前言
本文适合有一定java基础的同学,通过自定义持久层框架,可以更加清楚常用的mybatis等开源框架的原理。
JDBC操作回顾及问题分析
学习java的同学一定避免不了接触过jdbc,让我们来回顾下初学时期接触的jdbc操作吧
以下代码连接数据库查询用户表信息,用户表字段分别为用户id,用户名username。
Connection connection = null;
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;
User user = new User();
try {
// 加载数据库驱动
//Class.forName("com.mysql.jdbc.Driver");
Class.forName("com.mysql.cj.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", "mimashi3124");
// 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, "盖伦");
// 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();
}
}
}
查看代码我们可以发现使用JDBC操作数据库存在以下问题:
问题解决思路
自己动手写个持久层框架
接下来,我们来一个个解决上面的问题
数据库连接池我们可以直接使用c3p0提供的ComboPooledDataSource即可
为了解决sql硬编码问题,我们要把sql写到xml文件中,那自然是要定义一个xml文件了。
光有sql肯定不行,毕竟我们要先连接数据库,sql语句才有存在的意义。所以xml中得先定义数据配置信息,然后才是sql语句。
1.定义配置xml文件
我们新建一个sqlMapConfig.xml,定义数据源信息、并且增加两个sql语句,parameterType为sql执行参数,resultType为方法返回实体。
代码如下(数据库不同版本使用驱动类可能不同):
select * from user where id = #{id} and username =#{username}
select * from user
现在xml文件数据库信息也有了,sql语句定义也有了,还有什么问题呢?
我们实际中对sql的操作会涉及到不同的表,所以我们改进一下,把每个表的sql语句单独放在一个xml里,这样结构更清晰就容易维护。
优化以后的xml配置现在是这样了
sqlMapConfig.xml
mapper.xml
select * from user where id = #{id} and username =#{username}
select * from user
顺便定义一下业务实体User
public class User {
private int id;
private String username;
public int getId() {
return id;
}
public void setId(int 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 + '\'' +
'}';
}
}
2.读取配置文件
读取完成以后以流的形式存在,不好操作,所以我们要做解析拿到信息,创建实体对象来存储。
xml解析我们使用dom4j
首先引入maven依赖
代码如下(mysql驱动版本根据实际使用mysql版本调整):
8
8
mysql
mysql-connector-java
8.0.22
c3p0
c3p0
0.9.1.2
log4j
log4j
1.2.12
junit
junit
4.10
dom4j
dom4j
1.6.1
jaxen
jaxen
1.1.6
数据库配置实体 Configuration
public class Configuration {
//数据源
private DataSource dataSource;
//map集合: key:statementId value:MappedStatement
private Map mappedStatementMap = new HashMap();
public DataSource getDataSource() {
return dataSource;
}
public Configuration setDataSource(DataSource dataSource) {
this.dataSource = dataSource;
return this;
}
public Map getMappedStatementMap() {
return mappedStatementMap;
}
public Configuration setMappedStatementMap(Map mappedStatementMap) {
this.mappedStatementMap = mappedStatementMap;
return this;
}
}
Sql语句信息实体
public class MappedStatement {
//id
private String id;
//sql语句
private String sql;
//输⼊参数
private String parameterType;
//输出参数
private String resultType;
public String getId() {
return id;
}
public MappedStatement setId(String id) {
this.id = id;
return this;
}
public String getSql() {
return sql;
}
public MappedStatement setSql(String sql) {
this.sql = sql;
return this;
}
public String getParameterType() {
return parameterType;
}
public MappedStatement setParameterType(String parameterType) {
this.parameterType = parameterType;
return this;
}
public String getResultType() {
return resultType;
}
public MappedStatement setResultType(String resultType) {
this.resultType = resultType;
return this;
}
}
顺便定义一个Resources类来读取xml文件流
public class Resources {
public static InputStream getResourceAsSteam(String path) {
return Resources.class.getClassLoader().getResourceAsStream(path);
}
}
接下来就是实际的解析了,因为解析代码比较多,我们考虑封装类单独处理解析
定义XMLConfigBuilder类解析数据库配置信息
public class XMLConfigBuilder {
private Configuration configuration;
public XMLConfigBuilder() {
this.configuration = new Configuration();
}
public Configuration parserConfiguration(InputStream inputStream) throws DocumentException, PropertyVetoException, ClassNotFoundException {
Document document = new SAXReader().read(inputStream);
Element rootElement = document.getRootElement();
List 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);
}
//解析到数据库配置信息,设置数据源信息
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);
//将configuration传入XMLMapperBuilder中做sql语句解析。
