这是我参与8月更文挑战的第20天，活动详情查看：8月更文挑战

时序图

sequenceDiagram
participant A as XMLMapperBuilder
participant B as XMLStatementBuilder
participant C as XMLLanguageDriver
participant D as MapperBuilderAssistant

A ->> A : buildStatementFromContext
A ->> B : parseStatementNode
B ->> C : createSqlSource
C -->> B : SqlSource
B ->> D : addMappedStatement

详细步骤

XMLMapperBuilder#configurationElement

/**
 * 解析映射文件的下层节点
 * @param context 映射文件根节点
 */
private void configurationElement(XNode context) {
    try {
        // 读取当前映射文件namespace
        String namespace = context.getStringAttribute("namespace");
        if (namespace == null || namespace.equals("")) {
            throw new BuilderException("Mapper's namespace cannot be empty");
        }
        builderAssistant.setCurrentNamespace(namespace);
        // 映射文件中其他配置节点的解析
        // 解析缓存标签
        cacheRefElement(context.evalNode("cache-ref"));
        cacheElement(context.evalNode("cache"));
        // 解析参数映射
        parameterMapElement(context.evalNodes("/mapper/parameterMap"));
        // 解析结果集映射
        resultMapElements(context.evalNodes("/mapper/resultMap"));
        // 解析sql标签
        sqlElement(context.evalNodes("/mapper/sql"));
        // 处理各个数据库操作语句
        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);
    }
}

XMLMapperBuilder#buildStatementFromContext

private void buildStatementFromContext(List<XNode> list) {
    // 全局databaseId属性不为空
    if (configuration.getDatabaseId() != null) {
        buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
}

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
        // 创建解析对象
        final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
        try {
            // 解析
            statementParser.parseStatementNode();
        } catch (IncompleteElementException e) {
            // 将未处理完成的放入容器中，后续统一进行重试
            configuration.addIncompleteStatement(statementParser);
        }
    }
}

XMLStatementBuilder#parseStatementNode

/**
 * 解析select、insert、update、delete这四类节点
 */
public void parseStatementNode() {
    // 读取当前节点的id与databaseId
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    // 验证id与databaseId是否匹配。MyBatis允许多数据库配置，因此有些语句只对特定数据库生效
    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
        return;
    }

    // 读取节点名称
    String nodeName = context.getNode().getNodeName();
    // 读取和判断语句类型
    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);

    // 处理语句中的Include节点
    // Include Fragments before parsing
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // 参数类型
    String parameterType = context.getStringAttribute("parameterType");
    /**
     * 1. 根据属性值去别名映射器中查找对应的类
     * 2. 如果别名映射器中没有，则创建此类名对应的Class对象
     */
    Class<?> parameterTypeClass = resolveClass(parameterType);

    // 语句类型，默认值是XMLLanguageDriver（在Configuration默认构造方法中进行创建）
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    // 处理SelectKey节点，在这里会将KeyGenerator加入到Configuration.keyGenerators中
    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // 此时，<selectKey> 和 <include> 节点均已被解析完毕并被删除，开始进行SQL解析
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    // 判断是否已经有解析好的KeyGenerator
    if (configuration.hasKeyGenerator(keyStatementId)) {
        keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
        // 全局或者本语句只要启用自动key生成，则使用key生成
        keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
                configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
                ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

    // 根据获取的LanguageDriver来创建SqlSource
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    // 读取各个配置属性
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String resultType = context.getStringAttribute("resultType");
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultSetType = context.getStringAttribute("resultSetType");
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    if (resultSetTypeEnum == null) {
        resultSetTypeEnum = configuration.getDefaultResultSetType();
    }
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    String resultSets = context.getStringAttribute("resultSets");

    // 在MapperBuilderAssistant的帮助下创建MappedStatement对象，并写入到Configuration中
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
            fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
            resultSetTypeEnum, flushCache, useCache, resultOrdered,
            keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}

以上便是Mybatis解析select|insert|update|delete标签的流程。