Mybatis（三）

1 Mybaits--动态SQL

• 动态SQL是Mybatis强大特性之一。极大的简化我们拼装SQL的操作。
• 动态SQL元素和使用JSTL或其他类似基于XML的文本处理器相似。
• Mybatis采用功能强大的基于OGNL的表达式来简化操作。
  • if
  • choose(when ,otherwise)　
  • trim(where,set)
  • foreach

1.1 动态SQL之if

package cn.demo2;

/**
 * 描述：POJO
 */
public class Employee {
    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    private Department department;

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;

    }

    @Override
    public String toString() {
        return "Employee{" +
                "email='" + email + '\'' +
                ", gender='" + gender + '\'' +
                ", lastName='" + lastName + '\'' +
                ", id=" + id +
                '}';
    }
}

package cn.demo2;

import java.util.List;

public interface IEmployeeDAO {

    public List<Employee> getConditionIf(Employee employee);

}

<?xml version="1.0" encoding="UTF-8" ?>
        <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.demo2.IEmployeeDAO">

    <select id="getConditionIf" resultType="cn.demo2.Employee" >
      SELECT id,last_name lastName,email,gender from employee
      WHERE
      <if test="id != null">
        id = #{id}
      </if>
      <if test="lastName != null and lastName != ''">
          and last_name like #{lastName}
      </if>
      <if test="email != null and email.trim() != ''">
          and email = #{email}
      </if>
       <if test="gender == '男' or gender == '女'">
          and gender = #{gender}
       </if>
    </select>

</mapper>

package cn.test;

import cn.demo2.Employee;
import cn.demo2.IEmployeeDAO;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MybatisTest2 {

    /**
     * 测试查询
     * @throws IOException
     */
    @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);
        Employee employee = new Employee();
        employee.setId(5);
        List<Employee> employees = dao.getConditionIf(employee);
        System.out.print(employees);

        session.close();
    }
}

1.2 动态SQL之where

• 如果上面的程序中的id没有传入，那么拼接的SQL会多出and。
  • 解决方案一：在where之后加上1=1。
  • 解决方案二：Mybatis推荐使用where标签来将所有的查询条件包括在内，Mybatis就会将where标签中多出的and或or删除自动删除。并且where标签只会去掉第一个多出的and或or等。

package cn.demo2;

/**
 * 描述：POJO
 */
public class Employee {
    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    private Department department;

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;

    }

    @Override
    public String toString() {
        return "Employee{" +
                "email='" + email + '\'' +
                ", gender='" + gender + '\'' +
                ", lastName='" + lastName + '\'' +
                ", id=" + id +
                '}';
    }
}

<?xml version="1.0" encoding="UTF-8" ?>
        <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.demo2.IEmployeeDAO">

    <select id="getConditionIf" resultType="cn.demo2.Employee" >
      SELECT id,last_name lastName,email,gender from employee
      <where>

          <if test="id != null">
              id = #{id}
          </if>
          <if test="lastName != null and lastName != ''">
              and last_name like #{lastName}
          </if>
          <if test="email != null and email.trim() != ''">
              and email = #{email}
          </if>
          <if test="gender == '男' or gender == '女'">
              and gender = #{gender}
          </if>

      </where>

    </select>

</mapper>

package cn.test;

import cn.demo2.Employee;
import cn.demo2.IEmployeeDAO;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MybatisTest2 {

    /**
     * 测试查询
     * @throws IOException
     */
    @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);
        Employee employee = new Employee();
        employee.setLastName("%呵%");
        List<Employee> employees = dao.getConditionIf(employee);
        System.out.print(employees);

        session.close();
    }
}

1.2 动态SQL之trim

package cn.demo2;

/**
 * 描述：POJO
 */
public class Employee {
    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    private Department department;

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;

    }

    @Override
    public String toString() {
        return "Employee{" +
                "email='" + email + '\'' +
                ", gender='" + gender + '\'' +
                ", lastName='" + lastName + '\'' +
                ", id=" + id +
                '}';
    }
}

package cn.demo2;

import java.util.List;

public interface IEmployeeDAO {

    public List<Employee> getConditionIf(Employee employee);

}

<?xml version="1.0" encoding="UTF-8" ?>
        <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.demo2.IEmployeeDAO">