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(this.configuration);
List mapperElements = rootElement.selectNodes("//mapper");
for (Element mapperElement : mapperElements) {
String mapperPath = mapperElement.attributeValue("resource");
InputStream resourceAsStream = this.getClass().getClassLoader().getResourceAsStream(mapperPath);
xmlMapperBuilder.parse(resourceAsStream);
}
return configuration;
}
}
定义XMLMapperBuilder类解析数据库配置信息
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 select = rootElement.selectNodes("select");
for (Element element : select) { //id的值
String id = element.attributeValue("id");
String parameterType = element.attributeValue("parameterType"); //输⼊参数
String resultType = element.attributeValue("resultType"); //返回参数
//statementId,后续调用通过statementId,找到对应的sql执行
String key = namespace + "." + id;
//sql语句
String textTrim = element.getTextTrim();
//封装 mappedStatement
MappedStatement mappedStatement = new MappedStatement();
mappedStatement.setId(id);
mappedStatement.setParameterType(parameterType);
mappedStatement.setResultType(resultType);
mappedStatement.setSql(textTrim);
//填充 configuration
configuration.getMappedStatementMap().put(key, mappedStatement);
}
}
}
现在我们可以通过调用配置解析的方法拿到Configuration对象了。但是我们实际使用,肯定是希望我给你配置信息、sql语句,再调用你的方法就返回结果了。所以我们还需要定义一个数据库操作接口(类)
3.定义sql操作接口SqlSession
public interface SqlSession {
//查询多个
public List selectList(String statementId, Object... param) throws Exception;
//查询一个
public T selectOne(String statementId,Object... params) throws Exception;
}
对操作接口SqlSession做具体实现,这里主要是通过statementId找到对应的sql信息,进行执行
代码中simpleExcutor做真正的数据库语句执行、返回参数封装等操作
public class DefaultSqlSession implements SqlSession {
private Configuration configuration;
private Executor simpleExcutor = new SimpleExecutor();
public DefaultSqlSession(Configuration configuration) {
this.configuration = configuration;
}
@Override
public List selectList(String statementId, Object... param) throws Exception {
MappedStatement mappedStatement =
configuration.getMappedStatementMap().get(statementId);
List query = simpleExcutor.query(configuration, mappedStatement, param);
return query;
}
@Override
public T selectOne(String statementId, Object... params) throws Exception {
List objects = selectList(statementId, params);
if (objects.size() == 1) {
return (T) objects.get();
} else {
throw new RuntimeException("返回结果过多");
}
}
}
4.编写数据库执行逻辑
数据库操作类DefaultSqlSession中的selectList方法调用到了simpleExcutor.query()方法
public class SimpleExecutor implements Executor {
private Connection connection = null;
@Override
public List query(Configuration configuration, MappedStatement mappedStatement, Object[] params) throws Exception {
//获取连接
connection = configuration.getDataSource().getConnection();
// select * from user where id = #{id} and username = #{username} String sql =
String sql = mappedStatement.getSql();
//对sql进⾏处理
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 parameterType = mappedStatement.getParameterType();
Class parameterTypeClass = getClassType(parameterType);
List parameterMappingList = boundSql.getParameterMappingList();
for (int i = ; 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[]);
preparedStatement.setObject(i+1,o);
}
// 5. 执行sql
ResultSet resultSet = preparedStatement.executeQuery();
String resultType = mappedStatement.getResultType();
Class resultTypeClass = getClassType(resultType);
ArrayList objects = new ArrayList();
// 6. 封装返回结果集
while (resultSet.next()){
Object o =resultTypeClass.newInstance();
//元数据
ResultSetMetaData metaData = resultSet.getMetaData();
for (int i = 1; i aClass = Class.forName(parameterType);
return aClass;
}
return null;
}
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 parameterMappings =
