Hibernate教程-Hibernate 懒加载集合
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懒加载集合在需要时加载子对象,这用于提高性能。从 Hibernate 3.0 开始,懒加载集合默认是启用的。
要使用懒加载集合,可以在集合中可选地使用 lazy="true"
属性。它默认是 true
,所以通常不需要特别设置。如果将其设置为 false
,所有的子对象将在初始加载,这在处理大数据时会降低性能。
让我们看看使用 lazy="true"
属性的 Hibernate 映射文件。
<list name="answers" lazy="true">
<key column="qid"/>
<index column="type"/>
<one-to-many class="cn.javatiku.Answer"/>
</list>
示例:使用懒加载的持久类
Question.java
package cn.javatiku;
import javax.persistence.*;
import java.util.List;
@Entity
@Table(name="question")
public class Question {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id;
private String qname;
@OneToMany(mappedBy="question", cascade=CascadeType.ALL)
@LazyCollection(LazyCollectionOption.TRUE)
private List<Answer> answers;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getQname() {
return qname;
}
public void setQname(String qname) {
this.qname = qname;
}
public List<Answer> getAnswers() {
return answers;
}
public void setAnswers(List<Answer> answers) {
this.answers = answers;
}
}
Answer.java
package cn.javatiku;
import javax.persistence.*;
@Entity
@Table(name="answer")
public class Answer {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id;
private String answername;
private String postedBy;
@ManyToOne
@JoinColumn(name="qid")
private Question question;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getAnswername() {
return answername;
}
public void setAnswername(String answername) {
this.answername = answername;
}
public String getPostedBy() {
return postedBy;
}
public void setPostedBy(String postedBy) {
this.postedBy = postedBy;
}
public Question getQuestion() {
return question;
}
public void setQuestion(Question question) {
this.question = question;
}
}
配置文件
hibernate.cfg.xml
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 5.3//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-5.3.dtd">
<hibernate-configuration>
<session-factory>
<property name="hbm2ddl.auto">update</property>
<property name="dialect">org.hibernate.dialect.Oracle9Dialect</property>
<property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>
<property name="connection.username">system</property>
<property name="connection.password">jtp</property>
<property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
<mapping class="cn.javatiku.Question"/>
<mapping class="cn.javatiku.Answer"/>
</session-factory>
</hibernate-configuration>
存储和获取数据的用户类
Store.java
package cn.javatiku;
import org.hibernate.*;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import java.util.ArrayList;
import java.util.List;
public class Store {
public static void main(String[] args) {
StandardServiceRegistry ssr = new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
Metadata meta = new MetadataSources(ssr).getMetadataBuilder().build();
SessionFactory factory = meta.getSessionFactoryBuilder().build();
Session session = factory.openSession();
Transaction t = session.beginTransaction();
Answer ans1 = new Answer();
ans1.setAnswername("Java is a programming language");
ans1.setPostedBy("Ravi Malik");
Answer ans2 = new Answer();
ans2.setAnswername("Java is a platform");
ans2.setPostedBy("Sudhir Kumar");
List<Answer> list = new ArrayList<Answer>();
list.add(ans1);
list.add(ans2);
Question q1 = new Question();
q1.setQname("What is Java?");
q1.setAnswers(list);
ans1.setQuestion(q1);
ans2.setQuestion(q1);
session.persist(q1);
t.commit();
session.close();
System.out.println("success");
}
}
Fetch.java
package cn.javatiku;
import java.util.Iterator;
import java.util.List;
import javax.persistence.TypedQuery;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
public class Fetch {
public static void main(String[] args) {
StandardServiceRegistry ssr = new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
Metadata meta = new MetadataSources(ssr).getMetadataBuilder().build();
SessionFactory factory = meta.getSessionFactoryBuilder().build();
Session session = factory.openSession();
TypedQuery<Question> query = session.createQuery("from Question q");
List<Question> list = query.getResultList();
Iterator<Question> itr = list.iterator();
while (itr.hasNext()) {
Question q = itr.next();
System.out.println("Question: " + q.getQname());
List<Answer> answers = q.getAnswers();
for (Answer a : answers) {
System.out.println(a.getAnswername() + " - " + a.getPostedBy());
}
}
session.close();
System.out.println("success");
}
}
输出
运行 Store.java
时输出:
success
运行 Fetch.java
时输出:
Question: What is Java?
Java is a programming language - Ravi Malik
Java is a platform - Sudhir Kumar
success