自定义持久层框架

MyBatis框架原理

最新推荐文章于 2024-04-24 09:09:22 发布

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⾃定义持久层框架

1.1 分析JDBC操作问题

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.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问题总结：
原始jdbc开发存在的问题如下：

1、 数据库连接创建、释放频繁造成系统资源浪费，从⽽影响系统性能。

2、 Sql语句在代码中硬编码，造成代码不易维护，实际应⽤中sql变化的可能较⼤，sql变动需要改变java代码。

3、 使⽤preparedStatement向占有位符号传参数存在硬编码，因为sql语句的where条件不⼀定，可能多也可能少，修改sql还要修改代码，系统不易维护。

4、 对结果集解析存在硬编码(查询列名)，sql变化导致解析代码变化，系统不易维护，如果能将数据库记录封装成pojo对象解析⽐较⽅便

1.2 问题解决思路

①使⽤数据库连接池初始化连接资源

②将sql语句抽取到xml配置⽂件中

③使⽤反射、内省等底层技术，⾃动将实体与表进⾏属性与字段的⾃动映射

在这里插入图片描述

1.3 ⾃定义框架设计

使⽤端：

提供核⼼配置⽂件：

sqlMapConfig.xml : 存放数据源信息，引⼊mapper.xml
Mapper.xml : sql语句的配置⽂件信息

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，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

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：
Builder构建者设计模式、⼯⼚模式、代理模式
在这里插入图片描述

1.4 ⾃定义框架实现

在这里插入图片描述

在使⽤端项⽬中创建配置配置⽂件

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:///mybatis_db"></property>
        <property name="username" value="root"></property>
        <property name="password" value="root"></property>
    </dataSource>

    <!--存放mapper.xml的全路径-->
    <!--引⼊sql配置信息-->
    <mapper resource="UserMapper.xml"></mapper>
</Configuration>

UserMapper.xml

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

    <select id="findByCondition" resultType="com.myProject.pojo.User" parameterType="com.myProject.pojo.User">
        select * from user where id = #{id} and username = #{username}
    </select>
</mapper>

User实体

package com.myProject.pojo;

public class User {
	// 主键标识
    private Integer id;
    // ⽤户名
    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;
    }
}

IUserDao

package com.myProject.dao;

import com.myProject.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;
}

UserDaoImpl

package com.myProject.dao.impl;

import com.myProject.dao.IUserDao;
import com.myProject.io.Resources;
import com.myProject.pojo.User;
import com.myProject.sqlSession.SqlSession;
import com.myProject.sqlSession.SqlSessionFactory;
import com.myProject.sqlSession.SqlSessionFactoryBuilder;
import org.dom4j.DocumentException;

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

public class UserDaoImpl implements IUserDao {
    @Override
    public List<User> findAll() throws Exception {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

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

    @Override
    public User findByCondition(User user) throws Exception {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        // 调用
        User user2 = sqlSession.selectOne("user.selectOne", user);
        return user2;
    }
}

IPersistenceTest

package com.myProject.test;

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

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

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

        // 调用
        User user = new User();
        user.setId(1);
        user.setUsername("lili1");

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

        List<User> all = userDao.findAll();

        User byCondition = userDao.findByCondition(user);
    }
}

再创建⼀个Maven⼦⼯程并且导⼊需要⽤到的依赖坐标

<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>

Configuration

package com.myProject.pojo;

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

public class Configuration {
    //数据源
    private DataSource dataSource;

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

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

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

    public DataSource getDataSource() {
        return dataSource;
    }

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

MappedStatement

package com.myProject.pojo;

public class MappedStatement {
    // id标识
    private String id;
    // 返回值类型(输出参数)
    private Class<?> resultType;
    // 参数值类型(输⼊参数)
    private Class<?> parameterType;
    // sql语句
    private String sql;

    public String getId() {
        return id;
    }

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

    public Class<?> getResultType() {
        return resultType;
    }

    public void setResultType(Class<?> resultType) {
        this.resultType = resultType;
    }

    public Class<?> getParameterType() {
        return parameterType;
    }

    public void setParameterType(Class<?> parameterType) {
        this.parameterType = parameterType;
    }

    public String getSql() {
        return sql;
    }

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

Resources

package com.myProject.io;

import javax.annotation.Resource;
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 getResourceAsStream(String path) {
        InputStream resourceAsStream = Resource.class.getClassLoader().getResourceAsStream(path);
        return resourceAsStream;
    }
}

DefaultSqlSession

package com.myProject.sqlSession;

import com.myProject.pojo.Configuration;
import com.myProject.pojo.MappedStatement;

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

public class DefaultSqlSession implements SqlSession {
    private Configuration configuration;

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

    // 处理器对象
    private Executor simpleExcutor = new SimpleExecutor();

    @Override
    public <E> List<E> selectList(String statementid, Object... params) throws Exception {
        // 将要完成对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 void close() throws SQLException {
        simpleExcutor.close();
    }

    @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 {
                // 底层都还是执行jdbc代码
                // 根据不同情况，来调用selectList或者selectOne
                // 准备参数 1:statementid sql语句的唯一标识 namespace.id = 接口全限定名.方法名
                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.myProject.sqlSession;

import com.myProject.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.myProject.sqlSession;

import com.myProject.pojo.Configuration;
import com.myProject.pojo.MappedStatement;

import java.sql.SQLException;
import java.util.List;

