Hibernate学习之检索策略（lazy，fetch，batch-size等）

马哥的淘宝店:https://shop592330910.taobao.com/

Hibernate学习之检索策略

马哥的淘宝店:https://shop592330910.taobao.com/
检索数据时的2个问题
不浪费内存：当hibernate从数据库中加载customer对象时，如果同时加载所有关联的order对象，
而程序实际上仅仅需要访问customer对象，那么这些关联的order对象就白白浪费许多内存。

更高的查询效率：发送尽可能少的sql语句。
马哥的淘宝店:https://shop592330910.taobao.com/

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类级别的检索策略

类级别的检索策略包括立即检索和延迟检索，默认是延迟检索。
立即检索：立即加载检索方法指定的对象。
延迟检索：延迟加载检索方法指定的对象，在使用具体的属性时，再进行加载。

类级别的检索策略可以通过class元素的lazy属性进行设置，设置为true或者false。
这2个策略何时使用呢？如果程序加载一个对象的目的是为了访问它的属性，可以采用立即检索。
如果程序加载一个持久化对象的目的仅仅为了获取他的引用，可以采取延迟加载。

注意：延迟加载有可能出现懒加载异常。

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马哥的淘宝店:https://shop592330910.taobao.com/

package com.mamh.hibernate.demo.entities;import java.util.HashSet;import java.util.Set;public class Customer {    private Integer customerId;    private String customerName;马哥的淘宝店:https://shop592330910.taobao.com/    private Set<Order> orders = new HashSet<Order>();    public Customer() {    }    public Customer(String name) {        this.customerName = name;    }    public Integer getCustomerId() {        return customerId;    }    public void setCustomerId(Integer customerId) {        this.customerId = customerId;    }    public String getCustomerName() {        return customerName;    }    public void setCustomerName(String customerName) {        this.customerName = customerName;    }    public Set<Order> getOrders() {        return orders;    }    public void setOrders(Set<Order> orders) {        this.orders = orders;    }    @Override    public String toString() {        return "Customer{" +                "customerId=" + customerId +                ", customerName='" + customerName + '\'' +                '}';    }}package com.mamh.hibernate.demo.entities;public class Order {    private Integer orderId;    private String orderName;马哥的淘宝店:https://shop592330910.taobao.com/    private Customer customer;    public Integer getOrderId() {        return orderId;    }    public void setOrderId(Integer orderId) {        this.orderId = orderId;    }    public String getOrderName() {        return orderName;    }    public void setOrderName(String orderName) {        this.orderName = orderName;    }    public Customer getCustomer() {        return customer;    }    public void setCustomer(Customer customer) {        this.customer = customer;    }    @Override    public String toString() {        return "Order{" +                "orderId=" + orderId +                ", orderName='" + orderName + '\'' +                ", customer=" + customer +                '}' + '\n';    }}

<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-mapping PUBLIC    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="com.mamh.hibernate.demo.entities">    <class name="Order" table="hb_order" schema="mage">        <id name="orderId">            <column name="hb_order_id" sql-type="int(11)"/>            <generator class="native"/>        </id>        <property name="orderName" type="string">            <column name="hb_order_name"/>        </property>        <!-- 映射多对一的关联关系 property不再试用这个多对一的关系了               name是多这一端关联的那个一那一端属性名称               class的类全名               column是多的一端这边数据表的一个列名，一般是外键        -->        <many-to-one name="customer" class="Customer" column="customer_id"/>    </class></hibernate-mapping>

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测试类级别的检索策略

    @Test马哥的淘宝店:https://shop592330910.taobao.com/    public void testClassLevelStrategy(){        //类级别的检索策略        Customer customer = (Customer) session.load(Customer.class, 1);        System.out.println(customer.getClass());    }

延迟加载lazy="true"<hibernate-mapping package="com.mamh.hibernate.demo.entities">    <class name="Customer" table="hb_customer" schema="mage" lazy="true">        <id name="customerId">            <column name="hb_customer_id" sql-type="int(11)"/>            <generator class="native"/>        </id>        <property name="customerName" type="string">            <column name="hb_customer_name"/>        </property>        <set name="orders" table="hb_order" inverse="true" cascade="save-update" order-by="hb_order_id">            <key column="customer_id"/>            <one-to-many class="Order"/>        </set>    </class></hibernate-mapping>输出：class com.mamh.hibernate.demo.entities.Customer_$$_javassist_1

