JAVA-MyBatis 当传入参数为Integer的0时,动态sql解析为空

之前开发时遇到前台传来参数为0 时,使用mybatis拼接动态sql时将0解析为空从而跳过if判断参数是否为空,无法拼接限制条件的问题,现在深入研究问题的原因

• 现象:

  当mybatis的xml文件中使用动态sql的if判段,如下:

  <if test="warningId !=null and warningId !=''">
            and l.warning_id = #{warningId}
  </if>
  

  warningId为Integer的0时,if判断会为false

• 表面原因:

  mybatis会将0解析为 '' 空 warningId!='' 为false跳过拼接条件

• 解决方法:

  前台处理:将传入参数改为字符串的0

  后台处理:判断条件加上等于0的时候,如

  <if test="warningId !=null and warningId !='' or warningId == 0 " >
            and l.warning_id = #{warningId}
  </if>
  

• 原理及源码:

  查询资料以及查看mybatis源码,搞清楚mybatis解析xml文件时对if元素的处理

  1. 当mybatis遍历节点时,遇到节点类型为元素,会使用NodeHandler处理各个类型的元素,NodeHandler是XMLScriptBuilder的一个内部接口，其实现类包括TrimHandler、WhereHandler、SetHandler、IfHandler、ChooseHandler,处理后,会生成IfSqlNode

     private class IfHandler implements NodeHandler {
         public void handleNode(XNode nodeToHandle, List<SqlNode> targetContents) {
           List<SqlNode> contents = parseDynamicTags(nodeToHandle);
           MixedSqlNode mixedSqlNode = new MixedSqlNode(contents);
           String test = nodeToHandle.getStringAttribute("test");
           IfSqlNode ifSqlNode = new IfSqlNode(mixedSqlNode, test);
           targetContents.add(ifSqlNode);
         }
     }
     

  2. 当处理完所有节点后,开始构造sql

       @Override
       public BoundSql getBoundSql(Object parameterObject) {
         DynamicContext context = new DynamicContext(configuration, parameterObject);
         rootSqlNode.apply(context);//此处调用sqlnode的apply方法
         SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
         Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
         SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
         BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
         context.getBindings().forEach(boundSql::setAdditionalParameter);
         return boundSql;
       }
     

     rootSqlNode.apply(context);该处调用sql的apply方法,然后查看IfSqlNode的结构:

     public class IfSqlNode implements SqlNode {
       private final ExpressionEvaluator evaluator;
       private final String test;
       private final SqlNode contents;
     
       public IfSqlNode(SqlNode contents, String test) {
         this.test = test;
         this.contents = contents;
         this.evaluator = new ExpressionEvaluator();
       }
     
       @Override
       public boolean apply(DynamicContext context) {
         if (evaluator.evaluateBoolean(test, context.getBindings())) {
           contents.apply(context);
           return true;
         }
         return false;
       }
     
     }
     

     其中appl方法中使用evaluator.evaluateBoolean方法对表达式进行判断,evaluator是一个ExpressionEvaluator 类型,它解析表达式使用的是OGNL,对象导航图语言（Object Graph Navigation Language），简称OGNL，是应用于Java中的一个开源的表达式语言.

     public class ExpressionEvaluator {  
       public boolean evaluateBoolean(String expression, Object parameterObject) {  
         Object value = OgnlCache.getValue(expression, parameterObject);  
         if (value instanceof Boolean) return (Boolean) value;  
         if (value instanceof Number) return !new BigDecimal(String.valueOf(value)).equals(BigDecimal.ZERO);  
         return value != null;  
       }  
     
     

     查询OGNL的官方文档,

     Interpreting Objects as Booleans

     Any object can be used where a boolean is required. OGNL interprets objects as booleans like this:

     • If the object is a Boolean, its value is extracted and returned;

     • If the object is a Number, its double-precision floating-point value is compared with zero; non-zero is treated as true, zero as false;

     • If the object is a Character, its boolean value is true if and only if its char value is non-zero;

     • Otherwise, its boolean value is true if and only if it is non-null.

       其中说明,如果对象是一个Number的类型,值为0时将被解析为false，否则为true,即'' == 0 == false