JAVA-MyBatis 當(dāng)傳入?yún)?shù)為Integer的0時(shí),動(dòng)態(tài)sql解析為空

之前開發(fā)時(shí)遇到前臺(tái)傳來參數(shù)為0 時(shí),使用mybatis拼接動(dòng)態(tài)sql時(shí)將0解析為空從而跳過if判斷參數(shù)是否為空,無法拼接限制條件的問題,現(xiàn)在深入研究問題的原因

• 現(xiàn)象:

  當(dāng)mybatis的xml文件中使用動(dòng)態(tài)sql的if判段,如下:

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

  warningId為Integer的0時(shí),if判斷會(huì)為false

• 表面原因:

  mybatis會(huì)將0解析為 '' 空 warningId!='' 為false跳過拼接條件

• 解決方法:

  前臺(tái)處理:將傳入?yún)?shù)改為字符串的0

  后臺(tái)處理:判斷條件加上等于0的時(shí)候,如

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

• 原理及源碼:

  查詢資料以及查看mybatis源碼,搞清楚mybatis解析xml文件時(shí)對(duì)if元素的處理

  1. 當(dāng)mybatis遍歷節(jié)點(diǎn)時(shí),遇到節(jié)點(diǎn)類型為元素,會(huì)使用NodeHandler處理各個(gè)類型的元素,NodeHandler是XMLScriptBuilder的一個(gè)內(nèi)部接口鳖轰，其實(shí)現(xiàn)類包括TrimHandler、WhereHandler焰轻、SetHandler昆雀、IfHandler、ChooseHandler,處理后,會(huì)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. 當(dāng)處理完所有節(jié)點(diǎn)后,開始構(gòu)造sql

       @Override
       public BoundSql getBoundSql(Object parameterObject) {
         DynamicContext context = new DynamicContext(configuration, parameterObject);
         rootSqlNode.apply(context);//此處調(diào)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);該處調(diào)用sql的apply方法,然后查看IfSqlNode的結(jié)構(gòu):

     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方法對(duì)表達(dá)式進(jìn)行判斷,evaluator是一個(gè)ExpressionEvaluator 類型,它解析表達(dá)式使用的是OGNL,對(duì)象導(dǎo)航圖語(yǔ)言（Object Graph Navigation Language），簡(jiǎn)稱OGNL已球，是應(yīng)用于Java中的一個(gè)開源的表達(dá)式語(yǔ)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;  
       }  
     
     

     查詢OGNL的官方文檔,

     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.

       其中說明,如果對(duì)象是一個(gè)Number的類型,值為0時(shí)將被解析為false智亮，否則為true,即'' == 0 == false