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Hibernate学习之一级缓存

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

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