(六)Mybatis Sql如何解析执行(2)--MyBatis源码解析

上篇博客讲解了如何解析sql,不过SqlNode.apply()后，Sql还是个半成品。只处理了"${}"这种占位符，"#{}"这种占位符还没有处理，而且Sql执行时的参数也没有生成。

我们再回顾下DynamicSqlSource.getBoundSql()方法

public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    //这里只处理了"${}"占位符
    rootSqlNode.apply(context);
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> parameterType = parameterObject == null ? Object.class : 
    parameterObject.getClass();
    //这里就是处理"#{}"占位符的地方
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, 
    context.getBindings());
    //这个BoundSql就是数据库可执行的Sql,同时还包含了运行时的参数。
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {
      boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());
    }
    return boundSql;
}

sqlSourceParser.parse()方法，处理方式跟前面处理"${}"占位符的基本一致。

public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> 
                                                                additionalParameters) {
    //处理占位符的handler
    ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, 
                                        parameterType, additionalParameters);
    //要处理什么样的占位符
    GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);
    //开始处理
    String sql = parser.parse(originalSql); 
    //这个SqlSource就是一个简单的java对象
    return new StaticSqlSource(configuration, sql, handler.getParameterMappings());
}

再来看ParameterMappingTokenHandler是怎么处理占位符的

 public String handleToken(String content) {
      //从参数中获取具体的值，并加入parameterMappings中
      parameterMappings.add(buildParameterMapping(content));
     //直接替换成一个"?"
     //这里可以看到有多少个"#{}"占位符，就会生成对应个"?",同时还会生成对应的parameterMappings
      return "?";
    }
 
    private ParameterMapping buildParameterMapping(String content) {
      //这里content可以是这样子的:#{height,javaType=double,jdbcType=NUMERIC,numericScale=2}
      //parseParameterMapping()就是把这种复杂的复杂解析成Map方式
      Map<String, String> propertiesMap = parseParameterMapping(content);
      String property = propertiesMap.get("property");
      //解析参数的类型,String,int or boolean ...
      Class<?> propertyType;
      if (metaParameters.hasGetter(property)) { // issue #448 get type from additional params
        //在这里大部分的应该都能确定下来
        propertyType = metaParameters.getGetterType(property);
      } else if (typeHandlerRegistry.hasTypeHandler(parameterType)) {
        propertyType = parameterType;
      } else if (JdbcType.CURSOR.name().equals(propertiesMap.get("jdbcType"))) {
        propertyType = java.sql.ResultSet.class;
      } else if (property != null) {
        MetaClass metaClass = MetaClass.forClass(parameterType);
        if (metaClass.hasGetter(property)) {
          propertyType = metaClass.getGetterType(property);
        } else {
          propertyType = Object.class;
        }
      } else {
        propertyType = Object.class;
      }
      //构建一个ParameterMapping对象，ParameterMapping描述的是java对象的属性与sql执行参数的对应关系。跟ResultMapping对象差不多
      ParameterMapping.Builder builder = new ParameterMapping.Builder(configuration, property, propertyType);
      Class<?> javaType = propertyType;
      String typeHandlerAlias = null;
      for (Map.Entry<String, String> entry : propertiesMap.entrySet()) {
        String name = entry.getKey();
        String value = entry.getValue();
        //示例:#{height,javaType=double,jdbcType=NUMERIC,numericScale=2}
        if ("javaType".equals(name)) {
          javaType = resolveClass(value);
          builder.javaType(javaType);
        } else if ("jdbcType".equals(name)) {
          builder.jdbcType(resolveJdbcType(value));
        } else if ("mode".equals(name)) {
          builder.mode(resolveParameterMode(value));
        } else if ("numericScale".equals(name)) {
          builder.numericScale(Integer.valueOf(value));
        } else if ("resultMap".equals(name)) {
          builder.resultMapId(value);
        } else if ("typeHandler".equals(name)) {
          typeHandlerAlias = value;
        } else if ("jdbcTypeName".equals(name)) {
          builder.jdbcTypeName(value);
        } else if ("property".equals(name)) {
          // Do Nothing
        } else if ("expression".equals(name)) {
          throw new BuilderException("Expression based parameters are not supported yet");
        } else {
          throw new BuilderException("An invalid property '" + name + "' was found in 
            mapping #{" + content + "}.  Valid properties are " + parameterProperties);
        }
      }
      if (typeHandlerAlias != null) {
        builder.typeHandler(resolveTypeHandler(javaType, typeHandlerAlias));
      }
      return builder.build();
    }

至此，SqlSession在执行sql里已经生成了BoundSql：可执行的sql及相应的参数。下一步应该就是直接操作数据库了，如生成Statement，执行参数等等