mybatis 学习第⼀部分:⾃定义持久层框架
1.1 分析JDBC操作问
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
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");
}
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;
}
}
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
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);
}
});
return o;
}
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