一、问题描述
1. 代码片段
2. 问题清单:
如上图所示,传统的 JDBC
连接存在下列问题:
- 创建数据库连接存在硬编码问题
- 频繁的数据库连接与释放
- 组织查询
SQL
存在硬编码问题 SQL
参数个数的变化会导致代码的修改,不易维护- 封装结果集存在硬编码问题,
POJO
属性的修改会带来结果集封装代码的变更,不易维护
3. 解决思路
- 将数据库配置信息及
SQL
语句转移至xml
中(解决硬编码问题) - 使用数据库连接池来获取数据库连接(解决频繁操作数据库连接问题)
- 通过反射、自省功能对结果集进行封装
二、代码
既然问题已经罗列出来了,那么接下来就通过自己手写的 ROM
框架将传统 JDBC
中存在的问题来解决一下。
1. 编写自定义数据库连接配置文件及 SQL
查询文件
1.1 数据库配置文件: sqlMapConfig.xml
<configuration>
<dataSource>
<property name="driverClass" value="com.mysql.jdbc.Driver"></property>
<property name="jdbcUrl" value="jdbc:mysql://localhost:3306/stage_1_mybatis?characterEncoding=utf-8"></property>
<property name="user" value="root"></property>
<property name="password" value="root"></property>
</dataSource>
<!--添加 SQLMapper 文件信息,方便资源查找-->
<mapper resource="UserMapper.xml"></mapper>
</configuration>
1.2 SQL
配置文件: UserMapper.xml
<mapper nameSpace="com.idol.dao.IUserDao">
<select id="findAll" resultType="pojo.User">
select * from user
</select>
<select id="findByCondition" paramterType="pojo.User" resultType="pojo.User">
select * from user where id=#{id} and name=#{name}
</select>
</mapper>
2. 配置文件解析
2.1 xml
资源加载工具类: Resource
上面两个步骤已经将, JDBC
中硬编码部分提取成了 xml
文件,因此为了方便对 xml
配置文件进行加载,需要编写资源加载类。
import java.io.InputStream;
/**
* @author Supreme_Sir
* @version 1.0
* @className Resource
* @description
* @date 2020/9/24 22:39
**/
public class Resource {
/**
* xml 资源加载器
*/
public static InputStream getResourceAsStream(String path) {
InputStream resourceAsStream = Resource.class.getClassLoader().getResourceAsStream(path);
return resourceAsStream;
}
}
2.2 数据库配置文件: sqlMapConfig.xml
解析
import com.idol.io.Resource;
import com.idol.pojo.Configeration;
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;
/**
* 数据库配置文件解析类
*
* @author Supreme_Sir
* @version 1.0
* @className XMLConfigBuilder
* @description
* @date 2020/9/26 15:22
**/
public class XMLConfigBuilder {
private Configeration configeration;
public XMLConfigBuilder() {
this.configeration = new Configeration();
}
/**
* 该方法用于将 xml 解析成配置文件对象
* @param inputStream sqlMapConfig.xml 配置文件的 InputStream 对象
*/
public Configeration parseConfig(InputStream inputStream) throws DocumentException, PropertyVetoException {
// 解析 sqlMapConfig.xml
Document document = new SAXReader().read(inputStream);
Element rootElement = document.getRootElement();
List<Element> propertyList = rootElement.selectNodes("//property");
Properties dataSourceProperties = new Properties();
for (Element element : propertyList) {
String name = element.attributeValue("name");
String value = element.attributeValue("value");
dataSourceProperties.put(name, value);
}
// 创建 c3p0 数据库连接池
ComboPooledDataSource comboPooledDataSource = new ComboPooledDataSource();
comboPooledDataSource.setDriverClass(dataSourceProperties.getProperty("driverClass"));
comboPooledDataSource.setJdbcUrl(dataSourceProperties.getProperty("jdbcUrl"));
comboPooledDataSource.setUser(dataSourceProperties.getProperty("user"));
comboPooledDataSource.setPassword(dataSourceProperties.getProperty("password"));
this.configeration.setDataSource(comboPooledDataSource);
// 解析 Mapper.xml
List<Element> mapperList = rootElement.selectNodes("//mapper");
for (Element element : mapperList) {
String mapperPath = element.attributeValue("resource");
InputStream mapperStream = Resource.getResourceAsStream(mapperPath);
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(this.configeration);
xmlMapperBuilder.parse(mapperStream);
}
return this.configeration;
}
}
2.3 SQL
配置文件:UserMapper.xml
解析
import com.idol.pojo.Configeration;
import com.idol.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;
/**
* SQL 配置文件解析类
*
* @author Supreme_Sir
* @version 1.0
* @className XMLMapperBuilder
* @description
* @date 2020/9/26 15:45
**/
public class XMLMapperBuilder {
private Configeration configeration;
public XMLMapperBuilder(Configeration configeration) {
this.configeration = configeration;
}
public void parse(InputStream inputStream) throws DocumentException {
Document document = new SAXReader().read(inputStream);
Element rootElement = document.getRootElement();
String nameSpace = rootElement.attributeValue("nameSpace");
// 获取根节点下所有一级子节点
List<Element> sqlElement = rootElement.elements();
for (Element element : sqlElement) {
String id = element.attributeValue("id");
String resultType = element.attributeValue("resultType");
String paramterType = element.attributeValue("paramterType");
String sqlText = element.getTextTrim();
MappedStatement mappedStatement = new MappedStatement(id, paramterType, resultType, sqlText);
String mappedStatementName = nameSpace + "." + id;
this.configeration.putMappedStatement(mappedStatementName, mappedStatement);
}
}
}
3. 创建 SQL
执行函数
3.1 创建执行 SQL
的统一方法
import com.idol.config.BondSql;
import com.idol.pojo.Configeration;
import com.idol.pojo.MappedStatement;
import com.idol.utils.GenericTokenParser;
import com.idol.utils.ParameterMapping;
import com.idol.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;
/**
* 创建数据库连接并执行 SQL 语句
*
* @author Supreme_Sir
* @version 1.0
* @className SimpleExecutor
* @description
* @date 2020/9/26 16:48
**/
public class SimpleExecutor implements Executor {
/**
* 执行 SQL
*
* @param configeration 解析出来的数据库配置文件 DO 对象
* @param mappedStatement 解析出来的 SQL 配置文件 DO 对象
* @param params 条件查询参数
* @param <E>
* @return
* @throws Exception
*/
@Override
public <E> List<E> query(Configeration configeration, MappedStatement mappedStatement, Object... params) throws Exception {
// 创建数据库连接
Connection connection = configeration.getDataSource().getConnection();
String sql = mappedStatement.getSql();
// 将 SQL 配置文件中的 SQL 解析为可供 PreparedStatement 使用的 SQL 语句
BondSql bondSql = getBondSql(sql);
PreparedStatement preparedStatement = connection.prepareStatement(bondSql.getSqlText());
// 将条件查询参数配置到 SQL 语句中
String parameterType = mappedStatement.getParameterType();
Class<?> parameterTypeClass = getClassType(parameterType);
List<ParameterMapping> parameterMappingList = bondSql.getParameterMappingList();
for (int i = 0; i < parameterMappingList.size(); i++) {
ParameterMapping parameterMapping = parameterMappingList.get(i);
String name = parameterMapping.getContent();
Field field = parameterTypeClass.getDeclaredField(name);
// 暴力访问对象属性
field.setAccessible(true);
// 获取参数中指定属性的值
Object o = field.get(params[0]);
preparedStatement.setObject(i + 1, o);
}
// 执行 SQL 查询语句
ResultSet resultSet = preparedStatement.executeQuery();
// 开始封装查询结果
String resultType = mappedStatement.getResultType();
Class<?> resultTypeClass = getClassType(resultType);
List result = new ArrayList();
while (resultSet.next()) {
// 实例化查询结果对象
Object instance = resultTypeClass.newInstance();
ResultSetMetaData metaData = resultSet.getMetaData();
for (int i = 1; i <= metaData.getColumnCount() ; i++) {
String columnName = metaData.getColumnName(i);
Object val = resultSet.getObject(columnName);
// 为查询结果实例赋值
PropertyDescriptor propertyDescriptor = new PropertyDescriptor(columnName, resultTypeClass);
Method writeMethod = propertyDescriptor.getWriteMethod();
writeMethod.invoke(instance, val);
}
result.add(instance);
}
return result;
}
/**
* 根据 SQL 配置文件中的类路径获取该对象的 Class
*
* @param classPath
* @return
* @throws ClassNotFoundException
*/
private Class<?> getClassType(String classPath) throws ClassNotFoundException {
if (classPath != null) {
return Class.forName(classPath);
}
return null;
}
/**
* 将包含 #{} 的 SQL 解析为可供 PreparedStatement 使用的 SQL
*
* @param sql
* @return
*/
private BondSql getBondSql(String sql) {
// 参数标识处理类
ParameterMappingTokenHandler parameterMappingTokenHandler = new ParameterMappingTokenHandler();
// 解析 xml 文件中的 SQL
GenericTokenParser genericTokenParser = new GenericTokenParser("#{", "}", parameterMappingTokenHandler);
