mybatis系列-foreach 解析

在 org.apache.ibatis.builder.xml.XMLConfigBuilder#parseConfiguration 中进行配置解析，其中这一行就是解析 mappers

mapperElement(root.evalNode("mappers"));

具体的代码会执行到这

private void mapperElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
      if ("package".equals(child.getName())) {
        // 这里解析的不是 package
        String mapperPackage = child.getStringAttribute("name");
        configuration.addMappers(mapperPackage);
      } else {
        // 根据 resource 和 url 还有 mapperClass 判断
        String resource = child.getStringAttribute("resource");
        String url = child.getStringAttribute("url");
        String mapperClass = child.getStringAttribute("class");
        // resource 不为空其他为空的情况，就开始将 resource 读成输入流
        if (resource != null && url == null && mapperClass == null) {
          ErrorContext.instance().resource(resource);
          try(InputStream inputStream = Resources.getResourceAsStream(resource)) {
            // 初始化 XMLMapperBuilder 来解析 mapper
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          }

然后再是 parse 过程

public void parse() {
  if (!configuration.isResourceLoaded(resource)) {
    // 解析 mapper 节点，也就是下图中的mapper
    configurationElement(parser.evalNode("/mapper"));
    configuration.addLoadedResource(resource);
    bindMapperForNamespace();
  }

  parsePendingResultMaps();
  parsePendingCacheRefs();
  parsePendingStatements();
}

继续往下走

private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.isEmpty()) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      // 处理cache 和 cache 应用
      cacheRefElement(context.evalNode("cache-ref"));
      cacheElement(context.evalNode("cache"));
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      sqlElement(context.evalNodes("/mapper/sql"));
      // 因为我们是个 sql 查询，所以具体逻辑是在这里面
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

private void buildStatementFromContext(List<XNode> list) {
  if (configuration.getDatabaseId() != null) {
    buildStatementFromContext(list, configuration.getDatabaseId());
  }
  // 然后没有 databaseId 就走到这
  buildStatementFromContext(list, null);
}

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
  for (XNode context : list) {
    // 创建语句解析器
    final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
    try {
      // 解析节点
      statementParser.parseStatementNode();
    } catch (IncompleteElementException e) {
      configuration.addIncompleteStatement(statementParser);
    }
  }
}

这个代码比较长，做下简略，只保留相关代码

public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }

    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);


    // 简略前后代码，主要看这里，创建 sqlSource

    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    

然后根据 LanguageDriver，我们这用的 XMLLanguageDriver，先是初始化

  @Override
  public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
    XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script, parameterType);
    return builder.parseScriptNode();
  }
// 初始化有一些逻辑
  public XMLScriptBuilder(Configuration configuration, XNode context, Class<?> parameterType) {
    super(configuration);
    this.context = context;
    this.parameterType = parameterType;
    // 特别是这，我这次特意在 mapper 中加了 foreach，就是为了说下这一块的解析
    initNodeHandlerMap();
  }
// 设置各种类型的处理器
  private void initNodeHandlerMap() {
    nodeHandlerMap.put("trim", new TrimHandler());
    nodeHandlerMap.put("where", new WhereHandler());
    nodeHandlerMap.put("set", new SetHandler());
    nodeHandlerMap.put("foreach", new ForEachHandler());
    nodeHandlerMap.put("if", new IfHandler());
    nodeHandlerMap.put("choose", new ChooseHandler());
    nodeHandlerMap.put("when", new IfHandler());
    nodeHandlerMap.put("otherwise", new OtherwiseHandler());
    nodeHandlerMap.put("bind", new BindHandler());
  }

初始化解析器以后就开始解析了

public SqlSource parseScriptNode() {
  // 先是解析 parseDynamicTags
  MixedSqlNode rootSqlNode = parseDynamicTags(context);
  SqlSource sqlSource;
  if (isDynamic) {
    sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
  } else {
    sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
  }
  return sqlSource;
}

但是这里可能做的事情比较多

protected MixedSqlNode parseDynamicTags(XNode node) {
    List<SqlNode> contents = new ArrayList<>();
    // 获取子节点，这里可以把我 xml 中的 SELECT 语句分成三部分，第一部分是 select 到 in，然后是 foreach 部分，最后是\n结束符
    NodeList children = node.getNode().getChildNodes();
    for (int i = 0; i < children.getLength(); i++) {
      XNode child = node.newXNode(children.item(i));
      // 第一个节点是个纯 text 节点就会走到这
      if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
        String data = child.getStringBody("");
        TextSqlNode textSqlNode = new TextSqlNode(data);
        if (textSqlNode.isDynamic()) {
          contents.add(textSqlNode);
          isDynamic = true;
        } else {
          // 在 content 中添加这个 node
          contents.add(new StaticTextSqlNode(data));
        }
      } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { // issue #628
        // 第二个节点是个带 foreach 的，是个内部元素节点
        String nodeName = child.getNode().getNodeName();
        // 通过 nodeName 获取处理器
        NodeHandler handler = nodeHandlerMap.get(nodeName);
        if (handler == null) {
          throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
        }
        // 调用处理器来处理
        handler.handleNode(child, contents);
        isDynamic = true;
      }
    }
    // 然后返回这个混合 sql 节点
    return new MixedSqlNode(contents);
  }

