MyBatis之自定义持久化框架-1

1-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. 将数据库连接信息和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

1.3 ⾃定义框架设计

使⽤端：

引入自定义持久层框架的jar包
提供两部分配置信息：

• 数据库配置信息
• sql配置信息：SQL语句、参数类型、返回值类型

提供核⼼配置⽂件：
sqlMapConfig.xml : 存放数据源信息，引⼊mapper.xml（存放mapper.xml的全路径）
Mapper.xml : sql语句的配置⽂件信息

框架端：

本质上是对JDBC代码进行了封装

1. 读取配置⽂件
   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，创建Resource类，方法：InputStream getResourceAsStream(String path)
   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(sqlMapConfig.xml解析出来的内容)、Map<唯⼀标识，Mapper> 唯⼀标识：namespace + "." + id
   （2）MappedStatement：Mapper.xml解析出来的内容，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(InputStream in)：
   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DefaultSqlSessionFactory对象：生产SqlSession—会话对象（工厂模式）

3. 创建SqlSessionFactory：
   ⽅法：openSession() : 获取sqlSession接⼝的实现类实例对象（生产SqlSession）

4. 创建sqlSession接⼝及实现类：主要封装crud⽅法
   ⽅法：

   • selectList(String statementId,Object param)：查询所有
   • selectOne(String statementId,Object param)：查询单个
     具体实现：封装JDBC完成对数据库表的查询操作

5. 创建Executor接口及实现类SimpleExector实现类
   query(Configuration， MappedStatement， Object… params)：执行的就是JDBC代码

涉及到的设计模式：
Builder构建者设计模式、⼯⼚模式、代理模式

1.4 ⾃定义框架实现

在使⽤端项⽬中创建配置配置⽂件

创建 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:///zdy_mybatis"></property>
        <property name="username" value="root"></property>
        <property name="password" value="123456"></property>
    </dataSource>

    <!--存放mapper.xml的全路径-->
    <mapper resource="UserMapper.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 {  
  
 private Integer id;  
    private String username;  
    private String password;  
    private String birthday;  
  
  
    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;  
    }  
  
 public String getPassword() {  
 return password;  
    }  
  
 public void setPassword(String password) {  
 this.password = password;  
    }  
  
 public String getBirthday() {  
 return birthday;  
    }  
  
 public void setBirthday(String birthday) {  
 this.birthday = birthday;  
    }  
  
 @Override  
 public String toString() {  
 return "User{" +  
                "id=" + id +  
                ", username='" + username + '\'' +  
                ", password='" + password + '\'' +  
                ", birthday='" + birthday + '\'' +  
                '}';  
    }  
}

再创建⼀个Maven⼦⼯程并且导⼊需要⽤到的依赖坐标

<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>

Configuration

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

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;
    }

}

Resources

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 inputStream = Resources.class.getClassLoader().getResourceAsStream(path);
        return inputStream;
    }

}

SqlSessionFactoryBuilder

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;
    }

}

XMLConfigerBuilder

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);
        }

        // c3p0的连接池
        ComboPooledDataSource comboPooledDataSource = new ComboPooledDataSource();
        comboPooledDataSource.setDriverClass(properties.getProperty("driverClass"));
        comboPooledDataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));
        comboPooledDataSource.setUser(properties.getProperty("username"));
        comboPooledDataSource.setPassword(properties.getProperty("password"));

        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 resourceAsStream = Resources.getResourceAsStream(mapperPath);
            XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(configuration);
            xmlMapperBuilder.parse(resourceAsStream);
        }

        return configuration;
    }

}

XMLMapperBuilder

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);  
        }  
  
 }  
}

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;
    }

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

sqlSession 接⼝及 DefaultSqlSession 实现类

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);

}


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 returnType = method.getGenericReturnType();
                        // 判断是否进行了 泛型类型参数化
                        if (returnType instanceof ParameterizedType) {
                            List<Object> objectList = selectList(statementId, args);
                            return objectList;
                        }

                        return selectOne(statementId, args);
                    }
                });

        return (T) proxyInstance;
    }
}

Executor

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

SimpleExecutor

public class SimpleExecutor implements Executor {
    @Override
    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<?> parameterTypeClass = 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 = parameterTypeClass.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();

        // 6. 封装返回结果集
        String resultType = mappedStatement.getResultType();
        Class<?> resultTypeClass = getClassType(resultType);
        List<Object> objectList = new ArrayList<>();

        while (resultSet.next()) {
            Object instance = resultTypeClass.newInstance();
            //元数据(包含查询结果中的字段名称)
            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 columnValue = 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);
                // 相当于setXxx()方法
                Method writeMethod = propertyDescriptor.getWriteMethod();
                writeMethod.invoke(instance, columnValue);
            }
            objectList.add(instance);
        }
        return (List<E>) objectList;
    }

    private Class<?> getClassType(String className) throws ClassNotFoundException {

        if (className != null) {
            Class<?> clazz = Class.forName(className);
            return clazz;
        }
        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 BoundSql
     */
    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();
        // 标记解析器
        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;
    }
}

BoundSql

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;
    }

}

1.5 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(path：
                "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

//为Dao接口生成代理实现类  
public <T> T getMapper(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 returnType = method.getGenericReturnType();
                        // 判断是否进行了 泛型类型参数化
                        if (returnType instanceof ParameterizedType) {
                            List<Object> objectList = selectList(statementId, args);
                            return objectList;
                        }

                        return selectOne(statementId, args);
                    }
                });

        return (T) proxyInstance;
    }