上一篇文章（Hibernate的延迟加载 ，懒加载，lazy）说到Hibernate的延迟加载跟fetch的配置还有一定关系，下面就来讨论下fetch的用法。
抓取策略（fetch）是指当我们去查询一个对象里面所关联的其他对象时，按照哪种方法去抓取关联对象。
fetch策略一共有四种：select、subselect、join、batch，下面我们一一介绍。我们还是用上面介绍延迟加载的相关表和实体类。
Company表：
Employee表（employee_company_id为外键）
Company实体类：
import java.util.Set;
public class Company {
private int companyId;
private String companyName;
private Set<Employee> companyEmployees;
public int getCompanyId() {
return companyId;
}
public void setCompanyId(int companyId) {
this.companyId = companyId;
}
public String getCompanyName() {
return companyName;
}
public void setCompanyName(String companyName) {
this.companyName = companyName;
}
public Set<Employee> getCompanyEmployees() {
return companyEmployees;
}
public void setCompanyEmployees(Set<Employee> companyEmployees) {
this.companyEmployees = companyEmployees;
}
}Employee实体类：
public class Employee {
private int employeeId;
private String employeeName;
private Company employeeCompany;
public int getEmployeeId() {
return employeeId;
}
public void setEmployeeId(int employeeId) {
this.employeeId = employeeId;
}
public String getEmployeeName() {
return employeeName;
}
public void setEmployeeName(String employeeName) {
this.employeeName = employeeName;
}
public Company getEmployeeCompany() {
return employeeCompany;
}
public void setEmployeeCompany(Company employeeCompany) {
this.employeeCompany = employeeCompany;
}
}Company hbm配置：
<hibernate-mapping> <class name="com.jaeger.hibernatetest.day7.lazy.Company" table="company"> <id name="companyId" column="company_id"> <generator class="native"></generator> </id> <property name="companyName" column="company_name"/> <set name="companyEmployees" cascade="all"> <key column="employee_company_id"></key> <one-to-many class="com.jaeger.hibernatetest.day7.lazy.Employee"/> </set> </class> </hibernate-mapping>
Employee hbm配置：
<hibernate-mapping> <class name="com.jaeger.hibernatetest.day7.lazy.Employee" table="employee"> <id name="employeeId" column="employee_id"> <generator class="native"></generator> </id> <property name="employeeName" column="employee_name"/> <many-to-one name="employeeCompany" class="com.jaeger.hibernatetest.day7.lazy.Company" foreign-key="fk_employee_company" column="employee_company_id" cascade="save-update"> </many-to-one> </class> </hibernate-mapping>
fetch属性用在<many-to-one><one-to-many>等关联标签里，我们就用<many-to-one>来介绍，从数据库取数据就用get()方法，让外层对象不用延迟加载。
1. select
查询关联对象时，是拿外层对象的外键，向数据库再发送select语句去抓取关联对象。
修改上面Employee hbm的配置：
<many-to-one name="employeeCompany" class="com.jaeger.hibernatetest.day7.lazy.Company" foreign-key="fk_employee_company" column="employee_company_id" cascade="save-update" fetch="select"> </many-to-one>
在<many-to-one>标签中我们添加了fetch="select"，下面是测试代码：
Employee employee = (Employee)session.get(Employee.class, 1); //A System.out.println(employee.getEmployeeName()); Company company = employee.getEmployeeCompany(); //B System.out.println(company.getCompanyName()); //C
A：此处会生成sql语句去查询employee的信息。
select employee0_.employee_id as employee1_1_0_, employee0_.employee_name as employee2_1_0_, employee0_.employee_company_id as employee3_1_0_ from employee employee0_ where employee0_.employee_id=?
B：此处不会马上去取company的信息，因为<many-to-one>里面默认是lazy="proxy"支持懒加载的。
C：此处会根据employee的外键去查找company的信息，用的就是select fetch策略。
select company0_.company_id as company_1_0_0_, company0_.company_name as company_2_0_0_ from company company0_ where company0_.company_id=?
注意：<many-to-one><one-to-many>等关联标签默认是fetch="select"的，所以我们不配置也会采用多条select语句的方式去查询关联对象。
本文出自 “銅鑼衛門” 博客，请务必保留此出处http://jaeger.blog.51cto.com/11064196/1748526
原文地址：http://jaeger.blog.51cto.com/11064196/1748526
