Mybatis源码解析-sql执行

一、传统的jdbc操作步骤

• 获取驱动
• 获取jdbc连接
• 创建参数化预编译的sql
• 绑定参数
• 发送sql到数据库执行
• 将将获取到的结果集返回应用
• 关闭连接

传统的jdbc代码：

package com.zjp;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class JDBCTest {

    public static void main(String[] args) {
        try {
            Connection con = null; //定义一个MYSQL链接对象
            Class.forName("com.mysql.jdbc.Driver").newInstance(); //MYSQL驱动
            con = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mybatis?useUnicode=true", "root", "root"); //链接本地MYSQL

            //更新一条数据
            String updateSql = "UPDATE user_t SET user_name = 'test' WHERE id = ?";
            PreparedStatement pstmt = con.prepareStatement(updateSql);
            pstmt.setString(1, "1");
            long updateRes = pstmt.executeUpdate();
            System.out.print("UPDATE:" + updateRes);

            //查询数据并输出
            String sql = "select * from user_t where id = ?";
            PreparedStatement pstmt2 = con.prepareStatement(sql);
            pstmt2.setString(1, "1");
            ResultSet rs = pstmt2.executeQuery();
            while (rs.next()) { //循环输出结果集
                String id = rs.getString("id");
                String username = rs.getString("user_name");
                System.out.print("\r\n\r\n");
                System.out.print("id:" + id + ",username:" + username);
            }
            //关闭资源
            rs.close();
            pstmt.close();
            pstmt2.close();
            con.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

二、mybatis 执行sql

mybatis要执行sql，同样也需要获取到数据库连接，这个在mybatis里面就是sqlSession

// 使用MyBatis提供的Resources类加载mybatis的配置文件
Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
// 构建sqlSession的工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
SqlSession session = sqlSessionFactory.openSession();

2.1 获取mapper对象

获取到了session对象之后就是获取mapper对象了，在mybatis中是使用动态代理的方式获取

 @Override
  public <T> T getMapper(Class<T> type) {
    //最后会去调用MapperRegistry.getMapper
    return configuration.<T>getMapper(type, this);
  }

 /**
   * 返回代理类
   * @param type
   * @param sqlSession
   * @return
   */
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
      // MapperProxyFactory去把代理类做出来
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
        // 这里就是返回代理对象
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

@SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    //用JDK自带的动态代理生成映射器
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

• 通过session获取mapper，session通过MapperProxyFactory代理对象去获取mapper对象，最终在newInstance中我们看到了使用jdk自带的动态代理方式获取到了mapper对象。

2.2 执行sql语句

现在获取到了mapper对象，那么下一步就是执行sql语句了。

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //代理以后，所有Mapper的方法调用时，都会调用这个invoke方法
    //并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    //这里优化了，去缓存中找MapperMethod
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //执行 sql
    return mapperMethod.execute(sqlSession, args);
  }

所有的sql语句都会调用invoke方法，会后都要调用execute来执行sql语句

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //可以看到执行时就是4种情况，insert|update|delete|select，分别调用SqlSession的4大类方法
    if (SqlCommandType.INSERT == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      if (method.returnsVoid() && method.hasResultHandler()) {
        //如果有结果处理器
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        //如果结果有多条记录
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        //如果结果是map
        result = executeForMap(sqlSession, args);
      } else {
        //否则就是一条记录
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else {
      throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

这个方法也就是通过使用枚举的方式执行insert|update|delete|select，分别执行不同的方法。这里跟着select，这个也是最复杂的。在这里我们看到了很重要的一个方法

result = sqlSession.selectOne(command.getName(), param);

通过源码可知selectOne方法转而去调用selectList,很简单的，如果得到0条则返回null，得到1条则返回1条，得到多条报TooManyResultsException错，特别需要主要的是当没有查询到结果的时候就会返回null。因此一般建议在mapper中编写resultType的时候使用包装类型而不是基本类型，比如推荐使用Integer而不是int。这样就可以避免NPE

 //核心selectOne
  @Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>selectList(statement, parameter);
    if (list.size() == 1) {
      return list.get(0);
    } else if (list.size() > 1) {
      throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
  }

 public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //根据statement id找到对应的MappedStatement
      MappedStatement ms = configuration.getMappedStatement(statement);
      //转而用执行器来查询结果,注意这里传入的ResultHandler是null
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

从selectList方法中可以看出来，最终的查询还是交给了executor

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //得到绑定sql
    BoundSql boundSql = ms.getBoundSql(parameter);
    //创建缓存Key
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    //查询
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

从MappedStatement对象中获取到BoundSql对象，BoundSql对象包含了我们需要执行的sql语句。

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      //新建一个StatementHandler
      //这里看到ResultHandler传入了
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      //准备语句
      stmt = prepareStatement(handler, ms.getStatementLog());
      //StatementHandler.query
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

通过一些列的跟踪，定位到了doQuery方法，最终sql语句的执行交给了StatementHandler对象，这个对象也就是我们最常用的，封装的是PreparedStatement

 @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    // 结果交给了ResultSetHandler 去处理
    return resultSetHandler.<E> handleResultSets(ps);
  }

很明显这里就是使用的PreparedStatement进行处理。

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public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

    final List<Object> multipleResults = new ArrayList<Object>();

    int resultSetCount = 0;
    ResultSetWrapper rsw = getFirstResultSet(stmt);

    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    //一般resultMaps里只有一个元素
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }

    String[] resultSets = mappedStatement.getResulSets();
    if (resultSets != null) {
      while (rsw != null && resultSetCount < resultSets.length) {
        ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
        if (parentMapping != null) {
          String nestedResultMapId = parentMapping.getNestedResultMapId();
          ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
          handleResultSet(rsw, resultMap, null, parentMapping);
        }
        rsw = getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount++;
      }
    }

    return collapseSingleResultList(multipleResults);
  }

最后就是返回结果集了。