自定义持久层框架Mybatis-优快云博客

本文链接：https://blog.youkuaiyun.com/birenyin/article/details/124937614

一、前言

想要深入了解mybatis，就得自己自定义持久层框架，现在我们来试一下如何制定？

二、前期准备

jdk11

mysql 8

idea

我们创建两个maven项目

一个是自定义框架，一个是测试自定义框架

三、分析JDBC操作

我们知道Mybatis其实就是jdbc封装后的框架,先将其分为几个部分

package birenyin;

import java.sql.*;

/**
 * @ClassName jdbc
 * @Description TODO
 * @Author Birenyin
 * @Date 2022/5/23 23:09
 * @Version 1.0
 **/
public class jdbc {
	public static void main(String[] args) {
		Connection connection = null;
		PreparedStatement preparedStatement = null;
		ResultSet resultSet = null;
		try {
			// 加载数据库驱动
			Class.forName("com.mysql.jdbc.Driver");
			// 通过驱动管理类获取数据库链接
			connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/mybatis? " +
							"characterEncoding = utf - 8", " root", " root");
							// 定义sql语句？表示占位符
							String sql = "select * from user where username = ?";
			// 获取预处理statement
			preparedStatement = connection.prepareStatement(sql);
			// 设置参数，第⼀个参数为sql语句中参数的序号(从1开始)，第⼆个参数为设置的参数值
			preparedStatement.setString(1, "tom");
			// 向数据库发出sql执⾏查询，查询出结果集
			resultSet = preparedStatement.executeQuery();
			// 遍历查询结果集
			while (resultSet.next()) {
				int id = resultSet.getInt("id");
				String username = resultSet.getString("username");
				// 封装User
				User user = new User();
				user.setId(id);
				user.setUsername(username);
				System.out.println(user);
			}

		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			// 释放资源
			if (resultSet != null) {
				try {
					resultSet.close();
				} catch (SQLException e) {
					e.printStackTrace();
				}
			}
			if (preparedStatement != null) {
				try {
					preparedStatement.close();
				} catch (SQLException e) {
					e.printStackTrace();
				}
			}
			if (connection != null) {
				try {
					connection.close();
				} catch (SQLException e) {
					e.printStackTrace();
				}
			}

		}
	}
}

jdbc问题分析和解决方法：

1、数据库配置信息存在硬编码问题 : 配置文件

2、频繁创建释放数据库连接：连接池（druid)

3、sql语句、设置参数、获取结果集参数均存在硬编码问题：配置文件

4、手动封装返回结果集繁琐：进行反射/内省

四、自定义框架设计

1、测试端

提供核⼼配置⽂件：

sqlMapConfig.xml : 存放数据源信息，引⼊mapper.xml （为了能在同一代码中进行解析）存放

Mapper.xml : sql语句的配置⽂件信息

2、自定义框架端

1、读取配置文件

读取完成以后以流的形式存在，我们不能将读取到的配置信息以流的形式存放在内存中，因为不好操作，可以创建javaBean来存储

（1） Configuration : 存放数据库基本信息、Map<唯⼀标识，Mapper> 唯⼀标识：namespace + “.”+ id

（2） MappedStatement：sql语句、statement类型、输⼊参数java类型、输出参数java类型

2、解析配置⽂件

创建sqlSessionFactoryBuilder类：

⽅法：sqlSessionFactory build()：

第⼀：使⽤dom4j解析配置⽂件，将解析出来的内容封装到Configuration和MappedStatement中第⼆：创建SqlSessionFactory的实现类DefaultSqlSession

3、创建SqlSessionFactory：

⽅法：openSession() : 获取sqlSession接⼝的实现类实例对象

4、创建sqlSession接⼝及实现类：主要封装crud⽅法

⽅法：selectList(String statementId,Object param)：查询所有

selectOne(String statementId,Object param)：查询单个

具体实现：封装JDBC完成对数据库表的查询操作

涉及到的设计模式：

构建者设计模式、⼯⼚模式、代理模式

五、自定义框架实现

测试端:

sqlMapConfig.xml

<configuration>
        <!--数据库配置信息-->
    <dataSource>
        <property name="driverClass" value="com.mysql.jdbc.Driver"></property>
        <property name="jdbcUrl" value="jdbc:mysql://localhost:3306/test"></property>
        <property name="username" value="root"></property>
        <property name="password" value="new_password"></property>
    </dataSource>