立即加载lazy="false"<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="com.mamh.hibernate.demo.entities">    <class name="Customer" table="hb_customer" schema="mage" lazy="false">        <id name="customerId">            <column name="hb_customer_id" sql-type="int(11)"/>            <generator class="native"/>        </id>        <property name="customerName" type="string">            <column name="hb_customer_name"/>        </property>        <set name="orders" table="hb_order" inverse="true" cascade="save-update" order-by="hb_order_id">            <key column="customer_id"/>            <one-to-many class="Order"/>        </set>    </class></hibernate-mapping>输出：Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_0_,        customer0_.hb_customer_name as hb2_2_0_     from        mage.hb_customer customer0_     where        customer0_.hb_customer_id=?class com.mamh.hibernate.demo.entities.Customer=destroy=

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一对多和多对多的检索策略

1对多 或 多对多 的集合属性默认是使用懒加载检索策略的。
可以通过设置set元素的lazy属性来重新设置检索策略。默认是true，并不建议设置为false。
set中的lazy还可以取extra，增强型的延迟加载。取该值会尽可能的延迟集合的初始化。

    @Test马哥的淘宝店:https://shop592330910.taobao.com/    public void testOne2ManyLevelStrategy() {        Customer customer = (Customer) session.get(Customer.class, 1);        System.out.println(customer.getCustomerName());        //1. 1对多 或  多对多 的集合属性默认是使用懒加载检索策略的。        //2. 可以通过设置set元素的lazy属性来重新设置检索策略。        //System.out.println(customer.getOrders());    }

延迟加载：如果不访问任何order对象是不会取加载它的。Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_0_,        customer0_.hb_customer_name as hb2_2_0_     from        mage.hb_customer customer0_     where        customer0_.hb_customer_id=?aaProcess finished with exit code 0

延迟加载，如果 有这样的 调用 customer中的集合order属性的话就会加载oder对象。System.out.println(customer.getOrders());Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_0_,        customer0_.hb_customer_name as hb2_2_0_     from        mage.hb_customer customer0_     where        customer0_.hb_customer_id=?aaHibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id=?     order by        orders0_.hb_order_id[Order{orderId=1, orderName='aa', customer=Customer{customerId=1, customerName='aa'}}, Order{orderId=2, orderName='bb', customer=Customer{customerId=1, customerName='aa'}}, Order{orderId=3, orderName='cc', customer=Customer{customerId=1, customerName='aa'}}]Process finished with exit code 0

可以通过设置customer.hbm.xml对应的set元素的lazy属性来更改这个。这里lazy="false"就表示立即加载了。        <set name="orders" table="hb_order" inverse="true" cascade="save-update" order-by="hb_order_id" lazy="false">            <key column="customer_id"/>            <one-to-many class="Order"/>        </set>

    @Test    public void testOne2ManyLevelStrategy() {        Customer customer = (Customer) session.get(Customer.class, 1);        System.out.println(customer.getCustomerName());    } 这里lazy="false"就表示立即加载了。     Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_0_,        customer0_.hb_customer_name as hb2_2_0_     from        mage.hb_customer customer0_     where        customer0_.hb_customer_id=?Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id=?     order by        orders0_.hb_order_idaaProcess finished with exit code 0

这里lazy="extra"增强型延迟加载。         @Test    public void testOne2ManyLevelStrategy() {马哥的淘宝店:https://shop592330910.taobao.com/        Customer customer = (Customer) session.get(Customer.class, 1);        System.out.println(customer.getCustomerName());        //1. 1对多 或  多对多 的集合属性默认是使用懒加载检索策略的。        //2. 可以通过设置set元素的lazy属性来重新设置检索策略。        System.out.println(customer.getOrders().size());    }        <set name="orders" table="hb_order" inverse="true" cascade="save-update" order-by="hb_order_id" lazy="extra">            <key column="customer_id"/>            <one-to-many class="Order"/>        </set>这里访问了order集合的size，但是并没去初始化整个order集合，而是采用了sql语句来获取了集合的个数。Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_0_,        customer0_.hb_customer_name as hb2_2_0_     from        mage.hb_customer customer0_     where        customer0_.hb_customer_id=?aaHibernate:     select count(hb_order_id) from mage.hb_order  where  customer_id =?3

