長(zhǎng)文干貨 | 手寫(xiě)自定義持久層框架!

為何要手寫(xiě)自定義持久層框架？

1. JDBC 編碼的弊端

• 會(huì)造成硬編碼問(wèn)題(無(wú)法靈活切換數(shù)據(jù)庫(kù)驅(qū)動(dòng)) 頻繁創(chuàng)建和釋放數(shù)據(jù)庫(kù)連接造成系統(tǒng)資源浪費(fèi) 影響系統(tǒng)性能
• sql 語(yǔ)句存在硬編碼,造成代碼不易維護(hù),實(shí)際應(yīng)用中 sql 變化可能較大,變動(dòng) sql 需要改 Java 代碼
• 使用 preparedStatement 向占有位符號(hào)傳參數(shù)存在硬編碼, 因 sql 語(yǔ)句的 where 條件不確定甚至沒(méi)有where條件,修改 sql 還要修改代碼 系統(tǒng)不易維護(hù)
• 對(duì)結(jié)果集解析也存在硬編碼, sql變化導(dǎo)致解析代碼變化

2. 更有助于讀 mybatis 持久層框架源碼

JDBC代碼

public?class?jdbcConnection?{
????private?static?Connection?connection?=?null;
????private?static?PreparedStatement?preparedStatement?=?null;
????private?static?ResultSet?resultSet?=?null;

????public?static?void?main(String[]?args)?{
????????try?{
????????????//?加載數(shù)據(jù)庫(kù)驅(qū)動(dòng)
????????????Class.forName("com.mysql.jdbc.Driver");
????????????//?通過(guò)驅(qū)動(dòng)管理類(lèi)獲取數(shù)據(jù)庫(kù)連接
????????????connection?=?DriverManager.getConnection("jdbc:mysql://localhost:3306/huodd",?"root",?"1234");
????????????//?定義sql語(yǔ)句???表示占位符
????????????String?sql?=?"select?id,username?from?user?where?id?=??";
????????????//?獲取預(yù)處理對(duì)象?statement
????????????PreparedStatement?preparedStatement?=?(PreparedStatement)?connection.prepareStatement(sql);
????????????//?設(shè)置參數(shù)?第一個(gè)參數(shù)為?sql?語(yǔ)句中參數(shù)的序號(hào)(從1開(kāi)始)?第二個(gè)參數(shù)為?設(shè)置的參數(shù)值
????????????preparedStatement.setInt(1,?1);
????????????//?向數(shù)據(jù)庫(kù)發(fā)出sql執(zhí)行查詢?查詢出結(jié)果集
????????????resultSet?=?preparedStatement.executeQuery();
????????????//?遍歷查詢結(jié)果集
????????????while?(resultSet.next())?{
????????????????int?id?=?resultSet.getInt("id");
????????????????String?username?=?resultSet.getString("username");
????????????????//?封裝對(duì)象
????????????????User?user?=?new?User();
????????????????user.setId(id);
????????????????user.setUsername(username);
????????????????System.out.println(user);
????????????}
????????}?catch?(Exception?ex)?{
????????????ex.printStackTrace();
????????}?finally?{
????????????try?{
????????????????//?釋放資源
????????????????if?(resultSet?!=?null)?{
????????????????????resultSet.close();
????????????????}
????????????????if?(preparedStatement?!=?null)?{
????????????????????preparedStatement.close();
????????????????}
????????????????if?(connection?!=?null)?{
????????????????????connection.close();
????????????????}
????????????}?catch?(Exception?ex)?{
????????????????ex.printStackTrace();
????????????}
????????}
????}
}

解決問(wèn)題的思路

1. 數(shù)據(jù)庫(kù)頻繁創(chuàng)建連接、釋放資源 -> 連接池
2. sql語(yǔ)句及參數(shù)硬編碼 -> 配置文件
3. 手動(dòng)解析封裝結(jié)果集 -> 反射、內(nèi)省

編碼前思路整理

1. 創(chuàng)建、讀取配置文件

• sqlMapConfig.xml 存放數(shù)據(jù)庫(kù)配置信息
• userMapper.xml :存放sql配置信息
• 根據(jù)配置文件的路徑，加載配置文件成字節(jié)輸入流，存儲(chǔ)在內(nèi)存中Resources#getResourceAsStream(String path)
• 創(chuàng)建兩個(gè)JavaBean存儲(chǔ)配置文件解析出來(lái)的內(nèi)容
1. Configuration ：核心配置類(lèi) ，存放 sqlMapConfig.xml解析出來(lái)的內(nèi)容
2. MappedStatement：映射配置類(lèi)：存放mapper.xml解析出來(lái)的內(nèi)容

2. 解析配置文件(使用dom4j)

• 創(chuàng)建類(lèi)：SqlSessionFactoryBuilder#build(InputStream in) ? -> 設(shè)計(jì)模式之構(gòu)建者模式
• 使用dom4j解析配置文件，將解析出來(lái)的內(nèi)容封裝到容器對(duì)象(JavaBean)中

