Hibernate之get()和load()
get()和load()都是从数据库中检索对象，二者最大的区别就是load()支持延迟加载，而get()不支持延迟加载，是立即检索的策略。
分析get()和load()主要分析类级别的检索策略，立即检索和延迟检索。
类级别的默认检索方式有些书上写的是立即检索，事实证明默认检索方式是延迟检索。
1.立即检索
在xxx.hbm.xml配置文件中，配置成以下方式是立即检索的方式：
<class name="Customer" table="CUSTOMERS" select-before-update="true" lazy="false" >
当通过session的load()方法检索Customer对象或通过get()检索Customer对象时：
Customer customer = (Customer) session.load(Customer.class, new Long(1));
Hibernate会立刻执行select语句查询CUSTOMERS表，控制台打印的sql语句：
select customer0_.ID as ID1_0_0_, customer0_.NAME as NAME2_0_0_, customer0_.EMAIL as EMAIL3_0_0_, customer0_.PASSWORD as PASSWORD4_0_0_, customer0_.PHONE as PHONE5_0_0_, customer0_.ADDRESS as ADDRESS6_0_0_, customer0_.SEX as SEX7_0_0_, customer0_.IS_MARRIED as IS8_0_0_, customer0_.DESCRIPTION as DESCRIPT9_0_0_, customer0_.IMAGE as IMAGE10_0_0_, customer0_.BIRTHDAY as BIRTHDA11_0_0_, customer0_.REGISTERED_TIME as REGISTE12_0_0_, customer0_.HOME_PROVINCE as HOME13_0_0_, customer0_.HOME_CITY as HOME14_0_0_, customer0_.HOME_STREET as HOME15_0_0_, customer0_.HOME_ZIPCODE as HOME16_0_0_, customer0_.COMP_PROVINCE as COMP17_0_0_, customer0_.COMP_CITY as COMP18_0_0_, customer0_.COMP_STREET as COMP19_0_0_, customer0_.COMP_ZIPCODE as COMP20_0_0_ from CUSTOMERS customer0_ where customer0_.ID=?
这种立即检索的方式，get()和load()会立刻执行select语句，从数据库查询，初始化Customer对象。
关于立即检索的实例如下：
运行项目之前可以先创建表，或者通过hibernate完成表的创建。
CREATE TABLE `CUSTOMERS` ( `ID` bigint(20) NOT NULL, `NAME` varchar(25) NOT NULL, `EMAIL` varchar(255) NOT NULL, `PASSWORD` varchar(255) NOT NULL, `PHONE` int(11) DEFAULT NULL, `ADDRESS` varchar(255) DEFAULT NULL, `SEX` char(1) DEFAULT NULL, `IS_MARRIED` tinyint(1) DEFAULT NULL, `DESCRIPTION` longtext, `IMAGE` tinyblob, `BIRTHDAY` date DEFAULT NULL, `REGISTERED_TIME` datetime DEFAULT NULL, `HOME_PROVINCE` varchar(255) DEFAULT NULL, `HOME_CITY` varchar(255) DEFAULT NULL, `HOME_STREET` varchar(255) DEFAULT NULL, `HOME_ZIPCODE` smallint(6) DEFAULT NULL, `COMP_PROVINCE` varchar(255) DEFAULT NULL, `COMP_CITY` varchar(255) DEFAULT NULL, `COMP_STREET` varchar(255) DEFAULT NULL, `COMP_ZIPCODE` smallint(6) DEFAULT NULL, PRIMARY KEY (`ID`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8;
1.项目结构
2.创建实体
Customer.java:
package com.lanhuigu.hibernate.entity;
import java.io.Serializable;
import java.util.Date;
import com.sun.jmx.snmp.Timestamp;
/**
* 创建java持久化类，就是需要被持久化到数据库中的实体类
*/
public class Customer implements Serializable{
private static final long serialVersionUID = 9026723017886524518L;
private Long id;
private String name;
private String email;
private String password;
private int phone;
private boolean married;
private String address;
private char sex;
private String description;
private byte[] image;
private Date birthday;
private Timestamp registeredTime;
private Address homeAddress;
private Address compAddress;
public Customer(){}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public int getPhone() {
return phone;
}
public void setPhone(int phone) {
this.phone = phone;
}
public boolean isMarried() {
return married;
}
public void setMarried(boolean married) {
this.married = married;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public char getSex() {
return sex;
}
public void setSex(char sex) {
this.sex = sex;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public byte[] getImage() {
return image;
}
public void setImage(byte[] image) {
this.image = image;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public Timestamp getRegisteredTime() {
return registeredTime;
}
public void setRegisteredTime(Timestamp registeredTime) {
this.registeredTime = registeredTime;
}
public Address getHomeAddress() {
return homeAddress;
}
public void setHomeAddress(Address homeAddress) {
this.homeAddress = homeAddress;
}
public Address getCompAddress() {
