手寫Mybatis-v1.0

原始碼連結（包括v1.0與v2.0）: https://github.com/staticLin/customMyBatis.git

從上一個文章 ---Mybatis概述中瞭解到了Mybatis的主要架構與底層原理流程，結尾給出了一個巨集觀流程圖，可以知道，大致我們可以從三個模組入手：

1. SqlSession：含有屬性Configuration、Excutor，含有方法getMapper，selectOne（先實現一個查詢方法）
2. Configuration：含有MapperRegistry（mapper介面、方法、SQL），含有方法getMapper
3. Executor：含有方法doQuery查詢資料庫

這樣來看，我們的MyBatis1.0的大致脈絡已經出來了。接下來就coding吧~

先是自定義的SqlSession （這裡忽略SqlSessionFactory解析xml資源過程，1.0版本簡化）

/**
 * @description: 自定義的SqlSession
 * @author: linyh
 * @create: 2018-10-31 16:31
 **/
public class CustomSqlSession {

    //持有兩個關鍵物件
    private CustomConfiguration configuration;
    private CustomExecutor executor;

    /**
     * 用構造器將兩個物件形成關係
     */
    public CustomSqlSession(CustomConfiguration configuration, CustomExecutor executor) {
        this.configuration = configuration;
        this.executor = executor;
    }

    public CustomConfiguration getConfiguration() {
        return configuration;
    }

    /**
    * 委派configuration獲取mapper
    */
    public <T> T getMapper(Class<T> clazz){
        return configuration.getMapper(clazz, this);
    }

    /**
     * 委派executor查詢
     */
    public <T> T selectOne(String statement, String parameter){
        return executor.query(statement, parameter);
    }
}
 

然後是自定義Configuration實現getMapper方法，這裡也初始化一個MapperProxyFactory為了存放所有的Mapper。再寫一個驗證Mapper是否存在的方法和根據Class獲取對應mapper的MapperProxyFactory。

/**
 * @description:
 * @author: linyh
 * @create: 2018-10-31 16:32
 **/
public class CustomConfiguration {

    public final MapperRegistory mapperRegistory = new MapperRegistory();

    public static final Map<String, String> mappedStatements = new HashMap<>();

    //TODO 改用anontation掃描 (暫時HardCode)
    //初始化時Configuration載入所有Mapper方法與Sql語句
    public CustomConfiguration() {
        mapperRegistory.addMapper(TestMapper.class);
        mappedStatements.put("com.test.mybatis.v1.mapper.TestCustomMapper.selectByPrimaryKey"
        , "select * from test where id = %d");
    }

    //MapperProxy根據statementName查詢是否有對應SQL
    public boolean hasStatement(String statementName) {
        return mappedStatements.containsKey(statementName);
    }

    //MapperProxy根據statementID獲取SQL
    public String getMappedStatement(String id) {
        return mappedStatements.get(id);
    }

    public <T> T getMapper(Class<T> clazz, CustomSqlSession sqlSession) {
        return mapperRegistory.getMapper(clazz, sqlSession);
    }
}
 

這裡完善MapperProxy與MapperRegistory（註冊所有的Mapper的Map）

/**
 * @description: Mapper動態代理者
 * @author: linyh
 * @create: 2018-10-31 16:52
 **/
public class MapperProxy implements InvocationHandler{

    private CustomSqlSession sqlSession;

    public MapperProxy(CustomSqlSession sqlSession) {
        this.sqlSession = sqlSession;
    }

    /**
     * 每一個Mapper的每個方法都將執行invoke方法，此方法判斷方法名是否維護在Configuration中，如有則取出SQL
    */
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (sqlSession.getConfiguration().hasStatement(method.getDeclaringClass().getName()+"."+method.getName())) {
            String sql = sqlSession.getConfiguration().getMappedStatement(method.getDeclaringClass().getName()+"."+method.getName());
            return sqlSession.selectOne(sql, args[0].toString());
        }
        return method.invoke(proxy, args);
    }
}

/**
 * @description: mapper註冊類
 * @author: linyh
 * @create: 2018-10-31 16:51
 **/
public class MapperRegistory {

    //用一個Map維護所有Mapper
    private final Map<Class<?>, MapperProxyFactory> knownMappers = new HashMap<>();

    //TODO Configuration解析anontation之後呼叫方法初始化所有mapper
    public <T> void addMapper(Class<T> clazz){
        knownMappers.put(clazz, new MapperProxyFactory(clazz));
    }

