自定义mybatis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：

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：

1.读取配置⽂件

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、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

2.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

3.创建SqlSessionFactory：

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

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

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

<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 {
//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 Resources {
　　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 {
//1.解析配置⽂件，封装Configuration XMLConfigerBuilder
xmlConfigerBuilder = new
XMLConfigerBuilder(configuration);
Configuration configuration =
xmlConfigerBuilder.parseConfiguration(inputStream);
//2.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

 

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

}

}

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;

}

}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

public interface SqlSession {

public <T> T getMappper(Class<?> mapperClass);

实现类

@Override

public <T> T getMappper(Class<?> mapperClass) {

T o = (T) Proxy.newProxyInstance(mapperClass.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 key = className+"."+methodName;

MappedStatement mappedStatement =

configuration.getMappedStatementMap().get(key);

Type genericReturnType = method.getGenericReturnType();

ArrayList arrayList = new ArrayList<> ();

//判断是否实现泛型类型参数化

if(genericReturnType instanceof ParameterizedType){

return selectList(key,args);

return selectOne(key,args);

}

});