hibernate加载策略之lazy

一、hibernate的加载策略有两种：

1）即时加载--get加载数据

使用get获取数据，会立即查询，先到缓存中查找，找不到再到数据库中查找。

2）延迟加载（懒加载lazy）--load加载

不会立即查找，当需要的时候才会查找。

容易造成LazyInitializationException异常。如果出现这个异常，大部分是因为session被关闭了，还要从session中获取数据造成的。

但是懒加载也是有优势的，那就是效率高。因为我们查数据时不一定要把所有数据都查询出来。

注：load支持延迟加载，get不支持延迟加载。如果没有设置延迟加载，那么load也会立即加载对象。

二、class的lazy

class默认情况下是支持懒加载的（在*.hbm.xml中的class中有一个属性lazy="true"），所以在load方法时，是懒加载，但是当我们给成lazy="false"时，就不支持懒加载了，这里即使使用load方法，也会是即时加载。

我们来看项目案例：

Book实体类

public class Book {private int id;private String name;private double price;private String author;private Date pubDate;public Book() {}public Book(String name, double price) {super();this.name = name;this.price = price;}@Overridepublic String toString() {return "Book [id=" + id + ", name=" + name + ", price=" + price+ ", author=" + author + ", pubDate=" + pubDate + "]";}//省略get/set}

Book.hbm.xml配置文件

<hibernate-mapping package="com.test.pojo">      <class name="Book" table="book" lazy="false">          <id name="id">              <generator class="identity" />          </id>          <property name="author" />          <property name="name" column="book_name" />          <property name="price" />          <property name="pubDate" />      </class>  </hibernate-mapping> 

HibernateUtil类

public class HibernateUtil {private static Configuration cfg=null;private static SessionFactory factory=null;private static Session session=null;static{cfg=new Configuration().configure();factory=cfg.buildSessionFactory(new StandardServiceRegistryBuilder()        .applySettings(cfg.getProperties()).build());}public static Session getSession(){if(factory!=null)return session=factory.openSession();factory=cfg.buildSessionFactory(new StandardServiceRegistryBuilder()        .applySettings(cfg.getProperties()).build());return session=factory.openSession();}public static void closeSession(){if(session!=null && session.isOpen())session.close();}}

hibernate.cfg.xml配置文件

<hibernate-configuration><session-factory><property name="connection.driver_class">com.mysql.jdbc.Driver</property><property name="connection.url">jdbc:mysql:///hibernatetest</property><property name="connection.username">root</property><property name="connection.password">root</property><property name="dialect">org.hibernate.dialect.MySQLDialect</property><property name="show_sql">true</property><property name="format_sql">true</property><property name="hbm2ddl.auto">update</property><mapping resource="com/test/pojo/Book.hbm.xml" /></session-factory></hibernate-configuration>

HibernateTest测试类

public class HibernateTest {@Testpublic void testCreateDB(){Configuration cfg=new Configuration().configure();SchemaExport se=new SchemaExport(cfg);se.create(true, true);}@Testpublic void testSave(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Book book=new Book();book.setName("读者");book.setPrice(21.5);book.setAuthor("读者出版传媒有限公司");book.setPubDate(new Date());Book book1=new Book();book1.setName("蓝血人");book1.setPrice(50);book1.setAuthor("卫斯理");book1.setPubDate(new Date());Book book2=new Book();book2.setName("傲慢与偏见");book2.setPrice(80);book2.setAuthor("简.奥斯丁");book2.setPubDate(new Date());Book book3=new Book();book3.setName("中国历史");book3.setPrice(21.5);book3.setAuthor("人民");book3.setPubDate(new Date());Book book4=new Book();book4.setName("七龙珠");book4.setPrice(21.5);book4.setAuthor("不知道");book4.setPubDate(new Date());Book book5=new Book();book5.setName("名侦探柯南");book5.setPrice(50);book5.setAuthor("宫崎骏");book5.setPubDate(new Date());session.save(book);session.save(book1);session.save(book2);session.save(book3);session.save(book4);session.save(book5);tx.commit();HibernateUtil.closeSession();}@Testpublic void testGet(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Book book=(Book) session.get(Book.class, 1);System.out.println("书名："+book.getName());tx.commit();HibernateUtil.closeSession();System.out.println("书名："+book.getName());}@Testpublic void testLoad(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Book book=(Book) session.load(Book.class, 1);//System.out.println("书名："+book.getName());tx.commit();HibernateUtil.closeSession();//当Book.hbm.xml配置class中的lazy="true"时，以下操作会报错//open session in viewSystem.out.println("书名："+book.getName());}}

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三、现在我们再看另一种懒加载情况

项目结构如下：

Book实体类

public class Book {private int id;private String name;private double price;private String author;private Date pubDate;private Category category;public Book() {}public Book(String name, double price) {super();this.name = name;this.price = price;}@Overridepublic String toString() {return "Book [id=" + id + ", name=" + name + ", price=" + price+ ", author=" + author + ", pubDate=" + pubDate + "]";}//省略get/set}

Book.hbm.xml配置文件

<hibernate-mapping package="com.test.pojo">      <class name="Book" table="book">          <id name="id">              <generator class="identity" />          </id>          <many-to-one name="category" class="Category" column="category_id"            cascade="save-update"/>          <property name="author" />          <property name="name" column="book_name" />          <property name="price" />          <property name="pubDate" />      </class>  </hibernate-mapping>

