第一部分：自定义持久层框架

2022-06-03

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();
        User user = new User();
        //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.print(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

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
sql语句在代码中硬编译，造成代码不易维护，实际应用中sql变化的可能性较大，sql变动需要改变java代码。
使用preparedStatement向占有位符号传参数存在硬编码，因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。
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。

问题解决思路

使用数据库连接池初始化连接资源
将sql语句抽取到xml配置文件中
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

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

读取配置文件

读取完成后以流的形式存在，我们不能将读取的配置信息以流的形式存放到内存中，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

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

创建SqlSessionFactory

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

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

自定义框架实现

在使用端项目中创建配置文件
创建sqlMapConfig.xml

<configuration>
    <!--数据库连接信息-->
    <property name="driverClass" value="com.mysql.jdbc.Driver"></property>
    <property name="jdbcUrl" value="jdbc:mysql:///zdy_mybatis"></property>
    <property name="user" value="root"></property>
    <property name="password" value="root"></property>
    <! --引入sql配置信息-->
    <mapper resource="mapper.xml"></mapper>
</configuration>

mapper.xml

<mapper namespace="User">
    <select id="selectOne" paramterType="com.lagou.pojo.User"
            resultType="com.lagou.pojo.User">
        select * from user where id = #{id} and username =#{username}
    </select>
    <select id="selectList" resultType="com.lagou.pojo.User">
        select * from user
    </select>
</mapper>

User实体

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

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

public class Configuration {
    //数据源
    private DataSource dataSource;
    //map集合：key:statementId value:MappedStatement
    private 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;
    }
}

MappedStatement

public class MappedStatement {
    //id
    private Integer id;
    //sql语句
    private String sql;
    //输入参数
    private Class<?> paramterType;
    //输出参数
    private Class<?> resultType;

    public Integer getId() {
        return id;
    }

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

    public String getSql() {
        return sql;
    }

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

    public Class<?> getParamterType() {
        return paramterType;
    }

    public void setParamterType(Class<?> paramterType) {
        this.paramterType = paramterType;
    }

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

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

Resources

public class MappedStatement {
    public static InputStream getResourceAsSteam(String path){
        InputStream resourceAsStream = Resources.class.getClassLoader().getResourceAsStream((path));
        return resourceAsStream;
    }
}

SqlSessionFactoryBuilder

public class SqlSessionFactoryBuilder {
    private Configuration configuration;
    public SqlSessionFactoryBuilder(){
        this.configuration = new Configuration();
    }
    public SqlSessionFactory build(InputStream inputStream) throws
            DocumentException, PropertyVetoException, ClassNotFoundException {
        //解析配置文件，封装Configuration
        XMLConfigBuilder xmlConfigBuilder = new XMLConfigBuilder(configuration);
        Configuration configuration = xmlConfigBuilder.parseConfiguration(inputStream);
        //创建sqlSessionFactory
        SqlSessionFactory sqlSessionFactory = new DefaultSqlSessionFactory(configuration);
        return sqlSessionFactory;
    }
}

XMLConfigerBuilder

public class XMLConfigerBuilder {
    private Configuration configuration;

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

    public Configuration parseConfiguration(InputStream inputStream) throws
            DocumentException, PropertyVetoException, ClassNotFoundException {
        Document document = new SAXReader().read(inputStream); //<configuation>
        Element rootElement = document.getRootElement();
        List<Element> propertyElements = rootElement.selectNodes("//property");
        Properties properties = new Properties();
        for (Element propertyElement : propertyElements) {
            String name = propertyElement.attributeValue("name");
            String value = propertyElement.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 部分
        List<Element> mapperElements = rootElement.selectNodes("//mapper");
        XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(configuration);
        for (Element mapperElement : mapperElements) {
            String mapperPath = mapperElement.attributeValue("resource");
            InputStream resourceAsSteam =
                    Resources.getResourceAsSteam(mapperPath);
            xmlMapperBuilder.parse(resourceAsSteam);
        }
        return configuration;
    }
}

XMLMapperBuilder

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) {
            //id的值
            String id = element.attributeValue("id");
            String paramterType = element.attributeValue("paramterType");
            String resultType = element.attributeValue("resultType");
            //输入参数 class
            Class<?> paramterTypeClass = getClassType(paramterType);
            //返回结果class
            Class<?> resultTypeClass = getClassType(resultType);
            //statementId
            String key = namespace + "." + id;
            //sql语句
            String textTrim = element.getTextTrim();
            //封装 mappedStatement
            MappedStatement mappedStatement = new MappedStatement();
            mappedStatement.setId(id);
            mappedStatement.setParamterType(paramterTypeClass);
            mappedStatement.setResultType(resultTypeClass);
            mappedStatement.setSql(textTrim);
            //填充 configuration
            configuration.getMappedStatementMap().put(key, mappedStatement);
            private Class<?> getClassType (String paramterType) throws ClassNotFoundException {
                Class<?> aClass = Class.forName(paramterType);
                return aClass;
            }
        }
    }
}

sqlSessionFactory 接口及D efaultSqlSessionFactory 实现类

public interface SqlSessionFactory {
    public SqlSession openSession();
}

public class DefaultSqlSessionFactory implements SqlSessionFactory {
    private Configuration configuration;

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

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

sqlSession 接口及 DefaultSqlSession 实现类

public interface SqlSession {
    public <E> List<E> selectList(String statementId, Object... param) Exception;

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

    public void close() throws SQLException;
}

public class DefaultSqlSession implements SqlSession {
    private Configuration configuration;

    public DefaultSqlSession(Configuration configuration) {
        this.configuration = configuration;
        //处理器对象
        private Executor simpleExcutor = new SimpleExecutor();
        public <E> List <E> selectList(String statementId, Object...param) throws Exception {
            MappedStatement mappedStatement =
                    configuration.getMappedStatementMap().get(statementId);
            List<E> query = simpleExcutor.query(configuration, mappedStatement, param);
            return query;
        }
        //selectOne 中调用 selectList
        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("返回结果过多");
            }
        }
        public void close () throws SQLException {
            simpleExcutor.close();
        }
    }
}

Executor

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

    void close() throws SQLException;
}

SimpleExecutor

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

    public <E> List<E> query(Configuration configuration, MappedStatement
            mappedStatement, Object[] param) throws SQLException, NoSuchFieldException,
            IllegalAccessException, InstantiationException, IntrospectionException,
            InvocationTargetException {
        //获取连接
        connection = configuration.getDataSource().getConnection();
        // select * from user where id = #{id} and username = #{username} String
        sql = mappedStatement.getSql();
        //对sql进行处理
        BoundSql boundsql = getBoundSql(sql);
        // select * from where id = ? and username = ?
        String finalSql = boundsql.getSqlText();
        //获取传入参数类型
        Class<?> paramterType = mappedStatement.getParamterType();
        //获取预编译preparedStatement对象
        PreparedStatement preparedStatement =
                connection.prepareStatement(finalSql);
        List<ParameterMapping> parameterMappingList =
                boundsql.getParameterMappingList();
        for (int i = 0; i < parameterMappingList.size(); i++) {
            ParameterMapping parameterMapping = parameterMappingList.get(i);
            String name = parameterMapping.getName();
            //反射
            Field declaredField = paramterType.getDeclaredField(name);
            declaredField.setAccessible(true);
            //参数的值
            Object o = declaredField.get(param[0]);
            //给占位符赋值
            preparedStatement.setObject(i + 1, o);
        }
        ResultSet resultSet = preparedStatement.executeQuery();
        Class<?> resultType = mappedStatement.getResultType();
        ArrayList<E> results = new ArrayList<E>();
        while (resultSet.next()) {
            ResultSetMetaData metaData = resultSet.getMetaData();
            (E) resultType.newInstance();
            int columnCount = metaData.getColumnCount();
            for (int i = 1; i <= columnCount; i++) {
                //属性名
                String columnName = metaData.getColumnName(i);
                //属性值
                Object value = resultSet.getObject(columnName);
                //创建属性描述器，为属性生成读写方法
                PropertyDescriptor propertyDescriptor = new
                        PropertyDescriptor(columnName, resultType);
                //获取写方法
                Method writeMethod = propertyDescriptor.getWriteMethod();
                //向类中写入值
                writeMethod.invoke(o, value);
            }
            results.add(o);
        }
        return results;
    }

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

    private BoundSql getBoundSql(String sql) {
        //标记处理类：主要是配合通用标记解析器GenericTokenParser类完成对配置文件等的解析工作，其中TokenHandler主要完成处理
        ParameterMappingTokenHandler parameterMappingTokenHandler = new
                ParameterMappingTokenHandler();
        //GenericTokenParser :通用的标记解析器，完成了代码片段中的占位符的解析，然后再根 据给定的标记处理器(TokenHandler)来进行表达式的处理
        //三个参数：分别为openToken (开始标记)、closeToken (结束标记)、handler (标记处理器)
        GenericTokenParser genericTokenParser = new GenericTokenParser("# {", "}", parameterMappingTokenHandler);
        String parse = genericTokenParser.parse(sql);
        List<ParameterMapping> parameterMappings =
                parameterMappingTokenHandler.getParameterMappings();
        BoundSql boundSql = new BoundSql(parse, parameterMappings);
        return boundSql;
    }
}

BoundSql

public class BoundSql {
    //解析过后的sql语句
    private String sqlText;
    //解析出来的参数
    private List<ParameterMapping> parameterMappingList = new
            ArrayList<ParameterMapping>();

    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;
    }
}

自定义框架优化

通过上述我们的自定义框架，我们解决了JDBC操作数据库带来的一些问题：例如频繁创建释放数据库连接，硬编码，手动封装返回结果集等问题，但是现在我们继续来分析刚刚完成的自定义框架代码，有没有什么问题？
问题如下：

dao的实现类中存在重复的代码，整个操作的过程模板重复(创建sqlsession,调用sqlsession方法，关闭 sqlsession)
dao的实现类中存在硬编码，调用sqlsession的方法时，参数statement的id硬编码解决：使用代理模式来创建接口的代理对象

@Test
public void test2() throws Exception {
    InputStream resourceAsSteam = Resources.getResourceAsSteam("sqlMapConfig.xml")
    SqlSessionFactory build = new SqlSessionFactoryBuilder().build(resourceAsSteam);
    SqlSession sqlSession = build.openSession();
    User user = new User();
    user.setld(l);
    user.setUsername("tom");
    //代理对象
    UserMapper userMapper = sqlSession.getMappper(UserMapper.class);
    User userl = userMapper.selectOne(user);
    System.out.println(userl);
}

在sqlSession中添加方法

1
2
3

public interface SqlSession {
    public <T> T getMappper(Class<?> mapperClass);
}

实现类

@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;
}

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