(3)一起来看下使用mybatis框架的select语句的源码执行流程吧

本文是作者原创,版权归作者所有.若要转载,请注明出处.本文以简单的select语句为例,只贴我觉得比较重要的源码,其他不重要非关键的就不贴了

主流程和insert语句差不多,这里主要讲不同的流程,前面配置解析就不看了

SqlSession sqlSession = sqlSessionFactory.openSession();
    //通过动态代理实现接口 ,用动态代理对象去帮我们执行SQL
    //这里生成mapper实际类型是org.apache.ibatis.binding.MapperProxy
    DemoMapper mapper = sqlSession.getMapper(DemoMapper.class);
    //这里用生成的动态代理对象执行
    String projId="0124569b738e405fb20b68bfef37f487";
    String sectionName="标段";
    List<ProjInfo> projInfos = mapper.selectAll(projId, sectionName);
    System.out.println(projInfos);
    //sqlSession.commit();
    sqlSession.close();

点进去,看下方法sqlSession.getMapper(DemoMapper.class),这个调到下面这个方法

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //这里就是动态代理生成的代理对象
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

我们看下mapperProxyFactory.newInstance(sqlSession)这个方法

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

继续点进去看

protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

好,很明显了,这里用的jdk的动态代理,关于动态代理,这里就不赘述了,下次有机会在设计模式里专门讲一下.所以这里返回的是代理对象

继续往下看,我们看mapper.selectAll(projId, sectionName)这个方法debug进去,调的是jdk动态代理的invoke方法,里面就是具体的查询逻辑了

//这里method是接口DemoMapper的selectAll方法
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      // 如果目标方法是Object类继承来的，直接调用目标方法
      //method.getDeclaringClass()是接口DemoMapper的类对象,这里结果为false,跳过这一步
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
        //这里method是接口DemoMapper的selectAll方法,所以结果也为false,跳过这一步
      } else if (method.isDefault()) {
        if (privateLookupInMethod == null) {
          return invokeDefaultMethodJava8(proxy, method, args);
        } else {
          return invokeDefaultMethodJava9(proxy, method, args);
        }
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //这里将method放入缓存
    // 从缓存中获取MapperMethod 对象,如果没有就创建新的并添加
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //这是真正的执行方法
    return mapperMethod.execute(sqlSession, args);
  }

缓存的东西,我们下次再说,我们继续点进去看

//就是这个执行方法
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //command.getType()此时是select
    switch (command.getType()) {
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      //进入这个分支
      case SELECT:
        //这里结果为false,不进这里
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
          //这里结果为真,进这里
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
          //这里结果为false,不进这里
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
          //这里结果为false,不进这里
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          //
          Object param = method.convertArgsToSqlCommandParam(args);
          //处理参数完成后后param是hashmap类型,key有两种,一种是#{}里面的参数名,
          // 另一种是(param1, param2, ...),value只有一种,就是我们的实际参数
          //这里执行sql得到查询结果result,跟进去看下
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional()
              && (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = Optional.ofNullable(result);
          }
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        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;
  }

继续往下看

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
  List<E> result;
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    result = sqlSession.selectList(command.getName(), param, rowBounds);
  } else {
    //进这里,点进去看
    result = sqlSession.selectList(command.getName(), param);
  }
  // issue #510 Collections & arrays support
  if (!method.getReturnType().isAssignableFrom(result.getClass())) {
    if (method.getReturnType().isArray()) {
      return convertToArray(result);
    } else {
      return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
    }
  }
  return result;
}

继续

//这个是真正的sql执行方法了,statement是具体的方法名com.lusaisai.dao.DemoMapper.selectOne
  //parameter是参数名和真是的参数
  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //configuration对象根据statement,得到关于sql语句的相关信息
      //这里得到的ms包含sql语句
      MappedStatement ms = configuration.getMappedStatement(statement);
      //这里就是执行sql语句
      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();
    }
  }

继续跟

//执行语句这个方法,跟进去看下
  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    //这个跟进去看戏,应该是真正的jdbc操作了
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

继续

@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    //前面是缓存处理跟进去
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

这里继续

@Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //进这里,继续跟
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

继续

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    //key值就是待执行的sql
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      //跟进去
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

继续

@Override
  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();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

跟一下return那行

@Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    //这里强转成预编译对象PreparedStatement
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.handleResultSets(ps);
  }

好,我们看到PreparedStatement对象了,继续

public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
 
    final List<Object> multipleResults = new ArrayList<>();
 
    int resultSetCount = 0;
    ResultSetWrapper rsw = getFirstResultSet(stmt);
 
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      handleResultSet(rsw, resultMap, multipleResults, null);
      //这里已经执行完sql,生成结果集了
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }
 
    String[] resultSets = mappedStatement.getResultSets();
    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);
  }

继续跟我注释的那行

private ResultSetWrapper getNextResultSet(Statement stmt) {
    // Making this method tolerant of bad JDBC drivers
    try {
      if (stmt.getConnection().getMetaData().supportsMultipleResultSets()) {
        // Crazy Standard JDBC way of determining if there are more results
        //这里的stmt已经包含可执行的sql了
        if (!(!stmt.getMoreResults() && stmt.getUpdateCount() == -1)) {
          ResultSet rs = stmt.getResultSet();
          if (rs == null) {
            return getNextResultSet(stmt);
          } else {
            return new ResultSetWrapper(rs, configuration);
          }
        }
      }
    } catch (Exception e) {
      // Intentionally ignored.
    }
    return null;
  }

最后再贴张网上找的图

好,到这里我们已经看完执行流程,其实封装了很多层,并没有全都要搞懂的必要,这里的重点是用了动态代理,底层肯定是jdbc操作,当然它还加了很多缓存和各种验证的代码.关于缓存的源码,有机会下次再看吧.