    <!--存放mapper.xml的全路径-->
    <mapper resource="UserMapper.xml"></mapper>

</configuration>

UserMapper.xml

<mapper namespace="com.birenyin.dao.IUserDao">

    <!--sql的唯一标识：namespace.id来组成 ： statementId-->
    <select id="findAll" resultType="com.birenyin.pojo.User" >
        select * from user
    </select>

    <!--
        User user = new User()
        user.setId(1);
        user.setUsername("zhangsan")
    -->
    <select id="findByCondition" resultType="com.birenyin.pojo.User" paramterType="com.birenyin.pojo.User">
        select * from user where id = #{id} and username = #{username}
    </select>


</mapper>

IUserDao

package com.birenyin.dao;

import com.birenyin.pojo.User;

import java.util.List;

public interface IUserDao {

    //查询所有用户
    public List<User> findAll() throws Exception;


    //根据条件进行用户查询
    public User findByCondition(User user) throws Exception;


}

User

package com.birenyin.pojo;

public class User {

    private Integer id;
    private String username;


    public Integer getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

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

test

package com.birenyin.test;

import com.birenyin.dao.IUserDao;
import com.birenyin.io.Resources;
import com.birenyin.pojo.User;
import com.birenyin.sqlSession.SqlSession;
import com.birenyin.sqlSession.SqlSessionFactory;
import com.birenyin.sqlSession.SqlSessionFactoryBuilder;
import org.junit.Test;

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

public class Test {

    @Test
    public void test() throws Exception {
        InputStream resourceAsSteam = Resources.getResourceAsSteam("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsSteam);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        //调用
        User user = new User();
        user.setId(1);
        user.setUsername("张三");
      /*  User user2 = sqlSession.selectOne("user.selectOne", user);

        System.out.println(user2);*/

       /* List<User> users = sqlSession.selectList("user.selectList");
        for (User user1 : users) {
            System.out.println(user1);
        }*/

        IUserDao userDao = sqlSession.getMapper(IUserDao.class);

        List<User> all = userDao.findAll();
        for (User user1 : all) {
            System.out.println(user1);
        }


    }



}

自定义框架端

pom.xml

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.encoding>UTF-8</maven.compiler.encoding>
    <java.version>1.8</java.version>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
</properties>


<dependencies>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.17</version>
    </dependency>
    <dependency>
        <groupId>c3p0</groupId>
        <artifactId>c3p0</artifactId>
        <version>0.9.1.2</version>
    </dependency>
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.12</version>
    </dependency>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.10</version>
    </dependency>
    <dependency>
        <groupId>dom4j</groupId>
        <artifactId>dom4j</artifactId>
        <version>1.6.1</version>
    </dependency>
    <dependency>
        <groupId>jaxen</groupId>
        <artifactId>jaxen</artifactId>
        <version>1.1.6</version>
    </dependency>
</dependencies>

BoundSql

package com.birenyin.config;

import com.birenyin.utils.ParameterMapping;

import java.util.ArrayList;
import java.util.List;

public class BoundSql {

    private String sqlText; //解析过后的sql

    private List<ParameterMapping> parameterMappingList = new ArrayList<>();

    public BoundSql(String sqlText, List<ParameterMapping> parameterMappingList) {
        this.sqlText = sqlText;
        this.parameterMappingList = parameterMappingList;
    }

    public String getSqlText() {
        return sqlText;
    }

    public void setSqlText(String sqlText) {
        this.sqlText = sqlText;
    }

    public List<ParameterMapping> getParameterMappingList() {
        return parameterMappingList;
    }

    public void setParameterMappingList(List<ParameterMapping> parameterMappingList) {
        this.parameterMappingList = parameterMappingList;
    }
}

XMLConfigBuilder

package com.birenyin.config;

import com.birenyin.io.Resources;
import com.birenyin.pojo.Configuration;
import com.mchange.v2.c3p0.ComboPooledDataSource;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

import java.beans.PropertyVetoException;
import java.io.InputStream;
import java.util.List;
import java.util.Properties;

public class XMLConfigBuilder {

    private Configuration configuration;

    public XMLConfigBuilder() {
        this.configuration = new Configuration();
    }

    /**
     * 该方法就是使用dom4j对配置文件进行解析，封装Configuration
     */
    public Configuration parseConfig(InputStream inputStream) throws DocumentException, PropertyVetoException {