    /**
     * getMapper最底層執行者，獲取mapper的MapperProxyFactory物件
     */
    public <T> T getMapper(Class<T> clazz, CustomSqlSession sqlSession) {
        MapperProxyFactory proxyFactory = knownMappers.get(clazz);
        if (proxyFactory == null) {
            throw new RuntimeException("Type: " + clazz + " can not find");
        }
        return (T)proxyFactory.newInstance(sqlSession);
    }

    /**
     * 內部類實現一個Factory生成Mapper的代理
     */
    public class MapperProxyFactory<T>{

        private Class<T> mapperInterface;

        public MapperProxyFactory(Class<T> mapperInterface) {
            this.mapperInterface = mapperInterface;
        }

        public T newInstance(CustomSqlSession sqlSession) {
            return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, new MapperProxy(sqlSession));
        }
    }
}

到這裡getMapper就實現好了，每次getMapper都能生成對應的MapperProxy代理。接下來實現Executor的查詢方法。

先定義一個介面（2.0將加入CacheExecutor，面向介面程式設計便於擴充套件）。

/**
 * @description:
 * @author: linyh
 * @create: 2018-10-31 16:32
 **/
public interface CustomExecutor {
    <T> T query(String statement, String parameter);
}

然後是具體的實現類，其中使用JDBC查詢。

/**
 * @description: 自定義Executor
 * @author: linyh
 * @create: 2018-10-31 17:46
 **/
public class CustomDefaultExecutor implements CustomExecutor{

    @Override
    public <T> T query(String statement, String parameter) {
        Connection conn = null;
        PreparedStatement preparedStatement = null;
        Test test = null;
        try {
            conn = getConnection();

            //TODO ParameterHandler
            preparedStatement = conn.prepareStatement(String.format(statement, Integer.parseInt(parameter)));
            preparedStatement.execute();
            ResultSet rs = preparedStatement.getResultSet();

            //TODO ObjectFactory
            test = new Test();

            //TODO ResultSetHandler
            while (rs.next()) {
                test.setId(rs.getInt(1));
                test.setNums(rs.getInt(2));
                test.setName(rs.getString(3));
            }

        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return (T)test;
    }

    public Connection getConnection() throws SQLException {
        String driver = "com.mysql.jdbc.Driver";
        String url = "jdbc:mysql://127.0.0.1:3306/gp?serverTimezone=UTC";
        String username = "root";
        String password = "admin";
        Connection conn = null;
        try {
            Class.forName(driver); //classLoader,載入對應驅動
            conn = DriverManager.getConnection(url, username, password);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }
}

這樣查詢方法也大致完成了，建立實體與Mapper，然後開始測試一下吧~

/**
 * @description: 實體類
 * @author: linyh
 * @create: 2018-10-31 17:03
 **/
public class Test {

    private Integer id;
    private Integer nums;
    private String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getNums() {
        return nums;
    }

    public void setNums(Integer nums) {
        this.nums = nums;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Test{" +
                "id=" + id +
                ", nums=" + nums +
                ", name='" + name + '\'' +
                '}';
    }
}

/**
 * @Author:linyh
 * @Date: 2018/10/31 16:56
 * @Modified By:
 */
public interface TestCustomMapper {
    Test selectByPrimaryKey(int id);
}

/**
 * @description:
 * @author: linyh
 * @create: 2018-10-31 18:05
 **/
public class TestMybatis {
    public static void main(String[] args) {
        CustomSqlSession sqlSession = new CustomSqlSession(
                new CustomConfiguration(), new CustomDefaultExecutor());
        TestCustomMapper testCustomMapper = sqlSession.getMapper(TestCustomMapper.class);
        System.out.println(testCustomMapper.selectByPrimaryKey(2));
    }
}

控制檯列印：

這樣就完成了極簡版MyBatisv1.0了，其中還有很多不足之處，我用了TODO標記，下面總結幾點不足之處，統計2.0需要改進與增加的功能。

不足之處

1. 解析Mapper資訊時用了HardCode，需改用anontation方式去掃描Mapper與對應SQL語句。
2. Executor不夠單一職責，它不僅負責引數裝配還負責查詢語句加上結果集對映，不合理，需要細分職責。

增加的功能

1. 新增anontation功能動態掃描Mapper類，去掉HardCode。
2. 新增StatementHandler設定引數以及負責查詢工作。
3. 新增ObjectFactory動態建立實體類。
4. 新增ResultSetHandler將結果集對映到實體類中。
5. 增加一個Plugin功能。
6. 增加CacheExecutor裝飾者來為查詢增加一個快取功能。