自定义持久层框架------从零到一手写一个mybatis

今天不coding2024-10-10 7:10

JDBC操作问题

JDBC代码

java 复制代码 
import java.sql.*;
import java.util.ArrayList;
import java.util.List;

public class JDBC {
    public static void main(String[] args) {
        Connection connection = null;
        PreparedStatement preparedStatement = null;
        ResultSet resultSet = null;
        try {
            //加载驱动
            Class.forName("com.mysql.jdbc.Driver");
            //创建连接
            connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf-8", " root", " root");
            //写sql语句
            String sql = "select * from user where username = ?";
            //获取预处理对象
            preparedStatement = connection.prepareStatement(sql);
            //设置参数
            preparedStatement.setString(1, "tom");
            //执行sql，获取结果集
            resultSet = preparedStatement.executeQuery();
            //封装结果集
            List<User> userList = new ArrayList<>();
            while (resultSet.next()) {
                User user = new User();
                int id = resultSet.getInt("id");
                String username = resultSet.getString("username");
                user.setId(id);
                user.setUsername(username);
                userList.add(user);
            }
            System.out.println(userList);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            //关闭资源
            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

  1. 频繁创建、释放数据库连接，造成系统资源浪费
  2. sql语句在代码中硬编码，不易维护
  3. 传参存在硬编码，修改sql还要修改代码
  4. 手动解析结果集

JDBC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. 使用反射技术，将实体属性与表字段自动映射

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的全路径

Mapper.xml:sql语句的配置文件信息

框架端

  1. 读取配置文件

读取后以流的形式存在于内存中，后创建JavaBean存储

Configuration:存放数据库的基本信息、Map<唯一标识，Mapper>

唯一标识：namespace+"."+id

MappedStatement:sql语句、statement类型、输入参数类型、输出参数类型

  1. 解析配置文件

创建SqlSessionFactoryBuilder类

方法：SqlSessionFactory build()

使用dom4j解析配置文件，将解析出来的内容封装到Configuration和MappedStatement

创建SqlSessionFactory的实现类DefaultSqlSessionFactory。

  1. 创建SqlSessionFactory

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

  1. 创建sqlSession接口及其实现类

方法：selectOne(String statementId,Object... params)

selectList(String statementId,Object... params)

  1. 创建Executor接口及其实现类

封装JDBC代码实现CRUD

** 设计模式：建造者模式、工厂模式、代理模式**

自定义框架实现

在使用端创建配置文件

xml 复制代码 
<configuration>
    <dataSource>
        <property name="driverClass" value="com.mysql.jdbc.Driver"></property>
        <property name="jdbcUrl" value="jdbc:mysql:///test"></property>
        <property name="username" value="root"></property>
        <property name="password" value="@wdlm212027"></property>
    </dataSource>
    <mapper resource="UserMapper.xml"></mapper>
</configuration>
xml 复制代码 
<mapper namespace="user">
    <select id="selectList" resultType="com.wdlm.pojo.User">
        select * from user
    </select>
    <select id="selectOne" resultType="com.wdlm.pojo.User" parameterType="com.wdlm.pojo.User">
        select * from user where id=#{id} and username=#{username}
    </select>
</mapper>

在使用端创建实体类

java 复制代码 
public class User {
    private Integer id;
    private String username;

    public User() {
    }

    public User(Integer id, String username) {
        this.id = id;
        this.username = 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;
    }
}

在框架端导入Maven依赖

xml 复制代码 
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.wdlm.imybatis</groupId>
    <artifactId>Imybatis</artifactId>
    <version>1.0-SNAPSHOT</version>

    <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>

</project>

在框架端创建配置类

java 复制代码 
import javax.sql.DataSource;
import java.util.HashMap;
import java.util.Map;

public class Configuration {
    private DataSource dataSource;
    Map<String, MappedStatement> mappedStatementMap = new HashMap<String, MappedStatement>();

    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;
    }
}
java 复制代码 
public class MappedStatement {
    private String id;
    private String resultType;
    private String parameterType;
    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 getParameterType() {
        return parameterType;
    }

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

    public String getSql() {
        return sql;
    }

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

在框架端创建配置文件读取类

java 复制代码 
public class Resources {
    public static InputStream getResourceAsStream(String path) {
        InputStream resource = Resources.class.getClassLoader().getResourceAsStream(path);
        return resource;
    }
}

在框架端创建SqlSessionFactoryBuilder

java 复制代码 
public class SqlSessionFactoryBuilder {
    public SqlSessionFactory build(InputStream resource) throws PropertyVetoException, DocumentException {
        XMLConfigBuilder xmlConfigBuilder = new XMLConfigBuilder();
        Configuration configuration = xmlConfigBuilder.parseConfig(resource);
        DefaultSqlSessionFactory defaultSqlSessionFactory = new DefaultSqlSessionFactory(configuration);
        return defaultSqlSessionFactory;
    }
}

在框架端创建解析类

java 复制代码 
public class XMLConfigBuilder {
    private Configuration configuration;

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

    public Configuration parseConfig(InputStream resource) throws DocumentException, PropertyVetoException {
        Document document = new SAXReader().read(resource);
        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 dataSource = new ComboPooledDataSource();
        dataSource.setDriverClass(properties.getProperty("driverClass"));
        dataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));
        dataSource.setUser(properties.getProperty("username"));
        dataSource.setPassword(properties.getProperty("password"));

        configuration.setDataSource(dataSource);

        List<Element> mapperList = rootElement.selectNodes("//mapper");
        for (Element element : mapperList) {
            String mapperPath = element.attributeValue("resource");
            InputStream mapperResource = Resources.getResourceAsStream(mapperPath);
            XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(configuration);
            xmlMapperBuilder.parse(mapperResource);
        }

        return configuration;

    }
}
java 复制代码 
public class XMLMapperBuilder {
    private Configuration configuration;

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

    public void parse(InputStream resource) throws DocumentException {
        Document document = new SAXReader().read(resource);
        Element rootElement = document.getRootElement();
        List<Element> list = rootElement.selectNodes("//select");
        String namespace = rootElement.attributeValue("namespace");
        for (Element element : list) {
            String id = element.attributeValue("id");
            String resultType = element.attributeValue("resultType");
            String parameterType = element.attributeValue("parameterType");
            String sql = element.getTextTrim();
            MappedStatement mappedStatement = new MappedStatement();
            mappedStatement.setId(id);
            mappedStatement.setParameterType(parameterType);
            mappedStatement.setResultType(resultType);
            mappedStatement.setSql(sql);
            String key = namespace + "." + id;
            configuration.getMappedStatementMap().put(key, mappedStatement);
        }
    }
}

在框架端创建SqlSessionFactory接口及其实现类

java 复制代码 
public interface SqlSessionFactory {
    public SqlSession openSession();
}
java 复制代码 
public class DefaultSqlSessionFactory implements SqlSessionFactory {
    private Configuration configuration;

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

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

在框架端创建SqlSession接口及其实现类

java 复制代码 
public interface SqlSession {
    public <E> List<E> selectList(String statementId, Object... params);

    public <T> T selectOne(String statementId, Object... params) throws Exception;

}
java 复制代码 
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 {
        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("查询结果为空或者过多");
        }
    }
}

在框架端创建Executor接口及其实现类

java 复制代码 
public interface Executor {
    public <E>List<E> query(Configuration configuration, MappedStatement mappedStatement,Object... params);
}
java 复制代码 
public class SimpleExecutor implements Executor {
    @Override
    public <E> List<E> query(Configuration configuration, MappedStatement mappedStatement, Object... params) throws Exception {
        DataSource dataSource = configuration.getDataSource();
        Connection connection = dataSource.getConnection();
        String sql = mappedStatement.getSql();
        BoundSql boundSql = getBoundSql(sql);
        PreparedStatement preparedStatement = connection.prepareStatement(boundSql.getSql());
        List<ParameterMapping> parameterMappingList = boundSql.getParameterMappingList();
        String parameterType = mappedStatement.getParameterType();
        Class<?> parameterClass = getClassType(parameterType);
        for (int i = 0; i < parameterMappingList.size(); i++) {
            ParameterMapping parameterMapping = parameterMappingList.get(i);
            String content = parameterMapping.getContent();

            Field declaredField = parameterClass.getDeclaredField(content);
            declaredField.setAccessible(true);
            Object o = declaredField.get(params[0]);

            preparedStatement.setObject(i + 1, o);
        }

        ResultSet resultSet = preparedStatement.executeQuery();
        String resultType = mappedStatement.getResultType();
        Class<?> resultTypeClass = getClassType(resultType);
        Object o = resultTypeClass.newInstance();
        ArrayList<Object> objects = new ArrayList<>();
        while (resultSet.next()) {
            ResultSetMetaData metaData = resultSet.getMetaData();
            for (int i = 1; i <= metaData.getColumnCount(); i++) {
                String columnName = metaData.getColumnName(i);
                Object value = resultSet.getObject(columnName);
                PropertyDescriptor propertyDescriptor = new PropertyDescriptor(columnName, resultTypeClass);
                Method writeMethod = propertyDescriptor.getWriteMethod();
                writeMethod.invoke(o,value);
                objects.add(o);
            }
        }
        return (List<E>) objects;
    }

    private BoundSql getBoundSql(String sql) {
        ParameterMappingTokenHandler parameterMappingTokenHandler = new ParameterMappingTokenHandler();
        GenericTokenParser genericTokenParser = new GenericTokenParser("#{", "}", parameterMappingTokenHandler);
        String parseSql = genericTokenParser.parse(sql);
        List<ParameterMapping> parameterMappings = parameterMappingTokenHandler.getParameterMappings();
        BoundSql boundSql = new BoundSql(parseSql, parameterMappings);
        return boundSql;
    }

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

在框架端创建Sql解析类

java 复制代码 
public class BoundSql {
    private String sql;
    private List<ParameterMapping> parameterMappingList = new ArrayList<>();

    public BoundSql() {
    }

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

    public String getSql() {
        return sql;
    }

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

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

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

在框架端创建Sql解析工具类

java 复制代码 
public interface TokenHandler {
    String handleToken(String content);
}
java 复制代码 
public class GenericTokenParser {

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

  public GenericTokenParser(String openToken, String closeToken, com.wdlm.utils.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();
  }
}
java 复制代码 
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;
    }
}
java 复制代码 
public class ParameterMappingTokenHandler implements com.wdlm.utils.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;
	}

}
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