        Document document = new SAXReader().read(inputStream);
        //<configuration>
        Element rootElement = document.getRootElement();
        List<Element> list = rootElement.selectNodes("//property");
        Properties properties = new Properties();
        for (Element element : list) {
            String name = element.attributeValue("name");
            String value = element.attributeValue("value");
            properties.setProperty(name,value);
        }

        ComboPooledDataSource comboPooledDataSource = new ComboPooledDataSource();
        comboPooledDataSource.setDriverClass(properties.getProperty("driverClass"));
        comboPooledDataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));
        comboPooledDataSource.setUser(properties.getProperty("username"));
        comboPooledDataSource.setPassword(properties.getProperty("password"));

        configuration.setDataSource(comboPooledDataSource);

        //mapper.xml解析: 拿到路径--字节输入流---dom4j进行解析
        List<Element> mapperList = rootElement.selectNodes("//mapper");

        for (Element element : mapperList) {
            String mapperPath = element.attributeValue("resource");
            InputStream resourceAsSteam = Resources.getResourceAsSteam(mapperPath);
            XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(configuration);
            xmlMapperBuilder.parse(resourceAsSteam);

        }


        return configuration;
    }


}

XMLMapperBuilder

package com.birenyin.config;

import com.birenyin.pojo.Configuration;
import com.birenyin.pojo.MappedStatement;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

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

public class XMLMapperBuilder {

    private Configuration configuration;

    public XMLMapperBuilder(Configuration configuration) {
        this.configuration =configuration;
    }

    public void parse(InputStream inputStream) throws DocumentException {

        Document document = new SAXReader().read(inputStream);
        Element rootElement = document.getRootElement();

        String namespace = rootElement.attributeValue("namespace");

        List<Element> list = rootElement.selectNodes("//select");
        for (Element element : list) {
            String id = element.attributeValue("id");
            String resultType = element.attributeValue("resultType");
            String paramterType = element.attributeValue("paramterType");
            String sqlText = element.getTextTrim();
            MappedStatement mappedStatement = new MappedStatement();
            mappedStatement.setId(id);
            mappedStatement.setResultType(resultType);
            mappedStatement.setParamterType(paramterType);
            mappedStatement.setSql(sqlText);
            String key = namespace+"."+id;
            configuration.getMappedStatementMap().put(key,mappedStatement);

        }

    }


}

resources

package com.birenyin.io;

import java.io.InputStream;

public class Resources {

    // 根据配置文件的路径，将配置文件加载成字节输入流，存储在内存中
    public static InputStream getResourceAsSteam(String path){
        InputStream resourceAsStream = Resources.class.getClassLoader().getResourceAsStream(path);
        return  resourceAsStream;

    }



}

Configuration

package com.birenyin.pojo;

import javax.sql.DataSource;
import java.util.HashMap;
import java.util.Map;

public class Configuration {

    private DataSource dataSource;

    /*
    *   key: statementid  value:封装好的mappedStatement对象
     * */
    Map<String,MappedStatement> mappedStatementMap = new HashMap<>();

    public DataSource getDataSource() {
        return dataSource;
    }

    public void setDataSource(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    public Map<String, MappedStatement> getMappedStatementMap() {
        return mappedStatementMap;
    }

    public void setMappedStatementMap(Map<String, MappedStatement> mappedStatementMap) {
        this.mappedStatementMap = mappedStatementMap;
    }
}

mappedStatement

package com.birenyin.pojo;

public class MappedStatement {

    //id标识
    private String id;
    //返回值类型
    private String resultType;
    //参数值类型
    private String paramterType;
    //sql语句
    private String sql;

    public String getId() {
        return id;
    }

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

    public String getResultType() {
        return resultType;
    }

    public void setResultType(String resultType) {
        this.resultType = resultType;
    }

    public String getParamterType() {
        return paramterType;
    }

    public void setParamterType(String paramterType) {
        this.paramterType = paramterType;
    }

    public String getSql() {
        return sql;
    }

    public void setSql(String sql) {
        this.sql = sql;
    }
}

DefaultSqlSession

package com.birenyin.sqlSession;

import com.birenyin.pojo.Configuration;
import com.birenyin.pojo.MappedStatement;

import java.lang.reflect.*;
import java.util.List;

public class DefaultSqlSession implements SqlSession {

    private Configuration configuration;

    public DefaultSqlSession(Configuration configuration) {
        this.configuration = configuration;
    }

