Hibernate之多端加载优化

偏执的太偏执、 2023-10-17 11:39 3阅读 0赞

## Hibernate之多端加载优化 ##

**所谓多端加载优化是指：一方为主加载对象，而多方为从加载对象，对于多方加载时的进行的延迟加载配置，鉴于此，fetch（加载方式）、lazy（加载时机）应设置在一方的关联属性中，即在集合set标签中：举个栗子**  
场景：一个国家（country）对应多个部长（minister），让其关联关系为双向关联。

### 实体类（Country.java） ###

package com.hibernate.beans;
    
    import java.util.HashSet;
    import java.util.Set;
    
    public class Country {
        
    
        private Integer cid;
        private String cname;
        private Set<Minister> ministers;
        public Integer getCid() {
            return cid;
        }
        public void setCid(Integer cid) {
            this.cid = cid;
        }
    
    
        public Country() {
            ministers = new HashSet<Minister>();
        }
        public Country(String cname) {
            this();
            this.cname = cname;
        }
        @Override
        public String toString() {
            return "Country [cid=" + cid + ", cname=" + cname + "]";
        }
        public String getCname() {
            return cname;
        }
        public void setCname(String cname) {
            this.cname = cname;
        }
        public Set<Minister> getMinisters() {
            return ministers;
        }
        public void setMinisters(Set<Minister> ministers) {
            this.ministers = ministers;
        }
    
    
    
    }

### 实体类（Minister.java） ###

package com.hibernate.beans;
    /**
     * 部长类
     * @author Administrator
     *
     */
    public class Minister {
        
    
        private Integer mid;
        private Country country;
        private String mame;
        public Integer getMid() {
            return mid;
        }
        public void setMid(Integer mid) {
            this.mid = mid;
        }
    
        public Minister() {
            super();
        }
        public Minister(String mame) {
            super();
            this.mame = mame;
        }
        public String getMane() {
            return mame;
        }
        public void setMane(String mame) {
            this.mame = mame;
        }
    
        public Country getCountry() {
            return country;
        }
        /**
         * 双关联要求只有一方输出对方即可，否则就会形成递归（玩完）
         */
    
        @Override
        public String toString() {
            return "Minister [mid=" + mid + ", country=" + country + ", mame="
                    + mame + "]";
        }
        public void setCountry(Country country) {
            this.country = country;
        }
    }

### 映射文件（Country.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="com.hibernate.beans">
         <class name="Country" table="t_country">
            <id name="cid" column="t_cid">
                
                <generator class="native"></generator>
            </id>
            <property name="cname" column="t_cname"></property>
            
            <set name="ministers" cascade="save-update" lazy="true" fetch="select">
                <key column="t_cid"></key>
                <one-to-many class="Minister"/>
            </set>
         </class>
     </hibernate-mapping>

### 映射文件（Minister.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="com.hibernate.beans">
         <class name="Minister" table="t_minister">
            <id name="mid" column="t_mid">
                
                <generator class="native"></generator>
            </id>
            <property name="mane" column="t_mame"></property>
            <many-to-one name="country" class="Country" cascade="save-update" column="t_cid"/>
         </class>
     </hibernate-mapping>

### 测试代码 ###

public void testLayz(){
            Session session = HbnUtils.getSession();
            try{
                session.beginTransaction();
                Country country = session.get(Country.class, 1);
                Set<Minister> ministers = country.getMinisters();
                System.out.println(ministers.size()+"--------"+country.toString());
                for (Minister minister : ministers) {
                    System.out.println(minister);
                }
                session.getTransaction().commit();
            }catch(Exception e){
                e.printStackTrace();
                session.getTransaction().rollback();
            }
        }

### 分析原因 ###

我在`Country country = session.get(Country.class, 1)处设置一个断点，单步调试看其运行过程，发现`当运行完get方法时，只是查询了Country

select
            country0_.t_cid as t_cid1_0_0_,
            country0_.t_cname as t_cname2_0_0_ 
        from
            t_country country0_ 
        where
            country0_.t_cid=?

**细看调试信息**  
  
  
![这里写图片描述][SouthEast]  
**继续执行`Set<Minister> ministers = country.getMinisters();`仍旧没有查询**  
  
![这里写图片描述][SouthEast 1]  
**在往下：**

select
            ministers0_.t_cid as t_cid3_1_0_,
            ministers0_.t_mid as t_mid1_1_0_,
            ministers0_.t_mid as t_mid1_1_1_,
            ministers0_.t_mame as t_mame2_1_1_,
            ministers0_.t_cid as t_cid3_1_1_ 
        from
            t_minister ministers0_ 
        where
            ministers0_.t_cid=?

![这里写图片描述][SouthEast 2]  
  
**知道这个时候才进行了查询，因为访问到了详细信息——ministers.size()**

这是延迟加载，也叫懒加载，还有一种更懒的方式

**这个是能用聚合查询的就不用详情查询，真是懒到一定境界，优化就是这个意思，有优化性能才能更高，聚合查询查的是索引，效率不是一般的高！远远高于详情查询。**

[SouthEast]: https://img-blog.csdn.net/20170317164854075?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 1]: https://img-blog.csdn.net/20170317172626315?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 2]: https://img-blog.csdn.net/20170317172851041?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast