Hibernate--延迟加载lazy（类级别的延迟加载、关联级别的延迟加载）和抓取策略fetch详解

桃扇骨 2021-08-13 22:52 593阅读 0赞

#### 延迟加载 ####

延迟加载（lazy load）是（也成为懒加载）Hibernate关联关系对象默认的加载方式，延迟加载机制是为了避免一些无谓的性能开销而提出来的，所谓延迟加载就是当在真正需要数据的时候，才真正执行数据加载操作。

通常将延迟加载分为两类：\*\*一类叫做类级别延迟，另一个叫做关联级别的延迟。\*\*类级别的延迟指的是查询某个对象的时候，是否采用有延迟，这个通常在< class >标签上配置lazy属性。关联级别的延迟指的是，查询一个对象的关联对象的是否采用延迟加载。这个通常在< set >或 < many-to-one >上配置lazy属性。

#### 类级别的延迟加载 ####

使用load方法检索某个对象的时候，这个类是否采用延迟加载的策略，就是类级别的延迟。类级别的延迟一般在< class >上配置lazy属性，lazy的默认是true。默认是延迟加载的，所以使用load方法去查询的时候，不会马上发送SQL语句，当真正使用该对象的时候，才会发送SQL语句。

Customer customer = session.load(Customer.class, 1l);

如果不想使用延迟加载也有很多方法，最简单的就是将这个类的映射文件上的lazy属性设置为false，当然也可以将这个持久化类改为final修饰，这样该类就无法生成代理，使延迟加载失效。

#### 关联级别的延迟加载 ####

Customer customer = session.get(Customer.class, 1l);
    Set<LinkMan> linkMans = customer.getLinkMans();

通过客户查询其关联的联系人对象，在查询联系人的时候是否采用延迟加载成为关联级别的延迟。关联级别的延迟通常是在< set > 和 < many-to-one >上进行配置。

**< set >标签上的lazy通常有三个取值：**

*  true：默认值，采用延迟加载
 *  false：检索关联对象的时候，不采用延迟加载
 *  extra：及其懒惰的

**< many-to-one >标签上的lazy通常有三个取值：**

*  proxy：默认值，是否采用延迟取决于一的一方类上的lazy属性的值
 *  false：检索关联对象的时候，不采用延迟加载
 *  no-proxy：基本不使用

#### 抓取策略 ####

抓取策略是当应用程序需要在（Hibernate实体对象图的）关联关系间进行导航的时候，Hibernate如何获取关联对象的策略。

Hibernate的抓取策略是Hibernate提升性能的一种手段，可以在获取关联对象的时候，对发送的语句进行优化。

抓取策略通过在关联对象的标签上配置fetch属性完成。关联上分为是在< set > 和 < many-to-one >上，也都有不同的取值。

**< set >标签上的fetch通常有三个取值：**

*  select：默认值，发送的是普通的select语句查询
 *  join：发送一条迫切左外连接区查询
 *  subselect：发送一条子查询语句查询其关联对象

**< many-to-one >标签上的fetch有两个取值：**

*  select：默认值，发送一条普通的select语句查询关联对象
 *  join：发送一条迫切左外连接语句查询其关联对象

#### 测试 ####

< set >上配置的fetch有三个值，lazy有三个值，这样就会产生很多种效果，但是，如果fetch设置为join，lazy就会失效。

#### 创建表 ####

cst\_customer与cst\_linkman是一对多关系，cust\_id为外键。  
**cst\_customer表：**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70]  
**cst\_linkman表：**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 1]

#### 创建实体类 ####

**Customer类：**

public class Customer { 
    	//主键
    	private Long cust_id;
    	
    	private String cust_name;
    	private String cust_source;
    	private String cust_industry;
    	private String cust_level;
    	private String cust_linkman;
    	private String cust_phone;
    	private String cust_mobile;
    	
    	//使用set集合,表达一对多关系
    	private Set<LinkMan> linkMens = new HashSet<LinkMan>();
    	