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

    void close() throws SQLException;

}

SimpleExecutor

package com.myProject.sqlSession;

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

import java.beans.PropertyDescriptor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;

public class SimpleExecutor implements Executor{
    private Connection connection = null;

    @Override
    public <E> List<E> query(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception {
        // 1.注册驱动，获取驱动
        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 = ? 转换的过程中秒，还需要对#{}烈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
            // 获取了参数的全路径
        Class<?> parameterType = mappedStatement.getParameterType();
        Class<?> parameterTypeClass = getClassType(parameterType);
        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 = parameterTypeClass.getDeclaredField(content);
            // 暴力访问
            declaredField.setAccessible(true);
            Object o = declaredField.get(params[0]);

            // 给占位符赋值
            preparedStatement.setObject(i + 1, 0);
        }

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

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

        // 6. 封装返回结果集
        while(resultSet.next()) {
            Object o = resultSetClass.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，根据数据库表和实体的对应关系，完成封装
                PropertyDescriptor propertyDescriptor = new PropertyDescriptor(columnName, resultSetClass);
                Method writeMethod = propertyDescriptor.getWriteMethod();
                writeMethod.invoke(o, value);
            }
            objects.add(o);
        }
        return (List<E>) objects;
    }

    @Override
    public void close() throws SQLException {
        connection.close();
    }

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

    /**
     * 完成#{}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.myProject.sqlSession;

import java.sql.SQLException;
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;

    public void close() throws SQLException;

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

SqlSessionFactory

package com.myProject.sqlSession;

public interface SqlSessionFactory {
    public SqlSession openSession();
}

SqlSessionFactoryBuilder

package com.myProject.sqlSession;

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

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

public class SqlSessionFactoryBuilder {
    private Configuration configuration;

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

    public SqlSessionFactory build(InputStream in) throws DocumentException, PropertyVetoException, ClassNotFoundException {
        // 第一：使用dom4j解析配置文件，将解析出来的内容分封装到Configuration中
        // 1.解析配置⽂件，封装Configuration

        XMLConfigBuilder xmlConfigBuilder = new XMLConfigBuilder(configuration);

        Configuration configuration = xmlConfigBuilder.parseConfig(in);

        // 创建SqlSessionFactory对象：工厂类：生产sqlSession
        SqlSessionFactory sqlSessionFactory = new DefaultSqlSessionFactory(configuration);

        return sqlSessionFactory;
    }
}

BoundSql

package com.myProject.config;

import com.myProject.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;
    }
}

XMLConfigerBuilder

package com.myProject.config;

import com.mchange.v2.c3p0.ComboPooledDataSource;
import com.myProject.io.Resources;
import com.myProject.pojo.Configuration;
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(Configuration configuration) {
        this.configuration = configuration;
    }

    /**
     *  该方法就是使用DOM4j对配置文佳进行解析，封装Congiguration
     */
    public Configuration parseConfig(InputStream inputStream) throws DocumentException, PropertyVetoException {
        Document document = new SAXReader().read(inputStream);
        // 获取到了<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);
        }
        
        // 连接池
        ComboPooledDataSource comboPooledDataSource = new ComboPooledDataSource();
        
        comboPooledDataSource.setDriverClass(properties.getProperty("driverClass"));
        comboPooledDataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));
        comboPooledDataSource.setUser(properties.getProperty("username"));
        comboPooledDataSource.setPassword(properties.getProperty("password"));

        // 填充 configuration
        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");

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

        return configuration;
    }
}

XMLMapperBuilder

package com.myProject.config;

import com.myProject.pojo.Configuration;
import com.myProject.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, ClassNotFoundException {
        Document document = new SAXReader().read(inputStream);
        Element rootElement = document.getRootElement();

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

        List<Element> select = rootElement.selectNodes("//select");

        for (Element element : select) {
            String id = element.attributeValue("id");
            String resultType = element.attributeValue("resultType");
            String parameterType = element.attributeValue("parameterType");

            // 输入参数class
            Class<?> parameterTypeClass = getClassType(parameterType);

            // 返回结果class
            Class<?> resultTypeClass = getClassType(resultType);

            // statementId
            String key = namespace + "." + id;

            // 获取Sql语句
            String  sqlText = element.getTextTrim();

            // 封装mappedStatement
            MappedStatement mappedStatement = new MappedStatement();
            mappedStatement.setId(id);
            mappedStatement.setResultType(resultTypeClass);
            mappedStatement.setParameterType(parameterTypeClass);
            mappedStatement.setSql(sqlText);


            configuration.getMappedStatementMap().put(key,mappedStatement);
        }
    }

    private Class<?> getClassType(String parameterType) throws ClassNotFoundException {
        Class<?> aClass = Class.forName(parameterType);
        return aClass;
    }
}

GenericTokenParser

/**
 *    Copyright 2009-2017 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package com.myProject.utils;

/**
 * @author Clinton Begin
 */
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.myProject.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.myProject.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

/**
 *    Copyright 2009-2015 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package com.myProject.utils;

/**
 * @author Clinton Begin
 */
public interface TokenHandler {
  String handleToken(String content);
}