Category实体类

public class Category {private int id;private String name;private Set<Book> books=new HashSet<Book>();//省略get/set}

Category.hbm.xml配置文件

<hibernate-mapping package="com.test.pojo">      <class name="Category" >          <id name="id" >              <generator class="identity" />          </id>          <property name="name" />          <set name="books" inverse="true" lazy="true">              <key>                  <column name="category_id" />              </key>              <one-to-many class="Book" />          </set>      </class>    </hibernate-mapping>

HibernateTest

public class HibernateTest {@Testpublic void testCreateDB(){Configuration cfg=new Configuration().configure();SchemaExport se=new SchemaExport(cfg);se.create(true, true);}@Testpublic void testSave(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Category category=new Category();category.setName("文学");Category category1=new Category();category1.setName("科幻");Category category2=new Category();category2.setName("历史");Category category3=new Category();category3.setName("言情");Category category4=new Category();category4.setName("漫画");Category category5=new Category();category5.setName("悬疑");Book book=new Book();book.setName("读者");book.setPrice(21.5);book.setAuthor("读者出版传媒有限公司");book.setPubDate(new Date());book.setCategory(category);Book book1=new Book();book1.setName("蓝血人");book1.setPrice(50);book1.setAuthor("卫斯理");book1.setPubDate(new Date());book1.setCategory(category1);Book book2=new Book();book2.setName("傲慢与偏见");book2.setPrice(80);book2.setAuthor("简.奥斯丁");book2.setPubDate(new Date());book2.setCategory(category3);Book book3=new Book();book3.setName("中国历史");book3.setPrice(21.5);book3.setAuthor("人民");book3.setPubDate(new Date());book3.setCategory(category2);Book book4=new Book();book4.setName("七龙珠");book4.setPrice(21.5);book4.setAuthor("不知道");book4.setPubDate(new Date());book4.setCategory(category4);Book book5=new Book();book5.setName("名侦探柯南");book5.setPrice(50);book5.setAuthor("宫崎骏");book5.setPubDate(new Date());book5.setCategory(category5);session.save(book);session.save(book1);session.save(book2);session.save(book3);session.save(book4);session.save(book5);tx.commit();HibernateUtil.closeSession();}@Testpublic void testGet(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();/** * 当Category.hbm.xml中的set中的lazy属性是true（默认为true）时， * 先查询一端的Category，只会先查询Category数据； *  * 当执行下面for中的category.getBooks时才会去查询Book数据，这时就是懒加载 *  * 但是当我们把Category.hbm.xml中的set中的lazy属性设为false时， * 会在查询Category的时候，把Book也查询出来，这时就是即时加载 */Category category=(Category) session.get(Category.class, 1);System.out.println("分类名:"+category.getName());for(Iterator<Book> iter=category.getBooks().iterator();iter.hasNext();){System.out.println(iter.next().getName());}tx.commit();HibernateUtil.closeSession();}@Testpublic void testLoad(){Session session=HibernateUtil.getSession();Transaction tx=session.beginTransaction();Category category=(Category) session.get(Category.class, 1);System.out.println("分类名:"+category.getName());System.out.println("对应的书大小："+category.getBooks().size());tx.commit();HibernateUtil.closeSession();}}

断点运行testLoad，当运行到下面这句时，并没有查询数据的sql语句打印

Category category=(Category) session.get(Category.class, 1);

当执行到下面这句时，才会打印出sql语句

System.out.println("分类名:"+category.getName());

控制台打印sql语句如下：

Hibernate:     select        category0_.id as id1_0_0_,        category0_.name as name2_0_0_     from        Category category0_     where        category0_.id=?

继续运行

System.out.println("对应的书大小："+category.getBooks().size());

执行这句后，打印sql语句如下：

Hibernate:     select        books0_.category_id as category2_0_0_,        books0_.id as id1_1_0_,        books0_.id as id1_1_1_,        books0_.category_id as category2_1_1_,        books0_.author as author3_1_1_,        books0_.book_name as book_nam4_1_1_,        books0_.price as price5_1_1_,        books0_.pubDate as pubDate6_1_1_     from        book books0_     where        books0_.category_id=?

当把Category.hbm.xml中set的lazy改为extra时，再执行

System.out.println("对应的书大小："+category.getBooks().size());

打印sql语句如下：

Hibernate:     select        count(id)     from        book     where        category_id =?

总结：

1、set/list默认下是lazy=true的。支持懒加载，但是当使用size()的时候依然要去查询整个set集合的内容。

2、lazy=false时，立即查询所有集合的内容。

3、lazy=extra时，比较智能，支持懒加载，当使用size()的时候不会查询整个集合，仅仅查询集合元素的个数。当需要使用集合元素的内容时，再去查询集合内容。

四、单端关联上的lazy：（many-to-one,one-to-one），默认是支持懒加载的lazy=proxy。

五、property上也有懒加载，默认lazy=false。如果property要去支持懒加载，需要用到第三方增强，且大部分情况下用不到property懒加载，只有当是大对象Blob、Clob时才会用到。