3. 創(chuàng)建 SqlSessionFactory 接口及實(shí)現(xiàn)類(lèi)DefaultSqlSessionFactory

• SqlSessionFactory對(duì)象，生產(chǎn)sqlSession會(huì)話對(duì)象 ?-> 設(shè)計(jì)模式之工廠模式

4. 創(chuàng)建 SqlSession接口及實(shí)現(xiàn)類(lèi)DefaultSqlSession

• 定義對(duì)數(shù)據(jù)庫(kù)的CRUD操作
1. selectList()
2. selectOne()
3. update()
4. delete()

5. 創(chuàng)建Executor接口及實(shí)現(xiàn)類(lèi)SimpleExecutor實(shí)現(xiàn)類(lèi)

• query(Configuration configuration, MappedStatement mapStatement, Object... orgs) ?執(zhí)行的就是JDBC代碼

6. 測(cè)試代碼

用到的設(shè)計(jì)模式

• 構(gòu)建者模式
• 工廠模式
• 代理模式

進(jìn)入編碼

1.創(chuàng)建、讀取配置文件

sqlMapConfig.xml 存放數(shù)據(jù)庫(kù)配置信息

<configuration>
????<dataSource>
????????
????????<property?name="driverClass"?value="com.mysql.jdbc.Driver">property>
????????<property?name="jdbcUrl"?value="jdbc:mysql:///huodd">property>
????????<property?name="user"?value="root">property>
????????<property?name="password"?value="1234">property>
????dataSource>

????
????<mapper?resource="userMapper.xml">mapper>

configuration>

userMapper.xml 存放sql配置信息

<mapper?namespace="user">

????
????<select?id="selectList"?resultType="com.huodd.pojo.User"?paramterType="com.huodd.pojo.User">
????????select?*?from?user
????select>

????<select?id="selectOne"?paramterType="com.huodd.pojo.User"?resultType="com.huodd.pojo.User">
????????select?*?from?user?where?id?=?#{id}?and?username?=#{username}
????select>

mapper>

User.java

public?class?User?{
????private?Integer?id;
????private?String?username;

????...?省略getter?setter?方法
????...?省略?toString?方法
}

pom.xml 中引入依賴(lài)

<dependency>
????<groupId>mysqlgroupId>
????<artifactId>mysql-connector-javaartifactId>
????<version>5.1.17version>
dependency>
<dependency>
????<groupId>c3p0groupId>
????<artifactId>c3p0artifactId>
????<version>0.9.1.2version>
dependency>
<dependency>
????<groupId>log4jgroupId>
????<artifactId>log4jartifactId>
????<version>1.2.12version>
dependency>
<dependency>
????<groupId>junitgroupId>
????<artifactId>junitartifactId>
????<version>4.10version>
dependency>
<dependency>
????<groupId>dom4jgroupId>
????<artifactId>dom4jartifactId>
????<version>1.6.1version>
dependency>
<dependency>
????<groupId>jaxengroupId>
????<artifactId>jaxenartifactId>
????<version>1.1.6version>
dependency>

創(chuàng)建兩個(gè)JavaBean對(duì)象 用于存儲(chǔ)解析的配置文件的內(nèi)容（Configuration.java、MappedStatement.java）

public?class?Configuration?{

????//?數(shù)據(jù)源
????private?DataSource?dataSource;
????//map集合?key:statementId?value:MappedStatement
????private?Map?mappedStatementMap?=?new?HashMap<>();

????...?省略getter?setter?方法
}

public?class?MappedStatement?{

????//?id
????private?String?id;
????//?sql?語(yǔ)句
????private?String?sql;
????//?參數(shù)值類(lèi)型
????private?Class?paramterType;
????//?返回值類(lèi)型
????private?Class?resultType;

???...?省略getter?setter?方法
}

創(chuàng)建Resources工具類(lèi) 并編寫(xiě)靜態(tài)方法getResourceAsSteam(String path)

public?class?Resources?{

????/**
?????*?根據(jù)配置文件的路徑?將配置文件加載成字節(jié)輸入流?存儲(chǔ)在內(nèi)存中
?????*?@param?path
?????*?@return?InputStream
?????*/
????public?static?InputStream?getResourceAsStream(String?path)?{
????????InputStream?resourceAsStream?=?Resources.class.getClassLoader().getResourceAsStream(path);
????????return?resourceAsStream;
????}
}

2.解析配置文件(使用dom4j)

創(chuàng)建 SqlSessionFactoryBuilder類(lèi) 并添加 build 方法

public?class?SqlSessionFactoryBuilder?{

????public?SqlSessionFactory?build?(InputStream?in)?throws?DocumentException,?PropertyVetoException,?ClassNotFoundException?{
????????//?1.?使用?dom4j?解析配置文件?將解析出來(lái)的內(nèi)容封裝到Configuration中
????????XMLConfigerBuilder?xmlConfigerBuilder?=?new?XMLConfigerBuilder();

????????//?configuration?是已經(jīng)封裝好了sql信息和數(shù)據(jù)庫(kù)信息的對(duì)象
????????Configuration?configuration?=?xmlConfigerBuilder.parseConfig(in);