    @Override
    public <E> List<E> selectList(String statementid, Object... params) throws Exception {

        //将要去完成对simpleExecutor里的query方法的调用
        simpleExecutor simpleExecutor = new simpleExecutor();
        MappedStatement mappedStatement = configuration.getMappedStatementMap().get(statementid);
        List<Object> list = simpleExecutor.query(configuration, mappedStatement, params);

        return (List<E>) list;
    }

    @Override
    public <T> T selectOne(String statementid, Object... params) throws Exception {
        List<Object> objects = selectList(statementid, params);
        if(objects.size()==1){
            return (T) objects.get(0);
        }else {
            throw new RuntimeException("查询结果为空或者返回结果过多");
        }


    }

    @Override
    public <T> T getMapper(Class<?> mapperClass) {
        // 使用JDK动态代理来为Dao接口生成代理对象，并返回

        Object proxyInstance = Proxy.newProxyInstance(DefaultSqlSession.class.getClassLoader(), new Class[]{mapperClass}, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                // 底层都还是去执行JDBC代码 //根据不同情况，来调用selctList或者selectOne
                // 准备参数 1：statmentid :sql语句的唯一标识：namespace.id= 接口全限定名.方法名
                // 方法名：findAll
                String methodName = method.getName();
                String className = method.getDeclaringClass().getName();

                String statementId = className+"."+methodName;

                // 准备参数2：params:args
                // 获取被调用方法的返回值类型
                Type genericReturnType = method.getGenericReturnType();
                // 判断是否进行了 泛型类型参数化
                if(genericReturnType instanceof ParameterizedType){
                    List<Object> objects = selectList(statementId, args);
                    return objects;
                }

                return selectOne(statementId,args);

            }
        });

        return (T) proxyInstance;
    }


}

DefaultSqlSessionFactory

package com.birenyin.sqlSession;

import com.birenyin.pojo.Configuration;

public class DefaultSqlSessionFactory implements SqlSessionFactory {

    private Configuration configuration;

    public DefaultSqlSessionFactory(Configuration configuration) {
        this.configuration = configuration;
    }


    @Override
    public SqlSession openSession() {
        return new DefaultSqlSession(configuration);
    }
}

Executor

package com.birenyin.sqlSession;

import com.birenyin.pojo.Configuration;
import com.birenyin.pojo.MappedStatement;

import java.util.List;

public interface Executor {

    public <E> List<E> query(Configuration configuration,MappedStatement mappedStatement,Object... params) throws Exception;

}

simpleExecutor

package com.birenyin.sqlSession;


import com.birenyin.config.BoundSql;
import com.birenyin.pojo.Configuration;
import com.birenyin.pojo.MappedStatement;
import com.birenyin.utils.GenericTokenParser;
import com.birenyin.utils.ParameterMapping;
import com.birenyin.utils.ParameterMappingTokenHandler;

import java.beans.PropertyDescriptor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.util.ArrayList;
import java.util.List;

public class simpleExecutor implements  Executor {


    @Override                                                                                //user
    public <E> List<E> query(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception {
        // 1. 注册驱动，获取连接
        Connection connection = configuration.getDataSource().getConnection();

        // 2. 获取sql语句 : select * from user where id = #{id} and username = #{username}
            //转换sql语句： select * from user where id = ? and username = ? ，转换的过程中，还需要对#{}里面的值进行解析存储
        String sql = mappedStatement.getSql();
        BoundSql boundSql = getBoundSql(sql);

        // 3.获取预处理对象：preparedStatement
        PreparedStatement preparedStatement = connection.prepareStatement(boundSql.getSqlText());

        // 4. 设置参数
            //获取到了参数的全路径
         String paramterType = mappedStatement.getParamterType();
         Class<?> paramtertypeClass = getClassType(paramterType);

        List<ParameterMapping> parameterMappingList = boundSql.getParameterMappingList();
        for (int i = 0; i < parameterMappingList.size(); i++) {
            ParameterMapping parameterMapping = parameterMappingList.get(i);
            String content = parameterMapping.getContent();

            //反射
            Field declaredField = paramtertypeClass.getDeclaredField(content);
            //暴力访问
            declaredField.setAccessible(true);
            Object o = declaredField.get(params[0]);

            preparedStatement.setObject(i+1,o);

