[Java知识库] 自定义持久层框架Mybatis

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[Java知识库]自定义持久层框架Mybatis

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?个参数为sql语句中参数的序号(从1开始)，第?个参数为设置的参数值
			preparedStatement.setString(1, "tom");
			// 向数据库发出sql执?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?配置?件：

sqlMapConfig.xml : 存放数据源信息，引?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?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?标识，Mapper> 唯?标识：namespace + “.”+ id

（2） MappedStatement：sql语句、statement类型、输?参数java类型、输出参数java类型

2、解析配置?件

创建sqlSessionFactoryBuilder类：

?法：sqlSessionFactory build()：

第?：使?dom4j解析配置?件，将解析出来的内容封装到Configuration和MappedStatement中第?：创建SqlSessionFactory的实现类DefaultSqlSession

3、创建SqlSessionFactory：

?法：openSession() : 获取sqlSession接?的实现类实例对象

4、创建sqlSession接?及实现类：主要封装crud?法

?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、??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?款优秀的基于ORM的半?动轻量级持久层框架，它?持定制化SQL、存储过程以及?级映射。MyBatis避免了?乎所有的JDBC代码和?动设置参数以及获取结果集。MyBatis可以使?简单的XML或注解来配置和映射原?类型、接?和Java的POJO （Plain Old Java Objects,普通?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?向对象的编程语?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??向对象程序设计语?的简单易?性，?可以利?关系数据库的技术优势。ORM把关系数据库包装成?向对象的模型。ORM框架是?向对象设计语?与关系数据库发展不同步时的中间解决?案。采?ORM框架后，应?程序不再直接访问底层数据库，?是以?向对象的?式来操作持久化对象，?ORM框架则将这些?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