return compAddress;
}
public void setCompAddress(Address compAddress) {
this.compAddress = compAddress;
}
}
Address.java:
package com.lanhuigu.hibernate.entity;
import java.io.Serializable;
/**
* 地址实体类
*/
public class Address implements Serializable{
private static final long serialVersionUID = -1450327776636898762L;
private String province;
private String city;
private String street;
private Short zipcode;
private Customer customer;
public String getProvince() {
return province;
}
public void setProvince(String province) {
this.province = province;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getStreet() {
return street;
}
public void setStreet(String street) {
this.street = street;
}
public Short getZipcode() {
return zipcode;
}
public void setZipcode(Short zipcode) {
this.zipcode = zipcode;
}
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
}
3.Customer.hbm.xml配置文件（对象-映射文件）
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.lanhuigu.hibernate.entity" > <class name="Customer" table="CUSTOMERS" select-before-update="true" lazy="false" > <!-- 设置主键 --> <id name="id" column="ID" type="long"> <!-- 主键生成方式--> <generator class="increment"/> </id> <!-- 基本属性，基本属性必须设置在主键之口--> <property name="name" column="NAME" type="string" length="25" not-null="true" access="property"/> <property name="email" column="EMAIL" type="string" not-null="true" /> <property name="password" column="PASSWORD" type="string" not-null="true" /> <property name="phone" column="PHONE" type="int" /> <property name="address" column="ADDRESS" type="string" /> <property name="sex" column="SEX" type="character" /> <property name="married" column="IS_MARRIED" type="boolean" /> <property name="description" column="DESCRIPTION" type="text" /> <property name="image" column="IMAGE" type="binary" /> <property name="birthday" column="BIRTHDAY" type="date" /> <property name="registeredTime" column="REGISTERED_TIME" type="timestamp" insert="false" update="false"/> <!-- 部分类 --> <component name="homeAddress" class="Address"> <parent name="customer"/> <property name="province" type="string" column="HOME_PROVINCE"></property> <property name="city" type="string" column="HOME_CITY"></property> <property name="street" type="string" column="HOME_STREET"></property> <property name="zipcode" type="short" column="HOME_ZIPCODE"></property> </component> <component name="compAddress" class="Address"> <parent name="customer"/> <property name="province" type="string" column="COMP_PROVINCE"></property> <property name="city" type="string" column="COMP_CITY"></property> <property name="street" type="string" column="COMP_STREET"></property> <property name="zipcode" type="short" column="COMP_ZIPCODE"></property> </component> </class> </hibernate-mapping>
4.hibernate.cfg.xml配置文件
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <!-- hibernate-configuration为hibernate配置头文件 --> <hibernate-configuration> <!-- 配置session-factory, SessionFactory是Hibernate的工厂类， 该类负责Hibernate的配置和Hiberante的Session接口中save(),update(),delete(),load(),find()的操作 --> <session-factory> <!-- 1.数据库连接配置 --> <!-- 数据库连接url --> <property name="connection.url">jdbc:mysql://192.168.200.12:3306/hbtest</property> <!-- 数据库连接用户名 --> <property name="connection.username">root</property> <!-- 数据库连接口令 --> <property name="connection.password">root123</property> <!-- 数据库连接驱动 --> <property name="connection.driver_class">com.mysql.jdbc.Driver</property> <!-- 数据库方言 --> <property name="dialect">org.hibernate.dialect.MySQLDialect</property> <!-- 数据库连接池大小配置 --> <property name="connection.pool_size">1</property> <!-- 配置使用二级缓存的类 --> <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property> <!-- 一次读的数据库记录数 --> <property name="jdbc.fetch_size">50</property> <!-- 设定对数据库进行批量删除条数 --> <property name="jdbc.batch_size">30</property> <!-- 2.是否打印sql配置 --> <property name="show_sql">true</property> <!-- 3.对象-映射关系文件的位置 --> <mapping resource="com/lanhuigu/hibernate/entity/Customer.hbm.xml" /> </session-factory> </hibernate-configuration>