    	public Set<LinkMan> getLinkMens() { 
    		return linkMens;
    	}
    	public void setLinkMens(Set<LinkMan> linkMens) { 
    		this.linkMens = linkMens;
    	}
    	public Long getCust_id() { 
    		return cust_id;
    	}
    	public void setCust_id(Long cust_id) { 
    		this.cust_id = cust_id;
    	}
    	public String getCust_name() { 
    		return cust_name;
    	}
    	public void setCust_name(String cust_name) { 
    		this.cust_name = cust_name;
    	}
    	public String getCust_source() { 
    		return cust_source;
    	}
    	public void setCust_source(String cust_source) { 
    		this.cust_source = cust_source;
    	}
    	public String getCust_industry() { 
    		return cust_industry;
    	}
    	public void setCust_industry(String cust_industry) { 
    		this.cust_industry = cust_industry;
    	}
    	public String getCust_level() { 
    		return cust_level;
    	}
    	public void setCust_level(String cust_level) { 
    		this.cust_level = cust_level;
    	}
    	public String getCust_linkman() { 
    		return cust_linkman;
    	}
    	public void setCust_linkman(String cust_linkman) { 
    		this.cust_linkman = cust_linkman;
    	}
    	public String getCust_phone() { 
    		return cust_phone;
    	}
    	public void setCust_phone(String cust_phone) { 
    		this.cust_phone = cust_phone;
    	}
    	public String getCust_mobile() { 
    		return cust_mobile;
    	}
    	public void setCust_mobile(String cust_mobile) { 
    		this.cust_mobile = cust_mobile;
    	}
    }

**LinkMan类：**

//联系人实体
    public class LinkMan { 
    	//主键
    	private Long lkm_id;
    	
    	private Character lkm_gender;
    	private String lkm_name;
    	private String lkm_phone;
    	private String lkm_email;
    	private String lkm_qq;
    	private String lkm_mobile;
    	private String lkm_memo;
    	private String lkm_position;
    	
    	//表达多对一关系,外键
    	private Customer customer ;
    	
    	public Customer getCustomer() { 
    		return customer;
    	}
    	public void setCustomer(Customer customer) { 
    		this.customer = customer;
    	}
    	public Long getLkm_id() { 
    		return lkm_id;
    	}
    	public void setLkm_id(Long lkm_id) { 
    		this.lkm_id = lkm_id;
    	}
    	public Character getLkm_gender() { 
    		return lkm_gender;
    	}
    	public void setLkm_gender(Character lkm_gender) { 
    		this.lkm_gender = lkm_gender;
    	}
    	public String getLkm_name() { 
    		return lkm_name;
    	}
    	public void setLkm_name(String lkm_name) { 
    		this.lkm_name = lkm_name;
    	}
    	public String getLkm_phone() { 
    		return lkm_phone;
    	}
    	public void setLkm_phone(String lkm_phone) { 
    		this.lkm_phone = lkm_phone;
    	}
    	public String getLkm_email() { 
    		return lkm_email;
    	}
    	public void setLkm_email(String lkm_email) { 
    		this.lkm_email = lkm_email;
    	}
    	public String getLkm_qq() { 
    		return lkm_qq;
    	}
    	public void setLkm_qq(String lkm_qq) { 
    		this.lkm_qq = lkm_qq;
    	}
    	public String getLkm_mobile() { 
    		return lkm_mobile;
    	}
    	public void setLkm_mobile(String lkm_mobile) { 
    		this.lkm_mobile = lkm_mobile;
    	}
    	public String getLkm_memo() { 
    		return lkm_memo;
    	}
    	public void setLkm_memo(String lkm_memo) { 
    		this.lkm_memo = lkm_memo;
    	}
    	public String getLkm_position() { 
    		return lkm_position;
    	}
    	public void setLkm_position(String lkm_position) { 
    		this.lkm_position = lkm_position;
    	}
    }

#### 配置ORM元数据： ####

**Customer.hbm.xml：**

<?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="pers.zhang.domain" >
    	<class name="Customer" table="cst_customer" >
    		<id name="cust_id" >
    			<generator class="native"></generator>
    		</id>
    		<property name="cust_name" column="cust_name" ></property>
    		<property name="cust_source" column="cust_source" ></property>
    		<property name="cust_industry" column="cust_industry" ></property>
    		<property name="cust_level" column="cust_level" ></property>
    		<property name="cust_linkman" column="cust_linkman" ></property>
    		<property name="cust_phone" column="cust_phone" ></property>
    		<property name="cust_mobile" column="cust_mobile" ></property>
    
    		<set name="linkMens" inverse="true" cascade="all" >
    			<key column="lkm_cust_id" ></key>
    			<one-to-many class="LinkMan" />
    		</set>
    		
    	
    	</class>
    </hibernate-mapping>

**LinkMan.hbm.xml：**

<?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="pers.zhang.domain" >
    	<class name="LinkMan" table="cst_linkman" >
    		<id name="lkm_id" >
    			<generator class="native"></generator>
    		</id>
    		<property name="lkm_gender" ></property>
    		<property name="lkm_name" ></property>
    		<property name="lkm_phone" ></property>
    		<property name="lkm_email" ></property>
    		<property name="lkm_qq" ></property>
    		<property name="lkm_mobile" ></property>
    		<property name="lkm_memo" ></property>
    		<property name="lkm_position" ></property>
    		
    		<many-to-one name="customer" column="lkm_cust_id" class="Customer" ></many-to-one>
    		
