Hibernate学习之一级缓存

时间:2021-10-13 07:38:31

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Hibernate缓存:

  - 缓存是为了降低应用程序对物理数据源访问的频次,从而提供应用程序的运行性能的一种策略

  - Hibernate缓存是提升和优化Hibernate执行效率的重要手段

工作原理:

  1. 应用程序读取缓存数据

  2. 缓存命中则直接返回数据

  3. 缓存未命中则去数据库中查询数据,然后将数据放入缓存中并返回数据

一级缓存:

  1. Hibernate一级缓存又称为“Session缓存”、“会话级缓存”

  2. 通过Session从数据库查询实体时会把实体从内存中取出来,下一查询统一实体时不再从数据库获取,而是从内存中获取

  3. 一级缓存的声明周期和Session相同;Session销毁,它也销毁

  4. 一级缓存的数据可使用范围在当前会话之内

API:

  1. 一级缓存无法取消,用两个方法管理

    - evict():用于将对象从Session的一级缓存中清除

    - clear():用于将一级缓存中的所有对象清除

  2. 相关方法

    - query.list()

    - query.iterate()

实例:

1.项目结构

Hibernate学习之一级缓存

2.pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>org.struts</groupId>
<artifactId>Hibernate-PrimaryBuffer</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging> <properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<hibernate.version>5.1.7.Final</hibernate.version>
</properties> <dependencies>
<!-- Junit -->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
<!-- Hibernate -->
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>${hibernate.version}</version>
</dependency>
<!-- MySQL -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.42</version>
</dependency>
</dependencies> </project>

3.Student.java

package org.hibernate.model;

import java.util.Date;

/**
* 学生实体类
*
*/
public class Student { private long id;// 学号
private String username;// 姓名
private Date birthday;// 生日
private String sex;// 性别 public Student() {
}
public Student(long id, String username, Date birthday, String sex) {
this.id = id;
this.username = username;
this.birthday = birthday;
this.sex = sex;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
} }

4.Student.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> <class name="org.hibernate.model.Student" table="STUDENT">
<id name="id" type="java.lang.Long">
<column name="ID"/>
<generator class="assigned"/>
</id>
<property name="username" type="java.lang.String">
<column name="USERNAME"/>
</property>
<property name="birthday" type="date">
<column name="BIRTHDAY"/>
</property>
<property name="sex" type="java.lang.String">
<column name="SEX"/>
</property>
</class> </hibernate-mapping>

5.hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration> <!-- 配置SessionFactory -->
<session-factory>
<!-- 配置数据库连接信息 -->
<property name="connection.username">root</property>
<property name="connection.password">***</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">
jdbc:mysql:///hibernate?useSSL=true&amp;characterEncoding=UTF-8
</property>
<!-- 常用设置 -->
<property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property><!-- 方言 -->
<property name="show_sql">true</property><!-- 是否展示SQL -->
<property name="format_sql">false</property><!-- 格式化SQL -->
<property name="hbm2ddl.auto">update</property><!-- 自动创建表结构 -->
<!-- 引入映射文件 -->
<mapping resource="hbm/Student.hbm.xml"/>
</session-factory> </hibernate-configuration>

6.TestPrimartBuffer.java

package org.hibernate.test;

import java.util.Date;
import java.util.Iterator;
import java.util.List; import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.model.Student;
import org.junit.After;
import org.junit.Before;
import org.junit.Test; public class TestPrimaryBuffer { private SessionFactory sessionFactory;
private Session session;
private Transaction transaction; @Before
public void before() { sessionFactory = new Configuration().configure().buildSessionFactory();// 创建会话工厂对象
session = sessionFactory.openSession();// 创建会话
transaction = session.beginTransaction();// 开始事务 } @After
public void after() { transaction.commit();// 提交事务
session.close();// 关闭会话
sessionFactory.close();// 关闭会话工厂 } /**
* 初始化数据库
*/
@Test
public void init() { Student student = new Student(1, "张三", new Date(), "男");
session.save(student);
student = new Student(2, "李四", new Date(), "男");
session.save(student);
student = new Student(3, "王五", new Date(), "男");
session.save(student); } /**
* 同一个session
*/
@Test
public void testSameSession() { Student student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); } /**
* 不同session
*/
@Test
public void testDiffSession() { Student student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); session = sessionFactory.openSession();
student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); } /**
* Evict,同一个session
*/
@Test
public void testEvict() { Student student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); session.evict(student); student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); } /**
* Clear,同一个session
*/
@Test
public void testClear() { Student student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); session.clear(); student = session.get(Student.class, 1L);
System.out.println(student.getUsername()); } /**
* Query
*/
@Test
@SuppressWarnings("unchecked")
public void testQuery() { Query query = session.createQuery("from Student");
List<Student> list = query.list();
for (Student s: list) {
System.out.println(s.getUsername());
} System.out.println(); list = query.list();
for (Student s: list) {
System.out.println(s.getUsername());
} } /**
* Interate
*/
@Test
@SuppressWarnings("unchecked")
public void testIterate() { Query query = session.createQuery("from Student");
List<Student> list = query.list();
for (Student s: list) {
System.out.println(s.getUsername());
} System.out.println(); Iterator<?> iterator = query.iterate();
while (iterator.hasNext()) {
Student s = (Student) iterator.next();
System.out.println(s.getUsername());
} } /**
* Interate2
*/
@Test
public void testIterate2() { Query query = session.createQuery("from Student");
Iterator<?> iterator = query.iterate();
while (iterator.hasNext()) {
Student s = (Student) iterator.next();
System.out.println(s.getUsername());
} } }

7.效果预览(首先执行init()方法进行数据库表的初始化)

  7.1 执行testSameSession()方法

Hibernate学习之一级缓存

    说明:同一个Session中查询同一对象使用缓存,执行一次查询操作

  7.2 执行testDiffSession()方法

Hibernate学习之一级缓存

    说明:不同Session中查询同一对象没有使用缓存,执行多次查询操作

  7.3 执行testEvict()方法

Hibernate学习之一级缓存

    说明:同一Session中,由于使用evict()方法从缓存中清除了Student对象,因此也需查询多次

  7.4 执行testClear()方法

Hibernate学习之一级缓存

    说明:同一Session中,由于使用clear()方法清除了缓存中的所有对象,因此也需查询多次

  7.5 执行testQuery()方法

Hibernate学习之一级缓存

    说明:在同一Session中查询同一对象,依然执行了两次SQL,说明query.list()方法不使用缓存

  7.6 执行testIterate()方法

Hibernate学习之一级缓存

  7.7 执行testIterate2()方法

Hibernate学习之一级缓存

    说明:从7.6和7.7的结果可以得出,query.iterate()方法使用缓存,但是iterate()方法先从数据中查询所有的id,然后通过id去缓存中查找;若查找失败,则根据id去数据库查询详细信息

8.总结:

  1. 同一个session对象,多次查询同一个对象,因为缓存只会执行一次查询操作,查询效率较快

  2. 不同Session对象,多次查询同一对象,不存在缓存会多次执行查询操作,查询效率较慢

  3. query.list()虽然在同一个Session中,但是不会使用缓存

  4. query.iterate()在同一个Session中会使用缓存。但是iterate()会先从数据库中查询所有的id,然后通过id去缓存中查询对应对象;

    若缓存中不存在该id对应的对象,则会根据id去数据库中查询

参考:http://www.imooc.com/video/9016