        }


        // 5. 执行sql
        ResultSet resultSet = preparedStatement.executeQuery();
        String resultType = mappedStatement.getResultType();
        Class<?> resultTypeClass = getClassType(resultType);

        ArrayList<Object> objects = new ArrayList<>();

        // 6. 封装返回结果集
        while (resultSet.next()){
            Object o =resultTypeClass.newInstance();
            //元数据
            ResultSetMetaData metaData = resultSet.getMetaData();
            for (int i = 1; i <= metaData.getColumnCount(); i++) {

                // 字段名
                String columnName = metaData.getColumnName(i);
                // 字段的值
                Object value = resultSet.getObject(columnName);

                //使用反射或者内省，根据数据库表和实体的对应关系，完成封装
                PropertyDescriptor propertyDescriptor = new PropertyDescriptor(columnName, resultTypeClass);
                Method writeMethod = propertyDescriptor.getWriteMethod();
                writeMethod.invoke(o,value);


            }
            objects.add(o);

        }
            return (List<E>) objects;

    }

    private Class<?> getClassType(String paramterType) throws ClassNotFoundException {
        if(paramterType!=null){
            Class<?> aClass = Class.forName(paramterType);
            return aClass;
        }
         return null;

    }


    /**
     * 完成对#{}的解析工作：1.将#{}使用？进行代替，2.解析出#{}里面的值进行存储
     * @param sql
     * @return
     */
    private BoundSql getBoundSql(String sql) {
        //标记处理类：配置标记解析器来完成对占位符的解析处理工作
        ParameterMappingTokenHandler parameterMappingTokenHandler = new ParameterMappingTokenHandler();
        GenericTokenParser genericTokenParser = new GenericTokenParser("#{", "}", parameterMappingTokenHandler);
        //解析出来的sql
        String parseSql = genericTokenParser.parse(sql);
        //#{}里面解析出来的参数名称
        List<ParameterMapping> parameterMappings = parameterMappingTokenHandler.getParameterMappings();

        BoundSql boundSql = new BoundSql(parseSql,parameterMappings);
         return boundSql;

    }


}

SqlSession

package com.birenyin.sqlSession;

import java.util.List;

public interface SqlSession {

    //查询所有
    public <E> List<E> selectList(String statementid,Object... params) throws Exception;

    //根据条件查询单个
    public <T> T selectOne(String statementid,Object... params) throws Exception;


    //为Dao接口生成代理实现类
    public <T> T getMapper(Class<?> mapperClass);


}

SqlSessionFactory

package com.birenyin.sqlSession;

public interface SqlSessionFactory {

    public SqlSession openSession();


}

SqlSessionFactoryBuilder

package com.birenyin.sqlSession;

import com.birenyin.config.XMLConfigBuilder;
import com.birenyin.pojo.Configuration;
import org.dom4j.DocumentException;

import java.beans.PropertyVetoException;
import java.io.InputStream;

public class SqlSessionFactoryBuilder {

    public SqlSessionFactory build(InputStream in) throws DocumentException, PropertyVetoException {
        // 第一：使用dom4j解析配置文件，将解析出来的内容封装到Configuration中
        XMLConfigBuilder xmlConfigBuilder = new XMLConfigBuilder();
        Configuration configuration = xmlConfigBuilder.parseConfig(in);


        // 第二：创建sqlSessionFactory对象：工厂类：生产sqlSession:会话对象
        DefaultSqlSessionFactory defaultSqlSessionFactory = new DefaultSqlSessionFactory(configuration);

        return defaultSqlSessionFactory;
    }




}

GenericTokenParser

package com.birenyin.utils;

public class GenericTokenParser {

  private final String openToken; //开始标记
  private final String closeToken; //结束标记
  private final TokenHandler handler; //标记处理器

  public GenericTokenParser(String openToken, String closeToken, TokenHandler handler) {
    this.openToken = openToken;
    this.closeToken = closeToken;
    this.handler = handler;
  }

  /**
   * 解析${}和#{}
   * @param text
   * @return
   * 该方法主要实现了配置文件、脚本等片段中占位符的解析、处理工作，并返回最终需要的数据。
   * 其中，解析工作由该方法完成，处理工作是由处理器handler的handleToken()方法来实现
   */
  public String parse(String text) {
    // 验证参数问题，如果是null，就返回空字符串。
    if (text == null || text.isEmpty()) {
      return "";
    }