    <select id="getConditionIf" resultType="cn.demo2.Employee" >
      SELECT id,last_name lastName,email,gender from employee
      <!--
        where标签职能消除前面多余的and或者or之类的语句，而如果and或or在之后，那么where就无能为力了
        trim可以来解决这样的问题
            prefix表示在拼接的SQL之前加前缀
            prefixOverrides是将前面多余的and或or等消除
            suffix和prefix相反
            prefixOverrides和prefixOverrides相反
      -->
      <trim prefix="where" suffixOverrides="and">

          <if test="id != null">
              id = #{id} and
          </if>
          <if test="lastName != null and lastName != ''">
            last_name like #{lastName} and
          </if>
          <if test="email != null and email.trim() != ''">
             email = #{email} and
          </if>
          <if test="gender == '男' or gender == '女'">
              and gender = #{gender}
          </if>

      </trim>

    </select>

</mapper>

package cn.test;

import cn.demo2.Employee;
import cn.demo2.IEmployeeDAO;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MybatisTest2 {

    /**
     * 测试查询
     * @throws IOException
     */
    @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);
        Employee employee = new Employee();
        employee.setLastName("%呵%");
        List<Employee> employees = dao.getConditionIf(employee);
        System.out.print(employees);

        session.close();
    }
}

1.3 动态SQL之分支查询（和Java中的switch--case相似）

package cn.demo2;

/**
 * 描述：POJO
 */
public class Employee {
    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    private Department department;

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;

    }

    @Override
    public String toString() {
        return "Employee{" +
                "email='" + email + '\'' +
                ", gender='" + gender + '\'' +
                ", lastName='" + lastName + '\'' +
                ", id=" + id +
                '}';
    }
}

package cn.demo2;

import java.util.List;

public interface IEmployeeDAO {

    public List<Employee> getConditionIf(Employee employee);

}

<?xml version="1.0" encoding="UTF-8" ?>
        <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.demo2.IEmployeeDAO">

    <select id="getConditionIf" resultType="cn.demo2.Employee" >
      SELECT id,last_name lastName,email,gender from employee
      <where>
          <choose>
              <when test="id != null">
                  id = #{id}
              </when>
              <when test="lastName != null">
                  last_name like  #{lastName}
              </when>
              <when test="email != null">
                  email = #{email}
              </when>
              <otherwise>
                  gender = #{gender}
              </otherwise>
          </choose>
      </where>
    </select>

</mapper>

package cn.test;

import cn.demo2.Employee;
import cn.demo2.IEmployeeDAO;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MybatisTest2 {

    /**
     * 测试查询
     * @throws IOException
     */
    @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);
        Employee employee = new Employee();
        employee.setLastName("%呵%");
        List<Employee> employees = dao.getConditionIf(employee);
        System.out.print(employees);

        session.close();
    }
}

1.4 动态SQL之更新

 public void updateEmployee(Employee employee);

    <update id="updateEmployee" >
        update employee
        <set>
            <if test="email != null">
                email = #{email},
            </if>
            <if test="lastName != null">
                last_name = #{lastName}
            </if>
            <if test="gender != null">
                gender = #{gender}
            </if>
        </set>
        where id = #{id}
    </update>

 @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);
        Employee employee = new Employee();
        employee.setId(5);
        employee.setLastName("哈哈");
        dao.updateEmployee(employee);
        session.commit();
        session.close();
    }

1.5 动态SQL--forEach

public List<Employee> getEmpsByConditionForEach(List<Integer> ids);

<select id="getEmpsByConditionForEach" resultType="cn.demo2.Employee" >
        select * from employee where id IN
        <foreach collection="list" item="item_id" separator="," open="(" close=")" >
            #{item_id}
        </foreach>
    </select>

 /**
     * 测试查询
     * @throws IOException
     */
    @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);
        dao.getEmpsByConditionForEach(Arrays.asList(5,6,7));
        session.close();
    }

1.6 动态SQL--MySQL批量插入

• MySQL支持insert into 表名 values(),(),()。

package cn.demo2;

/**
 * 描述：POJO
 */
public class Employee {
    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    private Department department;

    public Employee(Integer id, Department department, String lastName, String gender, String email) {
        this.id = id;
        this.department = department;
        this.lastName = lastName;
        this.gender = gender;
        this.email = email;
    }

    public Employee() {
    }

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;