5.TestSessionLazy
package com.lanhuigu.hibernate.test;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import com.lanhuigu.hibernate.entity.Customer;
public class TestSessionLazy {
public static void main(String[] args) {
//1.读取hibernate.cfg.xml文件（完成数据库配置，映射文件的加载，以及SessionFactory创建）
Configuration cfg = new Configuration().configure();//默认读取/hibernate.cfg.xml配置文件
//2.获取配置文件创建的session工厂
SessionFactory sessionFactory = cfg.buildSessionFactory();
//3.从session工厂中拿到Session对象
Session session = sessionFactory.openSession();
//4.打开事务
Transaction tr = session.beginTransaction();
//5.调用session的方法
//get
//Customer customer = (Customer) session.get(Customer.class, new Long(1));
//load
Customer customer = (Customer) session.load(Customer.class, new Long(1));
System.out.println(customer.getName());
//System.out.println(customer.getId());
//6.提交事务
tr.commit();
//7.关闭session，清空缓存
session.close();
}
}
执行分析：
<class name="Customer" table="CUSTOMERS" select-before-update="true" lazy="false" >为立即检索设置，
load(),get()立刻执行select语句，初始化customer对象。
2.延迟检索
两种延迟检索的设置方式：
默认或者lazy设置成true:
<class name="Customer" table="CUSTOMERS" select-before-update="true" >
或
<class name="Customer" table="CUSTOMERS" select-before-update="true" lazy="true" >
get()方法对于延迟检索是没有影响的，就算设置成延迟检索，get()同样立刻执行select查询表，初始化customer对象。
load()方法并不会立刻执行select语句查询数据库，返回的是一个代理类实例，代理类具有以下特点：
（1）hibernate动态生成，扩展了Customer类，继承了Customer类的所有属性和方法，对于应用程序透明。
（2）hibernate创建Customer代理实例的时候仅仅初始化了OID属性，其他属性都是空的，这个代理类占用内存最少。
（3）当应用程序第一次访问代理类实例时，System.out.println(customer.getName());，会立刻执行select查询语句。
特别注意的是，当你访问id，即为：System.out.println(customer.getId());时，不会去执行select语句，初始化Customer对象。
get()对于延迟检索没有什么特别的，但是load()在运行时有很多特征，都是指延迟检索时：
（1）如果加载Customer对象时数据库中不存在，Session的load()不会抛异常，只有调用动态代理类对象时，才会抛异常。
我以上程序换一个数据库没有的数据，测试：
Customer customer = (Customer) session.load(Customer.class, new Long(8));
当你运行时是不会报错的，前提是你设置成延迟检索,lazy='true'或默认,如果lazy='false'会立刻报错。
但是，当你访问时，马上报错：
System.out.println(customer.getName());
控制台错误：
Hibernate: select customer0_.ID as ID1_0_0_, customer0_.NAME as NAME2_0_0_, customer0_.EMAIL as EMAIL3_0_0_, customer0_.PASSWORD as PASSWORD4_0_0_, customer0_.PHONE as PHONE5_0_0_, customer0_.ADDRESS as ADDRESS6_0_0_, customer0_.SEX as SEX7_0_0_, customer0_.IS_MARRIED as IS8_0_0_, customer0_.DESCRIPTION as DESCRIPT9_0_0_, customer0_.IMAGE as IMAGE10_0_0_, customer0_.BIRTHDAY as BIRTHDA11_0_0_, customer0_.REGISTERED_TIME as REGISTE12_0_0_, customer0_.HOME_PROVINCE as HOME13_0_0_, customer0_.HOME_CITY as HOME14_0_0_, customer0_.HOME_STREET as HOME15_0_0_, customer0_.HOME_ZIPCODE as HOME16_0_0_, customer0_.COMP_PROVINCE as COMP17_0_0_, customer0_.COMP_CITY as COMP18_0_0_, customer0_.COMP_STREET as COMP19_0_0_, customer0_.COMP_ZIPCODE as COMP20_0_0_ from CUSTOMERS customer0_ where customer0_.ID=? Exception in thread "main" org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.lanhuigu.hibernate.entity.Customer#8] at org.hibernate.internal.SessionFactoryImpl$1$1.handleEntityNotFound(SessionFactoryImpl.java:244) at org.hibernate.proxy.AbstractLazyInitializer.checkTargetState(AbstractLazyInitializer.java:261) at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:175) at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:285) at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185) at com.lanhuigu.hibernate.entity.Customer_$$_javassist_0.getName(Customer_$$_javassist_0.java) at com.lanhuigu.hibernate.test.TestSessionLazy.main(TestSessionLazy.java:25)
（2）如果在Session范围内，没有访问过Customer对象，Customer对象就没有被初始化，当你访问游离对象时，报错。
