背景：

在项目开发中，每个mapper xml都存在大量重复的基础CRUD操作，如 selectById、deleteById 等。这些模板化的SQL语句不仅增加了开发工作量，还容易出现不一致的问题。

解决方案：

通过动态解析和修改Mybatis XML文件，自动注入缺失的通用SQL语句，实现代码复用和标准化。

核心实现：

1.设置注入后的xml

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@Bean
    public SqlSessionFactory sqlSessionFactoryBean(DataSource dataSource) throws Exception {
        SqlSessionFactoryBean sessionFactory = new SqlSessionFactoryBean();
        sessionFactory.setDataSource(dataSource);
        List<Resource> resources = new ArrayList<>();
        for (Resource resource : mybatisProperties.resolveMapperLocations()){
            resources.add(MyBatisXmlProcessor.parse(resource));
        }
        sessionFactory.setMapperLocations(resources.toArray(new Resource[0]));
        return sessionFactory.getObject();
    }

2.预定义通用SQL模板，XML解析与语句注入

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@Slf4j
public class MyBatisXmlProcessor {
    private static final Map<String, String> DEFAULT_STATEMENTS = new HashMap<>();

    static {
        DEFAULT_STATEMENTS.put("selectById",
                "<select id=\"selectById\" resultMap=\"BaseResultMap\">\n" +
                        "    select * from <include refid=\"BaseTable\"/> where id = #{id}\n" +
                        "</select>");
    }

    public static Resource parse(Resource resource) {
        try {

            // 获取所有已存在的语句ID
            Set<String> existingIds = getExistingStatementIds(resource);

            // 检查哪些默认语句需要添加
            Map<String, String> statementsToAdd = DEFAULT_STATEMENTS.entrySet().stream().filter(entry -> !existingIds.contains(entry.getKey())).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

            // 如果没有需要添加的语句，直接返回原内容
            if (statementsToAdd.isEmpty()) {
                return resource;
            }

            // 在</mapper>前插入新语句
            String modifiedXml = insertStatementsBeforeMapperEnd(resource, statementsToAdd);

            log.info("Modified XML:\n{}", modifiedXml);

            return new ByteArrayResource(modifiedXml.getBytes(StandardCharsets.UTF_8));
        } catch (Exception e) {
            throw new RuntimeException("Failed to parse and modify MyBatis XML", e);
        }
    }

    /**
     * 获取XML中所有已存在的语句ID
     */
    private static Set<String> getExistingStatementIds(Resource resource) throws ParserConfigurationException, IOException, SAXException {

        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = factory.newDocumentBuilder();

        // 解析XML
        Document document = builder.parse(resource.getInputStream());
        Element root = document.getDocumentElement();

        if (!"mapper".equals(root.getTagName())) {
            throw new IllegalArgumentException("Invalid MyBatis XML: root element is not <mapper>");
        }

        NodeList children = root.getChildNodes();

        return IntStream.range(0, children.getLength())
                .mapToObj(children::item)
                .filter(node -> node.getNodeType() == Node.ELEMENT_NODE)
                .map(node -> ((Element) node).getAttribute("id"))
                .filter(id -> id != null && !id.isEmpty())
                .collect(Collectors.toSet());
    }

    /**
     * 在</mapper>前插入语句
     */
    private static String insertStatementsBeforeMapperEnd(Resource resource, Map<String, String> statementsToAdd) throws IOException {
        String xmlContent = StreamUtils.copyToString(resource.getInputStream(), StandardCharsets.UTF_8);
        int mapperEndIndex = xmlContent.lastIndexOf("</mapper>");
        if (mapperEndIndex == -1) {
            throw new IllegalArgumentException("Invalid MyBatis XML: </mapper> tag not found");
        }

        StringBuilder modifiedXml = new StringBuilder();

        // 提取</mapper>前的部分
        modifiedXml.append(xmlContent, 0, mapperEndIndex);

        // 添加新语句
        statementsToAdd.values().forEach(o-> modifiedXml.append(o).append("\n"));

        modifiedXml.append(xmlContent.substring(mapperEndIndex));

        return modifiedXml.toString();
    }
}