    }

    @Override
    public String toString() {
        return "Employee{" +
                "email='" + email + '\'' +
                ", gender='" + gender + '\'' +
                ", lastName='" + lastName + '\'' +
                ", id=" + id +
                '}';
    }
}

package cn.demo2;

import cn.demo1.Employee;

import java.util.List;

public class Department {
    private Integer id;
    private String departmentName;
    private List<cn.demo1.Employee> emps;

    public Department(Integer id) {
        this.id = id;
    }

    public Department() {
    }

    public List<cn.demo1.Employee> getEmps() {
        return emps;
    }

    public void setEmps(List<Employee> emps) {
        this.emps = emps;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getDepartmentName() {
        return departmentName;
    }

    public void setDepartmentName(String departmentName) {
        this.departmentName = departmentName;
    }

    @Override
    public String toString() {
        return "Department{" +
                "id=" + id +
                ", departmentName='" + departmentName + '\'' +
                '}';
    }
}

package cn.demo2;

import org.apache.ibatis.annotations.Param;

import java.util.List;

public interface IEmployeeDAO {

    public void insertBatch(@Param("emps")List<Employee> emps);

}

<?xml version="1.0" encoding="UTF-8" ?>
        <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.demo2.IEmployeeDAO">

    <insert id="insertBatch">
        insert into employee (last_name,gender,email,dept_id)
        values
        <foreach collection="emps" item="emp" separator="," >
            (#{emp.lastName},#{emp.gender},#{emp.email},#{emp.department.id})
        </foreach>
    </insert>

</mapper>

package cn.test;

import cn.demo2.Department;
import cn.demo2.Employee;
import cn.demo2.IEmployeeDAO;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class MybatisTest2 {

    /**
     * 测试查询
     * @throws IOException
     */
    @Test
    public  void demo1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        IEmployeeDAO dao = session.getMapper(IEmployeeDAO.class);

        List<Employee> emps = new ArrayList<>();
        emps.add(new Employee(null,new Department(1),"好人","女","[e-mail]"));
        emps.add(new Employee(null,new Department(2),"坏人","男","[e-mail]"));

        dao.insertBatch(emps);

        session.commit();

        session.close();
    }
}

1.7 动态SQL插入--Oracle下的批量插入

• Oracle不支持insert into 表名 values(),(),()。
• Oracle支持的方式：多个insert语句放在begin和end之间。

 <insert id="insertBatch">
       <foreach collection="emps" item="emp" open="begin" close="end/">
           insert into employee(id,last_name,email)
           values(employee_seq.nextval,#{emp.lastName},#{emp.email})
       </foreach>
    </insert>

2 Mybatis的缓存机制 

• Mybatis包含一个非常强大的查询缓存特性，它可以非常方便的配置和定制。缓存可以极大的提升查询效率。
• Mybatis系统中默认定义了两级缓存。
  • 一级缓存和二级缓存。
  • 默认情况下，只有一级缓存开启。
  • 二级缓存需要手动开启和配置，它是基于namespace基本的缓存。
  • 为了提高扩展性。Mybatis定义了缓存接口Cache。我们可以通过实现Cache接口来自定义二级缓存。　　

　　

2.1 一级缓存

• 一级缓存，即本地缓存，作用域默认为sqlSession。当session flush或close后，该session中的所有Cache将被清空。
• 本地缓存不能被关闭，但可以调用clearCache()清空本地缓存。　　

2.2 一级缓存失效情况（没有用到当前一级缓存的情况，效果就是，还需要向数据库发出SQL语句）

• 不同的sqlSession对应不同的一级缓存。
• 同一个sqlSession但是查询条件不同。
• 同一个sqlSession两次查询期间执行了任何一次增删改操作。
• 同一个sqlSession两次查询期间手动清空了缓存。

2.3 二级缓存（基于namespace级别的缓存，一个namespace对应一个二级缓存）

• 二级缓存，全局作用域缓存。
• 二级缓存默认不开启，需要手动配置。
• Mybatis提供二级缓存的接口以及实现，缓存实现要求POJO实现Serializable接口。
• 二级缓存在一级缓存关闭或提交之后才会生效。

2.4 二级缓存使用步骤

• 全局配置文件中开启二级缓存

<settings>
       <setting name="cacheEnabled" value="true"/>
    </settings>

• 去映射文件中配置使用二级缓存

<cache/>

• POJO需要实现Serializable接口

2.5 二级缓存相关属性

<!--
          eviction=“ FIFO” ：缓存回收策略：
            • LRU – 最近最少使用的：移除最长时间不被使用的对象。
            • FIFO – 先进先出：按对象进入缓存的顺序来移除它们。
            • SOFT – 软引用：移除基于垃圾回收器状态和软引用规则的对象。
            • WEAK – 弱引用：更积极地移除基于垃圾收集器状态和弱引用规则的对象。
            • 默认的是 LRU。

          flushInterval ：刷新间隔，单位毫秒
            默认情况是不设置，也就是没有刷新间隔，缓存仅仅调用语句时刷新

          size ：引用数目，正整数
            代表缓存最多可以存储多少个对象，太大容易导致内存溢出

        readOnly ：只读，true/false
            true：只读缓存；会给所有调用者返回缓存对象的相同实例。因此这些对象不能被修改。这提供了很重要的性能优势。
            false：读写缓存；会返回缓存对象的拷贝（通过序列化）。这会慢一些，但是安全，因此默认是 false。
    -->

