MyBatis 添加元数据自定义元素标签

 现有系统中维护了一套业务表相关列、键的元数据，希望通过读取元数据实现自动封装 SQL 语句、自定义主键策略。实现方案为入侵式修改 MyBatis，增加元素标签 meta ，支持业务开发中可以在XML映射文件中使用。

meta元素设计如下：

<!-- meta标签 可根据参数获取到对应的表名 动态生成语句 -->
<!ELEMENT meta EMPTY>
<!ATTLIST meta
test CDATA #IMPLIED
type (update|insert|select|columns|pk-col|load|load-columns) #IMPLIED
ignore CDATA #IMPLIED
table CDATA #IMPLIED
func CDATA #IMPLIED
alias CDATA #IMPLIED
>
复制代码

期望示例如下：

<insert id="insertMap" useGeneratedKeys="true" generator="meta">
    <meta table="USER" type="insert"/>
</insert>

<update id="updateMap">
    <meta table="USER" type="update"/>
</update>

<select id="selectOneByPk" resultType="java.util.HashMap">
    select 
    <meta table="USER" type="columns"/> 
    from USER 
    where <meta table="USER" type="pk-col"/> = #{__PK_VALUE}
</select>
复制代码

开发准备

 新建项目并引入 mybatis 、 mybatis-spring 两个核心依赖。

<!-- mybatis -->
<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
</dependency>
<!-- mybatis-spring -->
<dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis-spring</artifactId>
</dependency>
复制代码

增加自定义元素

创建 MetaHandler 和 MetaSqlNode

public class MetaHandler implements NodeHandler {

    private final CustomConfiguration configuration;

    protected MetaHandler(CustomConfiguration configuration) {
        this.configuration = configuration;
    }

    @Override
    public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
        final String test = nodeToHandle.getStringAttribute("test");
        final String type = nodeToHandle.getStringAttribute("type");
        final String ignore = nodeToHandle.getStringAttribute("ignore");
        final String table = nodeToHandle.getStringAttribute("table");
        final String func = nodeToHandle.getStringAttribute("func");
        String alias = nodeToHandle.getStringAttribute("alias");
        if (!StringUtils.isEmpty(alias)) {
            alias = alias.trim();
            //是否无效 防止注入
            boolean invalid = alias.contains(" ") || alias.contains(".");
            if (invalid) {
                throw new RuntimeException("alias is invalid : " + alias);
            }
        }
        MetaSqlNode metaSqlNode = new MetaSqlNode(configuration, test, type, ignore, table, func, alias);
        targetContents.add(metaSqlNode);
    }
}
复制代码

public class MetaSqlNode implements SqlNode {

    /**
     * mybatis核心数据
     */
    private final CustomConfiguration configuration;
    /**
     * 判断语句校验器
     */
    private final ExpressionEvaluator evaluator;
    /**
     * 判断语句,同if标签
     */
    private final String test;
    /**
     * 生成语句类型 update|insert|select|columns|pk-col|load|load-columns
     */
    private final TypeEnum type;
    /**
     * 忽略的列
     */
    private final String ignore;
    /**
     * 表名，未指定则从调用参数中获取
     */
    private final String table;
    /**
     * 功能，未指定则从调用参数中获取
     */
    private final String func;
    /**
     * 动态列别名
     */
    private final String alias;

    public MetaSqlNode(CustomConfiguration configuration, String test, String type, String ignore, String table, String func, String alias) {
        this.evaluator = new ExpressionEvaluator();
        this.configuration = configuration;
        this.test = test;
        this.type = TypeEnum.parse(type);
        this.ignore = ignore;
        this.table = table;
        this.func = func;
        this.alias = alias;
    }

    @Override
    public boolean apply(DynamicContext context) {
        // TODO 解析type与table，向context中添加语句
        context.appendSql(" insert ······  ");
    }
}
复制代码

创建 CustomXMLScriptBuilder

 内容复制自 org.apache.ibatis.scripting.xmltags.XMLScriptBuilder ，在 initNodeHandlerMap 方法中添加 MetaHandler。

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());
    //增加元数据标签解析器
    if (configuration instanceof CustomConfiguration) {
        nodeHandlerMap.put("meta", new MetaHandler((CustomConfiguration) configuration));
    }
}
复制代码

创建 CustomXMLLanguageDriver

 内容复制自 org.apache.ibatis.scripting.xmltags.XMLLanguageDriver ，在 createSqlSource 方法中使用 CustomXMLScriptBuilder 来解析Xml生成 SqlSource。

@Override
public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
    CustomXMLScriptBuilder builder = new CustomXMLScriptBuilder(configuration, script, parameterType);
    return builder.parseScriptNode();
}
复制代码

