,即:不处理集合对象。但是我们只要的是:

  System.out.println(“id:”+category.getId()+” 
,name:”+category.getName()+”,
description:”+category.getDescription());
输出的是id,name和description属性值,其他的我们不管,所以Hibernate用了lazy
loading(延迟加载),带来的好处就是我们不关心的

数据,不用现在加载,当我们要用的时候,才去加载

测试代码二:

HibernateTest.java

代码:

/**
 *
 */
package com.b510.examples;

import java.util.Set;

import org.hibernate.Session;

/**
 *
 * @author XHW
 *
 * @date 2011-7-18
 *
 */
public class HibernateTest {
 public static void main(String[] args) {
  new HibernateTest().update();
 }
 public void update(){
  Session
session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
  session.beginTransaction();
  Category category=(Category)session.get(Category.class, 1);
  System.out.println(“id:”+category.getId()+” 
,name:”+category.getName()+”,
description:”+category.getDescription());
  
  Set<Product> products=category.getProducts();
  for(Product product:products){
   System.out.println(“ID:  “+product.getId()+” 
name:”+product.getName()+” price: “+product.getPrice());
  }  
  session.getTransaction().commit();  
 }
 
 
}

运行效果:

log4j:WARN No appenders could be found for logger
(org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
    select
        category0_.id as id1_0_,
        category0_.name as name1_0_,
        category0_.description as descript3_1_0_
    from
        users.category category0_
    where
        category0_.id=?
id:1  ,name:java, description:java好啊
Hibernate: 
    select
        products0_.category_id as
category2_1_,
        products0_.id as id1_,
        products0_.id as
id0_0_,
        products0_.category_id as
category2_0_0_,
        products0_.name as
name0_0_,
        products0_.price as
price0_0_,
        products0_.descripton as
descripton0_0_
    from
        users.product products0_

    where
       
products0_.category_id=?
ID:  1  name:java SE应用程序设计 price: 78.00
这里可以明确的告诉我们,当我们要加载Set集合的时候,这时候才去加载,而上面的例子,说明的是我们不加载的时候

Hibernate就延迟加载

取消延迟加载:

Category.hbm.xml

代码:

<?xml version=”1.0″ encoding=”utf-8″?>
<!DOCTYPE hibernate-mapping PUBLIC “-//Hibernate/Hibernate Mapping
DTD 3.0//EN”
“”>
<!–
    Mapping file autogenerated by MyEclipse Persistence Tools
–>
<hibernate-mapping>
    <class name=”com.b510.examples.Category” table=”category”
catalog=”users”>
        <id name=”id” type=”java.lang.Integer”>
            <column name=”id” />
            <generator class=”increment” />
        </id>
        <property name=”name” type=”java.lang.String”>
            <column name=”name” length=”500″ />
        </property>
        <property name=”description” type=”java.lang.String”>
            <column name=”description” length=”500″ />
        </property>
        <set name=”products” inverse=”true” lazy=”false”>
            <key>
                <column name=”category_id” />
            </key>
            <one-to-many class=”com.b510.examples.Product” />
        </set>
    </class>
</hibernate-mapping>

测试代码:

HIbernateTest.java

代码:

/**
 *
 */
package com.b510.examples;

import java.util.Set;

import org.hibernate.Session;

/**
 *
 * @author XHW
 *
 * @date 2011-7-18
 *
 */
public class HibernateTest {
 public static void main(String[] args) {
  new HibernateTest().update();
 }
 public void update(){
  Session
session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
  session.beginTransaction();
  Category category=(Category)session.get(Category.class, 1);
  System.out.println(“id:”+category.getId()+” 
,name:”+category.getName()+”,
description:”+category.getDescription());
  
  Set<Product> products=category.getProducts();
   
  session.getTransaction().commit();  
 }
 
 
}

运行效果:

log4j:WARN No appenders could be found for logger
(org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
    select
        category0_.id as id1_0_,
        category0_.name as name1_0_,
        category0_.description as descript3_1_0_
    from
        users.category category0_
    where
        category0_.id=?
Hibernate:
    select
        products0_.category_id as category2_1_,
        products0_.id as id1_,
        products0_.id as id0_0_,
        products0_.category_id as category2_0_0_,
        products0_.name as name0_0_,
        products0_.price as price0_0_,
        products0_.descripton as descripton0_0_
    from
        users.product products0_
    where
        products0_.category_id=?
id:1  ,name:java, description:java好啊
和测试代码一的运行结果相互比较,我们会发现,这次运行结果用了两条select语句。但是我们会发现

第二条select语句,对于我们的需求是没有必要的,他只有一个用处就是占用我们的程序执行时间。当然,

这是我们不希望看到的结果。

一般情况下,Hibernate会默认给我们设置延迟加载。lazy=”true”
,这样会提升我们的系统性能,所以一般情况下,我们不会去

设置lazy=”false”,当然在特殊的情况下,我们必须要取消延迟加载的时候,我们就把lazy=”false”,就可以了