MyBatis（三） xml文件解析流程 动态SQL解析

1.MyBatis将整个系统串联起来的就是Configure对象这个需要牢记。在前面MyBatis xml文件解析流程(二) Mapper解析中介绍了Mapper的解析入口，以及Mapper文件中各个节点的解析方法。里面有几个类需要总结下

BaseBuilder.png

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1.BaseBuilder:作为其他Builder类的基类
2.XMLConfigBuilder:在Configuration解析时介绍过，主要用来解析config配置文件下的Configuration节点，内部会使用XMLMapperBuilder用于解析xml文件
3.XMLMapperBuilder:在Mapper解析中也介绍过。主要用来解析Mapper文件的，里面用，内部会使用XMLStatementBuilder来处理节点
4.XMLStatementBuilder:解析select|insert|update|delete节点，内部会使用XMLScriptBuilder解析xml节点
5.XMLScriptBuilder:解析sql中各个其他的节点并把解析结果保存到SqlNode中。
6.MapperBuilderAssistant:这是mapper解析中的关键，他作为mapper解析的助理类，负责将解析出来的结果保存下来，并且通过configuration.addMappedStatement方法保存到conguration对象中，最终在调用dao是根据传入的参数动态生成sql。所以说configuration是MyBatis调用的调配中心。

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2.动态sql解析流程分析
2.1 从XMLMapperBuilder类中configurationElement方法是mapper文件解析开始。

private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (namespace.equals("")) {
          throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      cacheRefElement(context.evalNode("cache-ref"));
      cacheElement(context.evalNode("cache"));
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      sqlElement(context.evalNodes("/mapper/sql"));
//sql insert|select|update|delete节点解析入口
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
    }
  }

2.2调用buildStatementFromContext方法,将配置文件中的insert|select|update|delete保存到list集合中，并且通过buildStatementFromContext方法进行循环解析所有节点

 private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
      buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
  }

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

2.3此处XMLStatementBuilder(configuration, builderAssistant, context,requiredDatabaseId)引出来XMLStatementBuilder ， MapperBuilderAssistant类，上面介绍过这两个类的作用，**XMLStatementBuilder **的parseStatementNode方法负责即解析sql节点。

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

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

    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultType = context.getStringAttribute("resultType");
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    Class<?> resultTypeClass = resolveClass(resultType);
    String resultSetType = context.getStringAttribute("resultSetType");
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

    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);

    // Include Fragments before parsing
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // Parse selectKey after includes,
    // in case if IncompleteElementException (issue #291)
    List<XNode> selectKeyNodes = context.evalNodes("selectKey");
    if (configuration.getDatabaseId() != null) {
      parseSelectKeyNodes(id, selectKeyNodes, parameterTypeClass, langDriver, configuration.getDatabaseId());
    }
    parseSelectKeyNodes(id, selectKeyNodes, parameterTypeClass, langDriver, null);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    String resultSets = context.getStringAttribute("resultSets");
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? new Jdbc3KeyGenerator() : new NoKeyGenerator();
    }
//MapperBuilderAssistant将解析信息保存并且与configuration关联起来，在后面调用mapper接口时会根据这里保存的信息，以及用户传入的参数来进行动态生成sql后面会说到。
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered, 
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

2.4 其中 SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
很重要，我们看下这个SqlSouse是如何生成的。

sqlsource.png

langDriver我们看其子类XMLLanguageDriver。在XMLLanguageDriver使用XMLScriptBuilder并传入了Configuration ，以及XNode 节点parameterType

public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
    XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script);
    return builder.parseScriptNode();
  }

2.5XMLScriptBuilder的parseScriptNode方法

public SqlSource parseScriptNode() {
//进行解析节点并返回sqlnode集合
    List<SqlNode> contents = parseDynamicTags(context);
//MixedSqlNode 负责对所有不同的SqlNode进行遍历并且调用他们的apply方法此时并没有调用只是将其保存到sqlSource 中，等待将来用户调用时执行MixedSqlNode apply方法
    MixedSqlNode rootSqlNode = new MixedSqlNode(contents);
//返回的是动态DynamicSqlSource
    SqlSource sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
    return sqlSource;
  }

2.6 SqlNode介绍

SqlNode.png

如上图所示所有的“sqlNode”都是实现自SqlNode接口并实现它的apply方法
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1.TrimSqlNode:insert|update|select|delete，mapper配置下的trim节点对象
2.IfSqlNode:insert|update|select|delete，mapper配置下的if节点对象
3.ChooseSqlNode:insert|update|select|delete，mapper配置下的Choose节点对象
4.ForEachSqlNode:insert|update|select|delete，mapper配置下的ForEach节点对象
5.WhereSqlNode，SetSqlNode集成自trim节点，其实这两个完全可以用trim的形式写出来。等等。

2.7XMLScriptBuilder parseDynamicTags方法

private List<SqlNode> parseDynamicTags(XNode node) {
    List<SqlNode> contents = new ArrayList<SqlNode>();
    NodeList children = node.getNode().getChildNodes();
    for (int i = 0; i < children.getLength(); i++) {
      XNode child = node.newXNode(children.item(i));
      String nodeName = child.getNode().getNodeName();
//如果节点类型是CDATA类型或者是TEXT_NODE则创建TextNode
      if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE
          || child.getNode().getNodeType() == Node.TEXT_NODE) {
        String data = child.getStringBody("");
        contents.add(new TextSqlNode(data));
      } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE && !"selectKey".equals(nodeName)) { // issue #628
//如果是ELEMENT_NODE 类型则根据nodeName,根据nodename获取不同的NodeHandler
        NodeHandler handler = nodeHandlers.get(nodeName);
        if (handler == null) {
          throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
        }
        handler.handleNode(child, contents);
      }
    }
    return contents;
  }

获取不同的NodeHandler

private Map<String, NodeHandler> nodeHandlers = new HashMap<String, NodeHandler>() {
    private static final long serialVersionUID = 7123056019193266281L;

    {
      put("trim", new TrimHandler());
      put("where", new WhereHandler());
      put("set", new SetHandler());
      put("foreach", new ForEachHandler());
      put("if", new IfHandler());
      put("choose", new ChooseHandler());
      put("when", new IfHandler());
      put("otherwise", new OtherwiseHandler());
      put("bind", new BindHandler());
    }
  };

以上就完成了sql的解析操作
2.8 介绍MixedSqlNode ，其中apply用于遍历所有sqlnode节点

  public MixedSqlNode(List<SqlNode> contents) {
    this.contents = contents;
  }

  public boolean apply(DynamicContext context) {
    for (SqlNode sqlNode : contents) {
      sqlNode.apply(context);
    }
    return true;
  }