    // 下面继续验证是否包含开始标签，如果不包含，默认不是占位符，直接原样返回即可，否则继续执行。
    int start = text.indexOf(openToken, 0);
    if (start == -1) {
      return text;
    }

   // 把text转成字符数组src，并且定义默认偏移量offset=0、存储最终需要返回字符串的变量builder，
    // text变量中占位符对应的变量名expression。判断start是否大于-1(即text中是否存在openToken)，如果存在就执行下面代码
    char[] src = text.toCharArray();
    int offset = 0;
    final StringBuilder builder = new StringBuilder();
    StringBuilder expression = null;
    while (start > -1) {
     // 判断如果开始标记前如果有转义字符，就不作为openToken进行处理，否则继续处理
      if (start > 0 && src[start - 1] == '\\') {
        builder.append(src, offset, start - offset - 1).append(openToken);
        offset = start + openToken.length();
      } else {
        //重置expression变量，避免空指针或者老数据干扰。
        if (expression == null) {
          expression = new StringBuilder();
        } else {
          expression.setLength(0);
        }
        builder.append(src, offset, start - offset);
        offset = start + openToken.length();
        int end = text.indexOf(closeToken, offset);
        while (end > -1) {存在结束标记时
          if (end > offset && src[end - 1] == '\\') {//如果结束标记前面有转义字符时
            // this close token is escaped. remove the backslash and continue.
            expression.append(src, offset, end - offset - 1).append(closeToken);
            offset = end + closeToken.length();
            end = text.indexOf(closeToken, offset);
          } else {//不存在转义字符，即需要作为参数进行处理
            expression.append(src, offset, end - offset);
            offset = end + closeToken.length();
            break;
          }
        }
        if (end == -1) {
          // close token was not found.
          builder.append(src, start, src.length - start);
          offset = src.length;
        } else {
          //首先根据参数的key（即expression）进行参数处理，返回?作为占位符
          builder.append(handler.handleToken(expression.toString()));
          offset = end + closeToken.length();
        }
      }
      start = text.indexOf(openToken, offset);
    }
    if (offset < src.length) {
      builder.append(src, offset, src.length - offset);
    }
    return builder.toString();
  }
}

ParameterMapping

package com.birenyin.utils;

public class ParameterMapping {

    private String content;

    public ParameterMapping(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }
}

ParameterMappingTokenHandler

package com.birenyin.utils;

import java.util.ArrayList;
import java.util.List;


public class ParameterMappingTokenHandler implements TokenHandler {
   private List<ParameterMapping> parameterMappings = new ArrayList<ParameterMapping>();

   // context是参数名称 #{id} #{username}

   public String handleToken(String content) {
      parameterMappings.add(buildParameterMapping(content));
      return "?";
   }

   private ParameterMapping buildParameterMapping(String content) {
      ParameterMapping parameterMapping = new ParameterMapping(content);
      return parameterMapping;
   }

   public List<ParameterMapping> getParameterMappings() {
      return parameterMappings;
   }

   public void setParameterMappings(List<ParameterMapping> parameterMappings) {
      this.parameterMappings = parameterMappings;
   }

}

TokenHandler

public interface TokenHandler {
  String handleToken(String content);
}

六、Mybatis

MyBatis是⼀款优秀的基于ORM的半⾃动轻量级持久层框架，它⽀持定制化SQL、存储过程以及⾼级映射。MyBatis避免了⼏乎所有的JDBC代码和⼿动设置参数以及获取结果集。MyBatis可以使⽤简单的XML或注解来配置和映射原⽣类型、接⼝和Java的POJO （Plain Old Java Objects,普通⽼式Java对象）为数据库中的记录。

ORM全称Object/Relation Mapping：表示对象-关系映射的缩写
ORM完成⾯向对象的编程语⾔到关系数据库的映射。当ORM框架完成映射后，程序员既可以利⽤⾯向对象程序设计语⾔的简单易⽤性，⼜可以利⽤关系数据库的技术优势。ORM把关系数据库包装成⾯向对象的模型。ORM框架是⾯向对象设计语⾔与关系数据库发展不同步时的中间解决⽅案。采⽤ORM框架后，应⽤程序不再直接访问底层数据库，⽽是以⾯向对象的⽅式来操作持久化对象，⽽ORM框架则将这些⾯向对象的操作转换成底层SQL操作。ORM框架实现的效果：把对持久化对象的保存、修改、删除等操作，转换为对数据库的操作