parameterMappingTokenHandler.getParameterMappings();
return new BoundSql(parse, parameterMappings);
}
}
上面的注释比较详细,流程为
通过以上步骤,我们获取到了数据库配置、sql语句信息。定义了数据库操作类SqlSession,但是我们并没有在什么地方调用解析配置文件。
我们还需要一个东西把两者给串起来,这里我们可以使用工厂模式来生成SqlSession
使用工厂模式创建SqlSession
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);
}
}
同时为了屏蔽构建SqlSessionFactory工厂类时获取Configuration的解析过程,我们可以使用构建者模式来获得一个SqlSessionFactory类。
public class SqlSessionFactoryBuilder {
public SqlSessionFactory build(InputStream inputStream) throws PropertyVetoException, DocumentException, ClassNotFoundException {
XMLConfigBuilder xmlConfigerBuilder = new XMLConfigBuilder();
Configuration configuration = xmlConfigerBuilder.parserConfiguration(inputStream);
SqlSessionFactory sqlSessionFactory = new DefaultSqlSessionFactory(configuration);
return sqlSessionFactory;
}
}
5.调用测试
终于好了,通过以上几个步骤我们现在可以具体调用执行代码了。
public static void main(String[] args) throws Exception {
InputStream resourceAsSteam = Resources.getResourceAsSteam("sqlMapConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsSteam);
SqlSession sqlSession = sqlSessionFactory.openSession();
User user = new User();
user.setId(1);
user.setUsername("盖伦");
User user2 = sqlSession.selectOne("user.selectOne", user);
System.out.println(user2);
List users = sqlSession.selectList("user.selectList");
for (User user1 : users) {
System.out.println(user1);
}
}
代码正确执行,输出
⾃定义框架优化
上述⾃定义框架,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,现在我们继续来分析刚刚完成的⾃定义框架代码,有没有什么问题呢?
问题如下:
我们可以使用代理模式,生成代理对象,在调用之前获取到执行方法的方法名、具体类。这样我们就能获取到statementId。
为SqlSession类新增getMappper方法,获取代理对象
public interface SqlSession {
public List selectList(String statementId, Object... param) throws Exception;
public T selectOne(String statementId,Object... params) throws Exception;
//为Dao接口生成代理实现类
public T getMapper(Class mapperClass);
}
public class DefaultSqlSession implements SqlSession {
private Configuration configuration;
private Executor simpleExcutor = new SimpleExecutor();
public DefaultSqlSession(Configuration configuration) {
this.configuration = configuration;
}
@Override
public List selectList(String statementId, Object... param) throws Exception {
MappedStatement mappedStatement =
configuration.getMappedStatementMap().get(statementId);
List query = simpleExcutor.query(configuration, mappedStatement, param);
return query;
}
@Override
public T selectOne(String statementId, Object... params) throws Exception {
List objects = selectList(statementId, params);
if (objects.size() == 1) {
return (T) objects.get();
} else {
throw new RuntimeException("返回结果过多");
}
}
@Override
public 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 {
// selectOne
String methodName = method.getName();
// className:namespace
String className = method.getDeclaringClass().getName();
//statementId
String statementId = className+'.'+methodName;
Type genericReturnType = method.getGenericReturnType();
//判断是否实现泛型类型参数化
if (genericReturnType instanceof ParameterizedType){
List objects = selectList(statementId,args);
return objects;
}
return selectOne(statementId,args);
}
});
return (T) proxyInstance;
}
}
定义业务数据dao接口
public interface IUserDao {
//查询所有用户
public List findAll() throws Exception;
//根据条件进行用户查询
public User findByCondition(User user) throws Exception;
}
接下来我们只需获取到代理对象,调用方法即可。
public class Main2 {
public static void main(String[] args) throws Exception {
InputStream resourceAsSteam = Resources.getResourceAsSteam("sqlMapConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsSteam);
SqlSession sqlSession = sqlSessionFactory.openSession();
//获取到代理对象
IUserDao userDao = sqlSession.getMapper(IUserDao.class);
List all = userDao.findAll();
for (User user1 : all) {
System.out.println(user1);
}
}
}
总结
为了解决JDBC操作数据库存在的问题,我们主要干了这些事
以下为本文的大体流程图: