1+N就是在hibernate中第一次查一个所需要的表的内容，他会把别的相关表的内容也查询一遍。
解决办法有三种：
1，设置LAZY。
2，借鉴createCriteria的查询语句，from Topic t left join fetch t.category c，通过join fetch来屏蔽多于查询。
3，将多次查询整成一次查询。给多于表加上BatchSize注解。这种方法并没有解决问题，只是相对来说优化了一点。
解决办法3的写法：
package com.bjsxt.hibernate;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import org.hibernate.annotations.BatchSize;
@Entity
//@BatchSize(size=5)//加上BachtSize注解，如此，一次会查询五条相关内容
public class Category {
private int id;
private String name;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
测试类：
import java.util.Date;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;
public class HibernateQLTest {
private static SessionFactory sf;
@BeforeClass
public static void beforeClass() {
sf = new AnnotationConfiguration().configure().buildSessionFactory();
}
@AfterClass
public static void afterClass() {
sf.close();
}
@Test
public void testSchemaExport() {
new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
}
@Test
public void testSave() {
Session session = sf.openSession();
session.beginTransaction();
for(int i=0; i<10; i++) {
Category c = new Category();
c.setName("c" + i);
Topic t = new Topic();
t.setCategory(c);
t.setTitle("t" + i);
t.setCreateDate(new Date());
session.save(c);
session.save(t);
}
session.getTransaction().commit();
session.close();
}
//N+1
@Test
public void testQuery1() {
Session session = sf.openSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
List<Topic> topics = (List<Topic>)session.createQuery("from Topic").list();
for(Topic t : topics) {
System.out.println(t.getId() + "-" + t.getTitle());
}
session.getTransaction().commit();
session.close();
}
@Test
public void testQuery2() {
Session session = sf.openSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();//关联关系不设lazy也不会全部查询，因为使用的是关联表方式
List<Topic> topics = (List<Topic>)session.createQuery("from Topic").list();//关联关系不设lazy会全部查询
for(Topic t : topics) {
System.out.println(t.getId() + "-" + t.getTitle());
System.out.println(t.getCategory().getName());
}
session.getTransaction().commit();
session.close();
}
//@BatchSize
@Test
public void testQuery3() {
Session session = sf.openSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
List<Topic> topics = (List<Topic>)session.createQuery("from Topic").list();
for(Topic t : topics) {
System.out.println(t.getId() + "-" + t.getTitle());
System.out.println(t.getCategory().getName());
}
session.getTransaction().commit();
session.close();
}
//join fetch
@Test
public void testQuery4() {
Session session = sf.openSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
List<Topic> topics = (List<Topic>)session.createQuery("from Topic t left join fetch t.category c").list();
for(Topic t : topics) {
System.out.println(t.getId() + "-" + t.getTitle());
System.out.println(t.getCategory().getName());
}
session.getTransaction().commit();
session.close();
}
public static void main(String[] args) {
beforeClass();
}
}