    	</class>
    </hibernate-mapping>

**准备数据：**  
![在这里插入图片描述][20191007133047866.png]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 2]  
**测试默认情况(未设置抓取策略和延迟加载)：**

@Test
    public void fun8() { 
    	Session session = HibernateUtils.openSession();
    	Transaction tx = session.beginTransaction();
    	
    	//查询一号客户,发送一条select语句查询一号客户
    	Customer customer = session.get(Customer.class, 1l);
    	
    	//获得一号客户的联系人数量，发送一条select语句查询1号客户的所有联系人
    	System.err.println(customer.getLinkMens().size());
    	
    	tx.commit();
    }

运行测试输出：

**< set >上配置fetch=“select” lazy=“true”:**

//集合级别的关联
    //fetch:select 单表查询
    //lazy:true 使用时才加载集合数据.
    @Test
    public void fun1(){ 
    	Session session = HibernateUtils.openSession();
    	Transaction tx = session.beginTransaction();
    	//----------------------------------------------------
    	
    	Customer c = session.get(Customer.class, 2l);
    	
    	Set<LinkMan> linkMens = c.getLinkMens();//关联级别
    	
    	System.out.println(linkMens);
    	
    	//----------------------------------------------------
    	tx.commit();
    	session.close();
    	
    }

通过在打印语句前设置断点可知，在打印前并未加载集合数据，确认为懒加载。

**< set >上配置fetch=“select” lazy=“false”:**

立即加载数据

**< set >上配置fetch=“select” lazy=“extra”:**

//fetch:select 单表查询
    //lazy:extra 极其懒惰.与懒加载效果基本一致. 如果只获得集合的size.只查询集合的size(count语句)
    @Test
    public void fun3(){ 
    	Session session = HibernateUtils.openSession();
    	Transaction tx = session.beginTransaction();
    	//----------------------------------------------------
    	Customer c = session.get(Customer.class, 2l);
    	Set<LinkMan> linkMens = c.getLinkMens();//关联级别
    	System.out.println(linkMens.size());
    	//----------------------------------------------------
    	tx.commit();
    	session.close();
    }

运行测试，Hibernate打印：

Hibernate: 
        select
            customer0_.cust_id as cust_id1_0_0_,
            customer0_.cust_name as cust_nam2_0_0_,
            customer0_.cust_source as cust_sou3_0_0_,
            customer0_.cust_industry as cust_ind4_0_0_,
            customer0_.cust_level as cust_lev5_0_0_,
            customer0_.cust_linkman as cust_lin6_0_0_,
            customer0_.cust_phone as cust_pho7_0_0_,
            customer0_.cust_mobile as cust_mob8_0_0_ 
        from
            cst_customer customer0_ 
        where
            customer0_.cust_id=?
    Hibernate: 
        select
            count(lkm_id) 
        from
            cst_linkman 
        where
            lkm_cust_id =?
    8

可见获得集合size就只查询集合的size

**< set >上配置fetch=“join” lazy=“true| false | extra”:**  
当fetch设置为join时，lazy属性失效，立即加载

**< set >上配置fetch=“subselect” lazy=“true”:**

@Test
    //fetch: subselect 子查询
    //lazy: false 立即加载
    public void fun6(){ 
    	Session session = HibernateUtils.openSession();
    	Transaction tx = session.beginTransaction();
    	//----------------------------------------------------
    		
    	String  hql = "from Customer";
    	
    	Query query = session.createQuery(hql);
    	
    	List<Customer> list = query.list();//设置断点，此处查询到所有Customer
    	
    	for(Customer c:list){ 
    		System.out.println(c);//Customer已经加载了，直接打印
    		System.out.println(c.getLinkMens().size());//通过子查询，查询所有linkMan
    		System.out.println(c.getLinkMens());//打印
    	}
    	
    	//----------------------------------------------------
    	tx.commit();
    	session.close();
    	
    }

Hibernate: 
        select
            customer0_.cust_id as cust_id1_0_,
            customer0_.cust_name as cust_nam2_0_,
            customer0_.cust_source as cust_sou3_0_,
            customer0_.cust_industry as cust_ind4_0_,
            customer0_.cust_level as cust_lev5_0_,
            customer0_.cust_linkman as cust_lin6_0_,
            customer0_.cust_phone as cust_pho7_0_,
            customer0_.cust_mobile as cust_mob8_0_ 
        from
            cst_customer customer0_
            //此处为断点
    Customer [cust_id=1, cust_name=百度, cust_level=null]
    Hibernate: 
        select
            linkmens0_.lkm_cust_id as lkm_cus10_1_1_,
            linkmens0_.lkm_id as lkm_id1_1_1_,
            linkmens0_.lkm_id as lkm_id1_1_0_,
            linkmens0_.lkm_gender as lkm_gend2_1_0_,
            linkmens0_.lkm_name as lkm_name3_1_0_,
            linkmens0_.lkm_phone as lkm_phon4_1_0_,
            linkmens0_.lkm_email as lkm_emai5_1_0_,
            linkmens0_.lkm_qq as lkm_qq6_1_0_,
            linkmens0_.lkm_mobile as lkm_mobi7_1_0_,
            linkmens0_.lkm_memo as lkm_memo8_1_0_,
            linkmens0_.lkm_position as lkm_posi9_1_0_,
            linkmens0_.lkm_cust_id as lkm_cus10_1_0_ 
        from
            cst_linkman linkmens0_ 
        where
            linkmens0_.lkm_cust_id in (
                select
                    customer0_.cust_id 
                from
                    cst_customer customer0_
            )