直接手动调用初始化集合对象

        Hibernate.initialize(customer.getOrders());

大部分取默认值，lazy= “true”

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set元素的batch-size属性

这个可以设定一次初始化集合的数量。

    @Test    public void testBatchSize(){马哥的淘宝店:https://shop592330910.taobao.com/        List<Customer> customers = session.createQuery("from Customer ").list();        System.out.println(customers.size());    }Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4

    @Test    public void testBatchSize() {马哥的淘宝店:https://shop592330910.taobao.com/        List<Customer> customers = session.createQuery("from Customer ").list();        System.out.println(customers.size());        for (Customer customer : customers) {            if (customer.getOrders() != null) {                System.out.println(customer.getOrders().size());            }        }    }Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id=?     order by        orders0_.hb_order_id3Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id=?     order by        orders0_.hb_order_id3Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id=?     order by        orders0_.hb_order_id3Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from马哥的淘宝店:https://shop592330910.taobao.com/        mage.hb_order orders0_     where        orders0_.customer_id=?     order by        orders0_.hb_order_id0Process finished with exit code 0

这里batch-size设置为4， batch-size="4",这样能检索select语句的个数，之前是4条，现在只有一条了。        <set name="orders" table="hb_order" inverse="true" cascade="save-update" order-by="hb_order_id" lazy="true" batch-size="4">            <key column="customer_id"/>            <one-to-many class="Order"/>        </set>Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where马哥的淘宝店:https://shop592330910.taobao.com/        orders0_.customer_id in (            ?, ?, ?, ?        )     order by        orders0_.hb_order_id3330Process finished with exit code 0

这里batch-size设置为2,batch-size="2",这样会有2条select语句。这里为什么是2了？应为customer表数据是4条，4除以batch-size就是等于2的。        <set name="orders" table="hb_order" inverse="true" cascade="save-update" order-by="hb_order_id" lazy="true" batch-size="2">            <key column="customer_id"/>            <one-to-many class="Order"/>        </set>Hibernate:马哥的淘宝店:https://shop592330910.taobao.com/     select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id in (            ?, ?        )     order by        orders0_.hb_order_id33Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id in ( ?, ? )     order by        orders0_.hb_order_id30Process finished with exit code 0

customer表格 hb_customer_id hb_customer_name 1 1 aa 2 2 bb 3 3 cc 4 4 dd order表格 hb_order_id hb_order_name customer_id 1 1 aa 1 2 2 bb 1 3 3 cc 1 4 4 dd 2 5 5 ee 2 6 6 ff 2 7 7 gg 3 8 8 hh 3 9 9 ii 3 10 10 jj NULL

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set元素的fetch属性，
当fetch取值为select或subselect，决定初始化order集合的查询语句形式，
若取值为join则决定orders集合被初始化的时机。

若把fetch设置为join，lazy属性取值将被忽略。

    @Test    public void testFetch() {马哥的淘宝店:https://shop592330910.taobao.com/        List<Customer> customers = session.createQuery("from Customer ").list();        System.out.println(customers.size());        for (Customer customer : customers) {            if (customer.getOrders() != null) {                System.out.println(customer.getOrders().size());            }        }    }

首先设置fetch是select，默认是select值。

Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id in ( ?, ? )     order by        orders0_.hb_order_id33Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id in ( ?, ? )     order by        orders0_.hb_order_id30Process finished with exit code 0

这里设置fetch是subselect。通过子查询的方式来初始化所有set集合。
次数lazy有效，但是batch-szie失效。

Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where马哥的淘宝店:https://shop592330910.taobao.com/        orders0_.customer_id in (            select customer0_.hb_customer_id             from mage.hb_customer customer0_        )     order by        orders0_.hb_order_id3330