创建 CustomConfiguration

 继承 org.apache.ibatis.session.Configuration ，内容复制自 Configuration。将构造方法中的 XMLLanguageDriver 修改为 CustomConfiguration。

public CustomConfiguration() {
    
    ······
    
    //默认使用自定义 LanguageDriver
    typeAliasRegistry.registerAlias("XML", CustomXMLLanguageDriver.class);
    
    ······
    
    //默认使用自定义 LanguageDriver
    languageRegistry.setDefaultDriverClass(CustomXMLLanguageDriver.class);
    
    ······
    
}
复制代码

创建 CustomXMLConfigBuilder

 内容复制自 org.apache.ibatis.builder.xml.XMLConfigBuilder ，支持通过 XML 配置来创建 CustomConfiguration。

public class CustomXMLConfigBuilder extends BaseBuilder {
    
    ······
    
    private CustomXMLConfigBuilder(XPathParser parser, String environment, Properties props) {
        // 使用 CustomConfiguration
        super(new CustomConfiguration());
        ErrorContext.instance().resource("SQL Mapper Configuration");
        this.configuration.setVariables(props);
        this.parsed = false;
        this.environment = environment;
        this.parser = parser;
    }
    
    ······
    
}
复制代码

创建 SqlSessionFactory

 复制自 org.mybatis.spring.SqlSessionFactoryBean ，将 buildSqlSessionFactory 方法中的 Configuration 替换为 CustomConfiguration。

protected SqlSessionFactory buildSqlSessionFactory() throws Exception {

    final Configuration targetConfiguration;

    CustomXMLConfigBuilder xmlConfigBuilder = null;
    if (this.configuration != null) {
        targetConfiguration = this.configuration;
        if (targetConfiguration.getVariables() == null) {
            targetConfiguration.setVariables(this.configurationProperties);
        } else if (this.configurationProperties != null) {
            targetConfiguration.getVariables().putAll(this.configurationProperties);
        }
    } else if (this.configLocation != null) {
        // 使用 CustomXMLConfigBuilder 创建 CustomConfiguration
        xmlConfigBuilder = new CustomXMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
        targetConfiguration = xmlConfigBuilder.getConfiguration();
    } else {
        LOGGER.debug(
                () -> "Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration");
        // 使用 CustomConfiguration
        targetConfiguration = new CustomConfiguration();
        Optional.ofNullable(this.configurationProperties).ifPresent(targetConfiguration::setVariables);
    }
    
    ······
    
    return this.sqlSessionFactoryBuilder.build(targetConfiguration);
}
复制代码

修改DTD约束

 MyBatis 的约束文件并不支持自定义的 meta 元素，需要使用 CDATA 来处理。示例如下：

<insert id="insertMap" useGeneratedKeys="true" generator="meta">
  <![CDATA[[
    <meta table="USER" type="insert"/>
  ]]>
</insert>
复制代码

 如果不想要 CDATA，则需要通过修改DTD约束来完成。

• 在 classes 下指定位置添加DTD约束文件 org/apache/ibatis/builder/xml/mybatis-3-config.dtd 达到覆盖 MyBatis DTD的效果。
• 重写代码来使用指定 DTD 。

创建 CustomXMLMapperEntityResolver

 复制自 org.apache.ibatis.builder.xml.XMLMapperEntityResolver ，将 MYBATIS_MAPPER_DTD 修改为指向本地 mybatis-3-mapper.dtd 文件，并在DTD文件中添加 meta 元素的约束。

public class CustomXMLMapperEntityResolver implements EntityResolver {
    
    ······
    
    private static final String MYBATIS_MAPPER_DTD = "com/my/ibatis/builder/xml/mybatis-3-mapper.dtd";
    
    ······
    
}
复制代码

<!-- meta标签 可根据参数获取到对应的表名 动态生成语句 -->
<!ELEMENT meta EMPTY>
<!ATTLIST meta
test CDATA #IMPLIED
type (update|insert|select|columns|pk-col|load|load-columns) #IMPLIED
ignore CDATA #IMPLIED
table CDATA #IMPLIED
func CDATA #IMPLIED
alias CDATA #IMPLIED
>
复制代码