**< set >上配置fetch=“subselect” lazy=“false”:**

@Test
    //fetch: subselect 子查询
    //lazy: false 立即加载
    public void fun6(){ 
    	Session session = HibernateUtils.openSession();
    	Transaction tx = session.beginTransaction();
    	//----------------------------------------------------
    		
    	String  hql = "from Customer";
    	
    	Query query = session.createQuery(hql);
    	
    	List<Customer> list = query.list();//断点，查询出所有的Customer及所有的LinkMan
    	
    	for(Customer c:list){ //直接打印即可
    		System.out.println(c);
    		System.out.println(c.getLinkMens().size());
    		System.out.println(c.getLinkMens());
    	}
    	
    	//----------------------------------------------------
    	tx.commit();
    	session.close();
    	
    }

**< set >上配置fetch=“subselect” lazy=“extra”:**

@Test
    //fetch: subselect 子查询
    //lazy: extra 极其懒惰
    public void fun7(){ 
    	Session session = HibernateUtils.openSession();
    	Transaction tx = session.beginTransaction();
    	//----------------------------------------------------
    		
    	String  hql = "from Customer";
    	
    	Query query = session.createQuery(hql);
    	
    	List<Customer> list = query.list();//断点，只查询Customer
    	
    	for(Customer c:list){ 
    		System.out.println(c);//打印
    		System.out.println(c.getLinkMens().size());//断点，只查询size
    		System.out.println(c.getLinkMens());//查询具体数据
    	}
    	
    	//----------------------------------------------------
    	tx.commit();
    	session.close();
    	
    }

**< many-to-one >上配置fetch=“select” lazy=“proxy” Customer上的lazy=“true”:(默认情况)**:

@Test
    	//fetch:select 单表查询
    	//lazy:proxy 
    	//customer-true 懒加载
    	public void fun1(){ 
    		Session session = HibernateUtils.openSession();
    		Transaction tx = session.beginTransaction();
    		//----------------------------------------------------
    		
    		LinkMan lm = session.get(LinkMan.class, 3l);//断点，只查询LinkMan
    		
    		Customer customer = lm.getCustomer();//获得关联的Customer，不加载
    		
    		System.out.println(customer);//使用Customer时加载
    		
    		//----------------------------------------------------
    		tx.commit();
    		session.close();
    		
    	}

**< many-to-one >上配置fetch=“select” lazy=“proxy” Customer上的lazy=“false”:**

@Test
    	//fetch:select 单表查询
    	//lazy:proxy 
    	//customer-false 立即加载
    	public void fun1(){ 
    		Session session = HibernateUtils.openSession();
    		Transaction tx = session.beginTransaction();
    		//----------------------------------------------------
    		
    		LinkMan lm = session.get(LinkMan.class, 3l);//断点，加载LinkMan和对应的Customer（单表查询）
    		
    		Customer customer = lm.getCustomer();
    		
    		System.out.println(customer);//直接使用
    		
    		//----------------------------------------------------
    		tx.commit();
    		session.close();
    		
    	}

**< many-to-one >上配置fetch=“join” lazy=“失效” :**

@Test
    	//fetch:join 多表
    	//lazy: 失效 
    	public void fun3(){ 
    		Session session = HibernateUtils.openSession();
    		Transaction tx = session.beginTransaction();
    		//----------------------------------------------------
    		
    		LinkMan lm = session.get(LinkMan.class, 3l);//多表查询出LinkMan和对应的Customer
    		
    		Customer customer = lm.getCustomer();
    		
    		System.out.println(customer);
    		
    		//----------------------------------------------------
    		tx.commit();
    		session.close();
    		
    	}

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70]: /images/20210813/0ff964275d4c46668755fc7c5c449703.png
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[20191007133047866.png]: /images/20210813/8aaa9f47ef784ee1951d923f39a93998.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70 2]: /images/20210813/6c44b24860974c6cb60a652d8160e321.png