????????//?2.?創(chuàng)建?SqlSessionFactory?對(duì)象??工廠類(lèi)?主要是生產(chǎn)sqlSession會(huì)話對(duì)象
????????DefaultSqlSessionFactory?defaultSqlSessionFactory?=?new?DefaultSqlSessionFactory(configuration);

????????return?defaultSqlSessionFactory;
????}
}

public?class?XMLConfigerBuilder?{

????private?Configuration?configuration;

????public?XMLConfigerBuilder()?{
????????this.configuration?=?new?Configuration();
????}

????/**
?????*?該方法?使用dom4j對(duì)配置文件進(jìn)行解析?封裝Configuration
?????*?@param?in
?????*?@return
?????*/
?????public?Configuration?parseConfig?(InputStream?in)?throws?DocumentException,?PropertyVetoException,?ClassNotFoundException?{
?????????Document?document?=?new?SAXReader().read(in);
?????????//?
?????????Element?rootElement?=?document.getRootElement();
?????????List?propertyElements?=?rootElement.selectNodes("http://property");
?????????Properties?properties?=?new?Properties();
?????????for?(Element?propertyElement?:?propertyElements)?{
?????????????properties.setProperty(propertyElement.attributeValue("name"),?propertyElement.attributeValue("value"));
?????????}
?????????//?連接池
?????????ComboPooledDataSource?comboPooledDataSource?=?new?ComboPooledDataSource();
?????????comboPooledDataSource.setDriverClass(properties.getProperty("driverClass"));
?????????comboPooledDataSource.setJdbcUrl(properties.getProperty("jdbcUrl"));
?????????comboPooledDataSource.setUser(properties.getProperty("user"));
?????????comboPooledDataSource.setPassword(properties.getProperty("password"));

?????????//?填充?configuration
?????????configuration.setDataSource(comboPooledDataSource);

?????????//?mapper?部分??拿到路徑?->?字節(jié)輸入流?->?dom4j進(jìn)行解析
?????????List?mapperElements?=?rootElement.selectNodes("http://mapper");
?????????XMLMapperBuilder?xmlMapperBuilder?=?new?XMLMapperBuilder(configuration);
?????????for?(Element?mapperElement?:?mapperElements)?{
?????????????String?mapperPath?=?mapperElement.attributeValue("resource");
?????????????InputStream?resourceAsStream?=?Resources.getResourceAsStream(mapperPath);
?????????????xmlMapperBuilder.parse(resourceAsStream);
?????????}

?????????return?configuration;
?????}
}

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("http://select");
????????for?(Element?element?:?select)?{
????????????//?獲取?id?的值
????????????String?id?=?element.attributeValue("id");
????????????String?paramterType?=?element.attributeValue("paramterType");
????????????String?resultType?=?element.attributeValue("resultType");
????????????//?輸入?yún)?shù)?class
????????????Class?paramterTypeClass?=?getClassType(paramterType);
????????????//?返回結(jié)果?class
????????????Class?resultTypeClass?=?getClassType(resultType);
????????????//?sql?語(yǔ)句
????????????String?sqlStr?=?element.getTextTrim();

????????????//?封裝?mappedStatement
????????????MappedStatement?mappedStatement?=?new?MappedStatement();
????????????mappedStatement.setId(id);
????????????mappedStatement.setParamterType(paramterTypeClass);
????????????mappedStatement.setResultType(resultTypeClass);
????????????mappedStatement.setSql(sqlStr);

????????????//?statementId
????????????String?key?=?namespace?+?"."?+?id;
????????????//?填充?configuration
????????????configuration.getMappedStatementMap().put(key,?mappedStatement);
????????}

????}

????private?Class?getClassType(String?paramterType)?throws?ClassNotFoundException?{
????????Class?aClass?=?Class.forName(paramterType);
????????return?aClass;
????}
}

3.創(chuàng)建 SqlSessionFactory 接口及實(shí)現(xiàn)類(lèi)DefaultSqlSessionFactory

public?interface?SqlSessionFactory?{
????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);
????}
}

4. 創(chuàng)建 SqlSession接口及實(shí)現(xiàn)類(lèi)DefaultSqlSession

public?interface?SqlSession?{

?????List?selectList(String?statementId,?Object...?param)?throws?Exception;

?????T?selectOne(String?statementId,?Object...?params)?throws?Exception;

????void?close()?throws?SQLException;
???
}

public?class?DefaultSqlSession?implements?SqlSession?{

????private?Configuration?configuration;

????//?處理器對(duì)象
????private?Executor?simpleExcutor?=?new?SimpleExecutor();

????public?DefaultSqlSession(Configuration?configuration)?{
????????this.configuration?=?configuration;
????}

????@Override
????public??List?selectList(String?statementId,?Object...?param)?throws?Exception?{
????????//?完成對(duì)?simpleExcutor里的query方法的調(diào)用
????????MappedStatement?mappedStatement?=?configuration.getMappedStatementMap().get(statementId);
????????List?list?=?simpleExcutor.query(configuration,?mappedStatement,?param);
????????return?list;
????}

????@Override
????public??T?selectOne(String?statementId,?Object...?params)?throws?Exception?{
????????List