CustomXMLLanguageDriver

  Mapper动态语句注解处理使用 CustomXMLMapperEntityResolver。

/**
 * Mapper动态语句注解调用
 * <p>
 * "<script>select * from user <if test=/"id !=null /">where id = #{id} </if></script>"
 *
 * @param configuration mybatis配置
 * @param script        动态语句字符串
 * @param parameterType 参数类型
 * @return org.apache.ibatis.mapping.SqlSource
 */
@Override
public SqlSource createSqlSource(Configuration configuration, String script, Class<?> parameterType) {
    // issue #3
    if (script.startsWith("<script>")) {
        //将动态语句字符串转换为XNode对象
        XPathParser parser = new XPathParser(script, false, configuration.getVariables(), new CustomXMLMapperEntityResolver());
        return createSqlSource(configuration, parser.evalNode("/script"), parameterType);
    } else {
        // issue #127
        script = PropertyParser.parse(script, configuration.getVariables());
        TextSqlNode textSqlNode = new TextSqlNode(script);
        if (textSqlNode.isDynamic()) {
            return new CustomDynamicSqlSource(configuration, textSqlNode);
        } else {
            return new RawSqlSource(configuration, script, parameterType);
        }
    }
}
复制代码

创建 CustomXMLMapperBuilder

  复制自 org.apache.ibatis.builder.xml.XMLMapperBuilder ，修改构造函数使用 CustomXMLMapperEntityResolver 解析xml。

public CustomXMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    this(new XPathParser(inputStream, true, configuration.getVariables(), new CustomXMLMapperEntityResolver()),
            configuration, resource, sqlFragments);
}
复制代码

SqlSessionFactory

 修改 buildSqlSessionFactory 方法，使用 CustomXMLMapperBuilder 来解析xml。

protected SqlSessionFactory buildSqlSessionFactory() throws Exception {

    ······
        try {
            //使用自定义 XMLMapperBuilder
            CustomXMLMapperBuilder xmlMapperBuilder = new CustomXMLMapperBuilder(mapperLocation.getInputStream(),
                    targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
            xmlMapperBuilder.parse();
        } catch (Exception e) {
            throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
        } finally {
            ErrorContext.instance().reset();
        }
        
    ······
    
}
复制代码

创建 CustomMapperAnnotationBuilder

 复制自 org.apache.ibatis.builder.annotation.MapperAnnotationBuilder ，修改 loadXmlResource 方法使用 CustomXMLMapperBuilder。

private void loadXmlResource() {
    if (!configuration.isResourceLoaded("namespace:" + type.getName())) {
        
    ······
    
        if (inputStream != null) {
            //使用自定义解析器支持自定义标签
            CustomXMLMapperBuilder xmlParser = new CustomXMLMapperBuilder(inputStream, assistant.getConfiguration(), xmlResource, configuration.getSqlFragments(), type.getName());
            xmlParser.parse();
        }
    }
}
复制代码

创建 CustomMapperRegistry

  复制自 org.apache.ibatis.binding.MapperRegistry ，修改 addMapper 方法使用 CustomMapperAnnotationBuilder。

@Override
public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
        
    ······
    
        try {
            knownMappers.put(type, new MapperProxyFactory<>(type));
            // It's important that the type is added before the parser is run
            // otherwise the binding may automatically be attempted by the
            // mapper parser. If the type is already known, it won't try.
            CustomMapperAnnotationBuilder parser = new CustomMapperAnnotationBuilder(config, type);
            parser.parse();
            loadCompleted = true;
        } finally {
            if (!loadCompleted) {
                knownMappers.remove(type);
            }
        }
    }
}
复制代码

CustomConfiguration

 修改 mapperRegistry 属性使用 CustomMapperRegistry。

public class CustomConfiguration extends Configuration {
        
    ······

    protected final MapperRegistry mapperRegistry = new CustomMapperRegistry(this);
        
    ······
    
}
复制代码

在Spring中使用

<!-- Mybatis SessionFactory-->
<bean id="sqlSessionFactory" class="com.my.ibatis.SqlSessionFactoryBean">
    <property name="dataSource" ref="dataSource" />
    <property name="configurationProperties" >
        <bean class="org.springframework.beans.factory.config.PropertiesFactoryBean">
            <property name="locations" value="classpath*:mybatis.properties"/>
        </bean>
    </property>
</bean>
复制代码

@Configuration
public class MybatisConfig {
    @Bean
    public PropertiesFactoryBean createPropertiesFactoryBean() throws IOException {
        PropertiesFactoryBean bean = new PropertiesFactoryBean();
        bean.setLocation(new ClassPathResource("mybatis.properties"));
        return bean;
    }

    @Bean("sqlSessionFactory")
    public SqlSessionFactoryBean createSqlSessionFactory(DataSource dataSource, PropertiesFactoryBean bean) throws IOException {
        SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
        factoryBean.setDataSource(dataSource);
        factoryBean.setConfigurationProperties(bean.getObject());
        return factoryBean;
    }
}
复制代码