String parsedSql = genericTokenParser.parse(sql);
// 获取 SQL 中的参数名称
List<ParameterMapping> parameterMappings = parameterMappingTokenHandler.getParameterMappings();
return new BondSql(parsedSql, parameterMappings);
}
}
3.2 创建具体的 SQL
查询方法
import com.idol.pojo.Configeration;
import java.lang.reflect.*;
import java.util.List;
/**
* 封装具体的 SQL 查询方法
*
* @author Supreme_Sir
* @version 1.0
* @className DefaultSqlSession
* @description
* @date 2020/9/26 16:34
**/
public class DefaultSqlSession implements SqlSession {
private Configeration configeration;
@Override
public <E> List<E> selectAll(String statementID, Object... params) throws Exception {
SimpleExecutor simpleExecutor = new SimpleExecutor();
return simpleExecutor.query(this.configeration, this.configeration.getMappedStatement(statementID), params);
}
@Override
public <T> T selectOne(String statementID, Object... params) throws Exception {
List<T> objectList = selectAll(statementID, params);
if (objectList.size() == 1) {
return objectList.get(0);
} else {
throw new RuntimeException("查询结果为空或查询结果不唯一!!!");
}
}
/**
* 通过 Java 动态代理执行 SQL
*
* @param mapperClass
* @param <T>
* @return
*/
@Override
public <T> T getMapper(Class<?> mapperClass) {
Object proxyInstance = Proxy.newProxyInstance(DefaultSqlSession.class.getClassLoader(), new Class[]{
mapperClass}, new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 由于代理对象无法获取 SQL 配置文件中的相关信息,因此通过类名和方法名的拼接,完成成对应 id SQL 的调用
String methodName = method.getName();
String className = method.getDeclaringClass().getName();
String statementID = className + "." + methodName;
// 判断返回结果是否存在泛型,如果存在泛型则表示改结果为集合,则调用 selectAll 方法
Type returnType = method.getGenericReturnType();
if (returnType instanceof ParameterizedType) {
return selectAll(statementID, args);
}
return selectOne(statementID, args);
}
});
return (T) proxyInstance;
}
public DefaultSqlSession(Configeration configeration) {
this.configeration = configeration;
}
}
4. 方法测试
4.1 创建数据表: user.sql
SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
-- Table structure for user
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
`id` int(10) NOT NULL,
`name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES (1, '张三');
INSERT INTO `user` VALUES (2, '李四');
INSERT INTO `user` VALUES (3, '王五');
INSERT INTO `user` VALUES (4, '赵六');
INSERT INTO `user` VALUES (5, '冯七');
SET FOREIGN_KEY_CHECKS = 1;
4.2 编写测试方法
import com.idol.dao.IUserDao;
import com.idol.io.Resource;
import com.idol.sql.session.SqlSession;
import com.idol.sql.session.SqlSessionFactory;
import com.idol.sql.session.SqlSessionFactoryBuilder;
import org.junit.Test;
import pojo.User;
import java.io.InputStream;
import java.util.List;
/**
* @author Supreme_Sir
* @version 1.0
* @className MyFrameworkTest
* @description
* @date 2020/9/24 22:44
**/
public class MyFrameworkTest {
@Test
public void getResourceAsStramTest() throws Exception {
InputStream resourceAsStream = Resource.getResourceAsStream("sqlMapConfig.xml");
SqlSessionFactory sqlsessionFactory = new SqlSessionFactoryBuilder().builder(resourceAsStream);
SqlSession sqlSession = sqlsessionFactory.openSession();
IUserDao userDao = sqlSession.getMapper(IUserDao.class);
List<User> userList = userDao.findAll();
System.out.println("SelectAll~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
for (User user : userList) {
System.out.println(user);
}
System.out.println("\r\nSelectOne~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
User user = userDao.findByCondition(new User(1, "张三"));
System.out.println(user);
}
}
4.3 执行结果
SelectAll~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
User{
id=1, name='张三'}
User{
id=2, name='李四'}
User{
id=3, name='王五'}
User{
id=4, name='赵六'}
User{
id=5, name='冯七'}
SelectOne~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
User{
id=1, name='张三'}
4.4 源码
-------------------- 你在朋友圈看到的华丽照片,P掉了多少生活中的苦难 --------------------