这里fetch取值join。
这里采用的是HQL查询。HQL查询忽略fetch=join的取值。

Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_,        customer0_.hb_customer_name as hb2_2_     from        mage.hb_customer customer0_4Hibernate:马哥的淘宝店:https://shop592330910.taobao.com/     select马哥的淘宝店:https://shop592330910.taobao.com/        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from        mage.hb_order orders0_     where        orders0_.customer_id in ( ?, ? )     order by        orders0_.hb_order_id33Hibernate:马哥的淘宝店:https://shop592330910.taobao.com/     select马哥的淘宝店:https://shop592330910.taobao.com/        orders0_.customer_id as customer3_2_1_,        orders0_.hb_order_id as hb1_1_,        orders0_.hb_order_id as hb1_3_0_,        orders0_.hb_order_name as hb2_3_0_,        orders0_.customer_id as customer3_3_0_     from马哥的淘宝店:https://shop592330910.taobao.com/        mage.hb_order orders0_     where马哥的淘宝店:https://shop592330910.taobao.com/        orders0_.customer_id in ( ?, ? )     order by        orders0_.hb_order_id30Process finished with exit code 0马哥的淘宝店:https://shop592330910.taobao.com/

这里值查询一个customer对象，这时值查询customer对象，这个时候order也初始化了，
这个时候lazy属性是没用的。
使用左外链接的方式检索。
HQL查询忽略fetch=join的取值。

    @Test    public void testFetch2() {马哥的淘宝店:https://shop592330910.taobao.com/        Customer customer = (Customer) session.get(Customer.class, 1);        System.out.println(customer.getOrders().size());    }Hibernate: 马哥的淘宝店:https://shop592330910.taobao.com/    select        customer0_.hb_customer_id as hb1_2_1_,        customer0_.hb_customer_name as hb2_2_1_,        orders1_.customer_id as customer3_2_3_,        orders1_.hb_order_id as hb1_3_,        orders1_.hb_order_id as hb1_3_0_,        orders1_.hb_order_name as hb2_3_0_,        orders1_.customer_id as customer3_3_0_     from马哥的淘宝店:https://shop592330910.taobao.com/        mage.hb_customer customer0_     left outer join        mage.hb_order orders1_             on customer0_.hb_customer_id=orders1_.customer_id     where马哥的淘宝店:https://shop592330910.taobao.com/        customer0_.hb_customer_id=?     order by        orders1_.hb_order_id3

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<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-mapping PUBLIC    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="com.mamh.hibernate.demo.entities">    <class name="Order" table="hb_order" schema="mage">        <id name="orderId">            <column name="hb_order_id" sql-type="int(11)"/>            <generator class="native"/>        </id>        <property name="orderName" type="string">            <column name="hb_order_name"/>        </property>        <!-- 映射多对一的关联关系 property不再试用这个多对一的关系了               name是多这一端关联的那个一那一端属性名称               class的类全名               column是多的一端这边数据表的一个列名，一般是外键        -->        <many-to-one name="customer" class="Customer" column="customer_id"/>    </class></hibernate-mapping>

    @Test    public void testManyToOneGet() {        Order order = (Order) session.get(Order.class, 1);        System.out.println(order.getOrderName() + ":  " + order.getOrderId());        //Customer customer = order.getCustomer();        //System.out.println(customer);    }

Hibernate:     select        order0_.hb_order_id as hb1_3_0_,        order0_.hb_order_name as hb2_3_0_,        order0_.customer_id as customer3_3_0_     from        mage.hb_order order0_     where        order0_.hb_order_id=?aa:  1

lazy属性设置为true或者false，表示延迟加载，还是立即加载。当设置了 lazy=”false”

        <many-to-one name="customer" class="Customer" column="customer_id"                     lazy="false"        />Hibernate:     select        order0_.hb_order_id as hb1_3_0_,        order0_.hb_order_name as hb2_3_0_,        order0_.customer_id as customer3_3_0_     from        mage.hb_order order0_     where        order0_.hb_order_id=?Hibernate:     select        customer0_.hb_customer_id as hb1_2_0_,        customer0_.hb_customer_name as hb2_2_0_     from        mage.hb_customer customer0_     where        customer0_.hb_customer_id=?aa:  1

fetch属性取值为join，表示采用迫切左外链接的方式初始化n关联的1的一端的属性。fetch=”join“

Hibernate:     select        order0_.hb_order_id as hb1_3_1_,        order0_.hb_order_name as hb2_3_1_,        order0_.customer_id as customer3_3_1_,        customer1_.hb_customer_id as hb1_2_0_,        customer1_.hb_customer_name as hb2_2_0_     from        mage.hb_order order0_     left outer join        mage.hb_customer customer1_             on order0_.customer_id=customer1_.hb_customer_id     where        order0_.hb_order_id=?aa:  1