再看下 handleNode 的逻辑

    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
      // 又会套娃执行这里的 parseDynamicTags
      MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
      String collection = nodeToHandle.getStringAttribute("collection");
      Boolean nullable = nodeToHandle.getBooleanAttribute("nullable");
      String item = nodeToHandle.getStringAttribute("item");
      String index = nodeToHandle.getStringAttribute("index");
      String open = nodeToHandle.getStringAttribute("open");
      String close = nodeToHandle.getStringAttribute("close");
      String separator = nodeToHandle.getStringAttribute("separator");
      ForEachSqlNode forEachSqlNode = new ForEachSqlNode(configuration, mixedSqlNode, collection, nullable, index, item, open, close, separator);
      targetContents.add(forEachSqlNode);
    }
// 这里走的逻辑不一样了
protected MixedSqlNode parseDynamicTags(XNode node) {
    List<SqlNode> contents = new ArrayList<>();
    // 这里是 foreach 内部的，所以是个 text_node
    NodeList children = node.getNode().getChildNodes();
    for (int i = 0; i < children.getLength(); i++) {
      XNode child = node.newXNode(children.item(i));
      // 第一个节点是个纯 text 节点就会走到这
      if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
        String data = child.getStringBody("");
        TextSqlNode textSqlNode = new TextSqlNode(data);
        // 判断是否动态是根据代码里是否有 ${}
        if (textSqlNode.isDynamic()) {
          contents.add(textSqlNode);
          isDynamic = true;
        } else {
          // 所以还是会走到这
          // 在 content 中添加这个 node
          contents.add(new StaticTextSqlNode(data));
        }
// 最后继续包装成 MixedSqlNode
// 再回到这里
    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
      MixedSqlNode mixedSqlNode = parseDynamicTags(nodeToHandle);
      // 处理 foreach 内部的各个变量
      String collection = nodeToHandle.getStringAttribute("collection");
      Boolean nullable = nodeToHandle.getBooleanAttribute("nullable");
      String item = nodeToHandle.getStringAttribute("item");
      String index = nodeToHandle.getStringAttribute("index");
      String open = nodeToHandle.getStringAttribute("open");
      String close = nodeToHandle.getStringAttribute("close");
      String separator = nodeToHandle.getStringAttribute("separator");
      ForEachSqlNode forEachSqlNode = new ForEachSqlNode(configuration, mixedSqlNode, collection, nullable, index, item, open, close, separator);
      targetContents.add(forEachSqlNode);
    }

public SqlSource parseScriptNode() {
  MixedSqlNode rootSqlNode = parseDynamicTags(context);
  SqlSource sqlSource;
  // 因为在 foreach 节点处理时直接是把 isDynamic 置成了 true
  if (isDynamic) {
    // 所以是个 DynamicSqlSource
    sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
  } else {
    sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
  }
  return sqlSource;
}

这里就做完了预处理工作，真正在执行的执行的时候还需要进一步解析

因为前面讲过很多了，所以直接跳到这里

  @Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    // 都知道是在这进去
    List<T> list = this.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;
    }
  }

  @Override
  public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }
  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    return selectList(statement, parameter, rowBounds, Executor.NO_RESULT_HANDLER);
  }
  private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      // 前面也讲过这个，
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
  // 包括这里，是调用的org.apache.ibatis.executor.CachingExecutor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)
  @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);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }
// 然后是获取 BoundSql
  public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }
// 因为前面讲了是生成的 DynamicSqlSource，所以也是调用这个的 getBoundSql
  @Override
  public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    // 重点关注着
    rootSqlNode.apply(context);
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    context.getBindings().forEach(boundSql::setAdditionalParameter);
    return boundSql;
  }
// 继续是这个 DynamicSqlNode 的 apply
  public boolean apply(DynamicContext context) {
    contents.forEach(node -> node.apply(context));
    return true;
  }
// 看下面的图

我们重点看 foreach 的逻辑

@Override
  public boolean apply(DynamicContext context) {
    Map<String, Object> bindings = context.getBindings();
    final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings,
      Optional.ofNullable(nullable).orElseGet(configuration::isNullableOnForEach));
    if (iterable == null || !iterable.iterator().hasNext()) {
      return true;
    }
    boolean first = true;
    // 开始符号
    applyOpen(context);
    int i = 0;
    for (Object o : iterable) {
      DynamicContext oldContext = context;
      if (first || separator == null) {
        context = new PrefixedContext(context, "");
      } else {
        context = new PrefixedContext(context, separator);
      }
      int uniqueNumber = context.getUniqueNumber();
      // Issue #709
      if (o instanceof Map.Entry) {
        @SuppressWarnings("unchecked")
        Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o;
        applyIndex(context, mapEntry.getKey(), uniqueNumber);
        applyItem(context, mapEntry.getValue(), uniqueNumber);
      } else {
        applyIndex(context, i, uniqueNumber);
        applyItem(context, o, uniqueNumber);
      }
      // 转换变量名，变成这种形式 select * from student where id in
      //   (  
      //  #{__frch_id_0}
      //   )
      contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber));
      if (first) {
        first = !((PrefixedContext) context).isPrefixApplied();
      }
      context = oldContext;
      i++;
    }
    applyClose(context);
    context.getBindings().remove(item);
    context.getBindings().remove(index);
    return true;
  }
// 回到外层就会调用 parse 方法， 把#{} 这段替换成 ?
public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> additionalParameters) {
    ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalParameters);
    GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);
    String sql;
    if (configuration.isShrinkWhitespacesInSql()) {
      sql = parser.parse(removeExtraWhitespaces(originalSql));
    } else {
      sql = parser.parse(originalSql);
    }
    return new StaticSqlSource(configuration, sql, handler.getParameterMappings());
  }

可以看到这里，然后再进行替换

真实的从 ? 替换成具体的变量值，是在这里
org.apache.ibatis.executor.SimpleExecutor#doQuery
调用了

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }
  @Override
  public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }
    @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
            // --------------------------> 
            // 替换变量
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException | SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }