解密Mybatis,手写Mybatis框架（二）

简化版Mybatis实现思路

• 1.创建SqlSessionFactory实例.
• 2.实例化过程中，加载配置文件创建configuration对象.
• 3.通过factory创建SqlSession对象,把configuaration传入SqlSession.
• 4.通过SqlSession获取mapper接口动态代理
• 5.通过代理对调sqlsession中查询方法;
• 6.sqlsession将查询方法转发给executor;
• 7.executor基于JDBC访问数据库获取数据;
• 8.executor通过反射将数据转换成POJO并返回给sqlsession;
• 9.数据返回给调用者

上节讲到快速入门mybatis的demo三大阶段

// 1.读取mybatis配置文件创SqlSessionFactory
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
inputStream.close();
//-------------第二阶段-------------
// 2.获取sqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
// 3.获取对应mapper
TUserMapper mapper = sqlSession.getMapper(TUserMapper.class);

//-------------第三阶段-------------
// 4.执行查询语句并返回结果
TUser user = mapper.selectByPrimaryKey(1);
System.out.println(user.toString());

第一阶段：

第一阶段先把配置文件加载到内存，包括数据库信息和mapper.xml。

针对mapper.xml我们定义一个MappedStatement类来存入相应信息.

public class MappedStatement {
    //此处忽略getset方法
    private String namespace;
    
    private String sourceId;//mapper接口路径+xml里面的每一个id
    
    private String sql;//sql语句
    
    private String resultType;//返回类型    

}

再定义一个全局配置信息即Configuration存放所有配置信息：

public class Configuration {
    //记录mapper xml文件存放的位置
    public static final String MAPPER_CONFIG_LOCATION = "config";
    //记录数据库连接信息文件存放位置
    public static final String DB_CONFIG_FILE = "db.properties";
    
    private String dbUrl;

    private String dbUserName;

    private String dbPassword;

    private String dbDriver;

    //mapper xml解析完以后select节点的信息存放在mappedStatements,key为MappedStatement里面 
     //的sourceId
    protected final Map<String, MappedStatement> mappedStatements = new HashMap<String, MappedStatement>();
    
    //为mapper接口生成动态代理的方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return MapperProxyFactory.getMapperProxy(sqlSession, type);
}
}

SqlSessionFactory实例化，并加载configuaration对象信息,这样就把所有的配置信息加载到内存里

public class SqlSessionFactory {
    //配置对象全局唯一 加载数据库信息和mapper文件信息
    private Configuration conf = new Configuration();

    public SqlSessionFactory() {
        //加载数据库信息
         loadDbInfo();
         //加载mapper文件信息
         loadMappersInfo();
    }
    
    private void loadMappersInfo() {
        URL resources =null;
        resources = SqlSessionFactory.class.getClassLoader().getResource(conf.MAPPER_CONFIG_LOCATION);
        File mappers = new File(resources.getFile());
        if(mappers.isDirectory()){
            File[] listFiles = mappers.listFiles();
            for (File file : listFiles) {
                loadMapperInfo(file);
            }
        }
    }

    private void loadMapperInfo(File file) {
        // 创建saxReader对象  
        SAXReader reader = new SAXReader();  
        // 通过read方法读取一个文件 转换成Document对象  
        Document document=null;
        try {
            document = reader.read(file);
        } catch (DocumentException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }  
        //获取根节点元素对象  
        Element node = document.getRootElement();
        //获取命名空间
        String namespace = node.attribute("namespace").getData().toString();
        //获取select子节点列表
        List<Element> selects = node.elements("select");
        for (Element element : selects) {//遍历select节点，将信息记录到MappedStatement对象，并登记到configuration对象中
            MappedStatement mappedStatement = new MappedStatement();
            String id = element.attribute("id").getData().toString();
            String resultType = element.attribute("resultType").getData().toString();
            String sql = element.getData().toString();
            String sourceId = namespace+"."+id;
            mappedStatement.setSourceId(sourceId);
            mappedStatement.setResultType(resultType);
            mappedStatement.setSql(sql);
            mappedStatement.setNamespace(namespace);
            conf.getMappedStatements().put(sourceId, mappedStatement);//登记到configuration对象中
        }
    }

    private void loadDbInfo() {
        InputStream dbIn = SqlSessionFactory.class.getClassLoader().getResourceAsStream(conf.DB_CONFIG_FILE);
         Properties p = new Properties();
         try {
            p.load(dbIn);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
         conf.setDbDriver(p.get("jdbc.driver").toString());
         conf.setDbPassword(p.get("jdbc.password").toString());
         conf.setDbUrl(p.get("jdbc.url").toString());
         conf.setDbUserName(p.get("jdbc.username").toString());
    }
    public SqlSession openSession(){
        SqlSession sqlSession  = new DefaultSqlSession(conf);
        return sqlSession;
    }
}

第二阶段

第二阶段为获取Sqlsession并且从sqlsession获取mapper动态代理.

Sqlsession

• mybatis暴露给外部的接口，实现增删改查的能力
• 1.对外提供数据访问的api
• 2.对内将请求转发给executor

• 3.executor基于JDBC访问数据库

  public class DefaultSqlSession implements SqlSession {
         
         //配置对象全局唯一 加载数据库信息和mapper文件信息
         private Configuration conf;
         
         //真正提供数据库访问能力的对象
         private Executor executor;
         
     
         public DefaultSqlSession(Configuration conf) {
             super();
             this.conf = conf;
             executor = new SimpleExecutor(conf);
         }
     
         public <T> T selectOne(String statement, Object parameter) {
             List<Object> selectList = this.selectList(statement, parameter);
             if(selectList==null||selectList.size()==0){
                 return null;
             }
             if(selectList.size()==1){
                 return (T) selectList.get(0);
             }else {
                 throw new RuntimeException("Too Many Result!");
             }
     
         }
     
         public <E> List<E> selectList(String statement, Object parameter) {
             MappedStatement mappedStatement = conf.getMappedStatement(statement);
             try {
                 return executor.query(mappedStatement, parameter);
             } catch (SQLException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
             return null;
         }
     
           @Override
           //获取当前mapper接口的动态代理
           public <T> T getMapper(Class<T> type) {
             return conf.<T>getMapper(type, this);
           }
     
     }

  Executor是Mybatis核心接口定义了数据库操作的基本方法,Sqlsession都是基于它来实现的

  public interface Executor {
  
  
   <E> List<E> query(MappedStatement ms, Object parameter) throws SQLException;
  
  
   <T> T selectOne(String statement,Object parameter);

  }

  Executor实现类:

public class SimpleExecutor implements Executor {

        
        private Configuration conf;
    
    
        public SimpleExecutor(Configuration conf) {
            this.conf = conf;
        }

        public <E> List<E> query(MappedStatement ms, Object parameter)
                throws SQLException {
            //获取mappedStatement对象，里面包含 sql 语句和目标对象等信息；
            MappedStatement mappedStatement = conf.getMappedStatement(ms.getSourceId());
            //1.获取Connection对象
            Connection conn = getConnect();
            //2.实例化StatementHandler对象，准备实例化Statement
            StatementHandler statementHandler = new DefaultStatementHandler(mappedStatement);
            //3.通过statementHandler和Connection获取PreparedStatement
            PreparedStatement prepare = statementHandler.prepare(conn);
            //4.实例化ParameterHandler对象，对Statement中sql语句的占位符进行处理
            ParameterHandler parameterHandler = new DefaultParameterHandler(parameter);
            parameterHandler.setParameters(prepare);
            //5.执行查询语句，获取结果集resultSet
            ResultSet resultSet = statementHandler.query(prepare);
            //6.实例化ResultSetHandler对象，对resultSet中的结果集进行处理，转化成目标对象
            ResultSetHandler resultSetHandler = new DefaultResultSetHandler(mappedStatement);
            return resultSetHandler.handleResultSets(resultSet);
        }
    
        @Override
        public <T> T selectOne(String statement, Object parameter) {
            MappedStatement mappedStatement =conf.getMappedStatements().get(statement);
            return null;
        }
    
    
        private Connection getConnect() {
            Connection conn =null;
            try {
                Class.forName(conf.getDbDriver());
                conn = DriverManager.getConnection(conf.getDbUrl(), conf.getDbUserName(), conf.getDbPassword());    
            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return conn;
        }
    
        public Configuration getConf() {
            return conf;
        }
    
        public void setConf(Configuration conf) {
            this.conf = conf;
        }
        
    }

mapper接口在我们工程里面没有实现类，是通过动态代理来执行方法的.

/**
 * mapper接口生成动态代理的工程类
 * 
 */
public class MapperProxyFactory<T> {

  
  public static <T> T getMapperProxy(SqlSession sqlSession,Class<T> mapperInterface){
      MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface);
      return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

InvocationHandler实现类：

public class MapperProxy<T> implements InvocationHandler {

    private SqlSession sqlSession;
    
    private final Class<T> mapperInterface;
    

    public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface) {
        super();
        this.sqlSession = sqlSession;
        this.mapperInterface = mapperInterface;
    }

    private <T> boolean isCollection(Class<T> type) {
        return Collection.class.isAssignableFrom(type);
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args)
            throws Throwable {
        if (Object.class.equals(method.getDeclaringClass())) {// 如果是Object本身的方法不增强
            return method.invoke(this, args);
        }

        Class<?> returnType = method.getReturnType();// 获取方法的返回参数class对象
        Object ret = null;
        if (isCollection(returnType)) {// 根据不同的返回参数类型调用不同的sqlsession不同的方法
            ret = sqlSession.selectList(mapperInterface.getName()+"."+ method.getName(), args);
        } else {
            ret = sqlSession.selectOne(mapperInterface.getName()+"."+ method.getName(), args);
        }

        return ret;
    }

}

第三阶段

第三阶段执行查询并返回结果.刚刚讲过我们执行数据库操作实际上是executor基于jdbc执行的。

jdbc三大巨头,Connection,PreparedStatement,ResultSet,

结果集 Result 再通过反射机制映射到对象上面，便做好了数据的映射(关于映射具体内容可查阅资料及源码),到这我们已经完成了一个简易的Mybatis框架了.

通过手写一个简单的Mybatis框架，我们就可以看得懂源码了，学习框架设计的思路并且增强我们 Java 的内功.