package com.lanhuigu.hibernate.test;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import com.lanhuigu.hibernate.entity.Customer;
public class TestSessionLazy {
public static void main(String[] args) {
//1.读取hibernate.cfg.xml文件（完成数据库配置，映射文件的加载，以及SessionFactory创建）
Configuration cfg = new Configuration().configure();//默认读取/hibernate.cfg.xml配置文件
//2.获取配置文件创建的session工厂
SessionFactory sessionFactory = cfg.buildSessionFactory();
//3.从session工厂中拿到Session对象
Session session = sessionFactory.openSession();
//4.打开事务
Transaction tr = session.beginTransaction();
//5.调用session的方法
//get
//Customer customer = (Customer) session.get(Customer.class, new Long(1));
//load
Customer customer = (Customer) session.load(Customer.class, new Long(1));
//System.out.println(customer.getName());
//System.out.println(customer.getId());
//6.提交事务
tr.commit();
//7.关闭session，清空缓存
session.close();
//session范围内没有访问Customer对象，没有被初始化，session关闭时，访问一个没有初始化的游离对象，报错
System.out.println(customer.getName());
}
}
错误如下：
Exception in thread "main" org.hibernate.LazyInitializationException: could not initialize proxy - no Session at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:164) at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:285) at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185) at com.lanhuigu.hibernate.entity.Customer_$$_javassist_0.getName(Customer_$$_javassist_0.java) at com.lanhuigu.hibernate.test.TestSessionLazy.main(TestSessionLazy.java:31)（3）前面说过的访问getId()时，不会触发Hibernate执行select语句。
System.out.println(customer.getId());
控制台可以看出不会有sql打印出，但是把获得的id打印出来。
（4）Hibernate.isInitialized(customer)，判断动态代理类是不是已经被初始化，初始化为true,否则为false。
Hibernate.initialize(customer);//初始化动态代理对象
package com.lanhuigu.hibernate.test;
import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import com.lanhuigu.hibernate.entity.Customer;
public class TestSessionLazy {
public static void main(String[] args) {
//1.读取hibernate.cfg.xml文件（完成数据库配置，映射文件的加载，以及SessionFactory创建）
Configuration cfg = new Configuration().configure();//默认读取/hibernate.cfg.xml配置文件
//2.获取配置文件创建的session工厂
SessionFactory sessionFactory = cfg.buildSessionFactory();
//3.从session工厂中拿到Session对象
Session session = sessionFactory.openSession();
//4.打开事务
Transaction tr = session.beginTransaction();
//5.调用session的方法
//get
//Customer customer = (Customer) session.get(Customer.class, new Long(1));
//load
Customer customer = (Customer) session.load(Customer.class, new Long(1));
if(Hibernate.isInitialized(customer)){//判断动态代理类是不是已经被初始化，初始化为true,否则为false
System.out.println("被初始化");
} else {
System.out.println("没有被初始化");
Hibernate.initialize(customer);//初始化动态代理对象
System.out.println(Hibernate.isInitialized(customer));
}
//System.out.println(customer.getName());
//System.out.println(customer.getId());
//6.提交事务
tr.commit();
//7.关闭session，清空缓存
session.close();
//session范围内没有访问Customer对象，没有被初始化，session关闭时，访问一个没有初始化的游离对象，报错
//System.out.println(customer.getName());
}
}
控制台打印结果：
没有被初始化 Hibernate: select customer0_.ID as ID1_0_0_, customer0_.NAME as NAME2_0_0_, customer0_.EMAIL as EMAIL3_0_0_, customer0_.PASSWORD as PASSWORD4_0_0_, customer0_.PHONE as PHONE5_0_0_, customer0_.ADDRESS as ADDRESS6_0_0_, customer0_.SEX as SEX7_0_0_, customer0_.IS_MARRIED as IS8_0_0_, customer0_.DESCRIPTION as DESCRIPT9_0_0_, customer0_.IMAGE as IMAGE10_0_0_, customer0_.BIRTHDAY as BIRTHDA11_0_0_, customer0_.REGISTERED_TIME as REGISTE12_0_0_, customer0_.HOME_PROVINCE as HOME13_0_0_, customer0_.HOME_CITY as HOME14_0_0_, customer0_.HOME_STREET as HOME15_0_0_, customer0_.HOME_ZIPCODE as HOME16_0_0_, customer0_.COMP_PROVINCE as COMP17_0_0_, customer0_.COMP_CITY as COMP18_0_0_, customer0_.COMP_STREET as COMP19_0_0_, customer0_.COMP_ZIPCODE as COMP20_0_0_ from CUSTOMERS customer0_ where customer0_.ID=? true