2.6 和缓存有关的设置/属性　　

• CacheEnalbed
  • true；开启二级缓存。
  • false:关闭二级缓存。
• 每个select标签默认有UseCache属性，而且属性值默认为true。
  • true；表示这个select使用二级缓存。
  • false：表示这个select不使用二级缓存。　　
• sql标签的flushCache属性：
  • 增删改标签默认flushCache=true。SQL执行以后，会同时清空一级缓存和二级缓存。查询默认flushCache=false。
• sqlSession.clearCache()：
  • 只是用来清除一级缓存。
• 当在某个作用域（一级缓存和二级缓存）进行了增删改操作，默认该作用域下所有select中的缓存将被clear。　　　　　　

2.7 缓存的原理

2.8 第三方缓存整合

• EhCache是一个纯Java的进程内的缓存框架，具有快速、精干等特点，是Hibernate中默认的CacheProvider。
• Mybatis定义了Cache接口方便我们进行自定义扩展。

• 步骤：
  • 导入ehcache包，以及整合包。日志包。
    • ehcache-core-2.6.8.jar
    • mybatis-ehcache-1.0.3.jar
    • slf4j-api-1.6.1.jar
    • slf4j-log4j12-1.6.2.jar
  • 编写ehcache.xml配置文件

<?xml version="1.0" encoding="UTF-8"?>
<ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:noNamespaceSchemaLocation="../config/ehcache.xsd">
 <!-- 磁盘保存路径 -->
 <diskStore path="D:\ehcache" />

 <defaultCache
   maxElementsInMemory="1"
   maxElementsOnDisk="10000000"
   eternal="false"
   overflowToDisk="true"
   timeToIdleSeconds="120"
   timeToLiveSeconds="120"
   diskExpiryThreadIntervalSeconds="120"
   memoryStoreEvictionPolicy="LRU">
 </defaultCache>
</ehcache>

<!--
属性说明：
l diskStore：指定数据在磁盘中的存储位置。
l defaultCache：当借助CacheManager.add("demoCache")创建Cache时，EhCache便会采用<defalutCache/>指定的的管理策略

以下属性是必须的：
l maxElementsInMemory - 在内存中缓存的element的最大数目
l maxElementsOnDisk - 在磁盘上缓存的element的最大数目，若是0表示无穷大
l eternal - 设定缓存的elements是否永远不过期。如果为true，则缓存的数据始终有效，如果为false那么还要根据timeToIdleSeconds，timeToLiveSeconds判断
l overflowToDisk - 设定当内存缓存溢出的时候是否将过期的element缓存到磁盘上

以下属性是可选的：
l timeToIdleSeconds - 当缓存在EhCache中的数据前后两次访问的时间超过timeToIdleSeconds的属性取值时，这些数据便会删除，默认值是0,也就是可闲置时间无穷大
l timeToLiveSeconds - 缓存element的有效生命期，默认是0.,也就是element存活时间无穷大
 diskSpoolBufferSizeMB 这个参数设置DiskStore(磁盘缓存)的缓存区大小.默认是30MB.每个Cache都应该有自己的一个缓冲区.
l diskPersistent - 在VM重启的时候是否启用磁盘保存EhCache中的数据，默认是false。
l diskExpiryThreadIntervalSeconds - 磁盘缓存的清理线程运行间隔，默认是120秒。每个120s，相应的线程会进行一次EhCache中数据的清理工作
l memoryStoreEvictionPolicy - 当内存缓存达到最大，有新的element加入的时候， 移除缓存中element的策略。默认是LRU（最近最少使用），可选的有LFU（最不常使用）和FIFO（先进先出）
 -->

• • 配置cache标签

<cache type= "org.mybatis.caches.ehcache.EhcacheCache"></cache>

• 参照缓存：如果想在命名空间*享相同的缓存配置和实力，可以使用cache-ref元素来引用另外一个缓存。