【异常处理】Mybatis报错 source is null for getProperty(null, “length“)

发现问题

      <select id="listArticle" resultType="top.ambtwill.blog.dao.pojo.Article">
            select * from ms_article
            <where>
                1=1
                <if test="categoryId != null">
                    and category_id=#{categoryId}
                </if>
                <if test="tagId != null">
                    and id in (select article_id from ms_article_tag where tag_id = #{tagId})
                </if>
                <if test=" month.length>0  and year.length>0">
                    and year(FROM_UNIXTIME(create_date/1000)) = #{year}
                    and month(FROM_UNIXTIME(create_date/1000)) = #{month}
                </if>
            </where>
            order by weight,create_date desc;
        </select>

    org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.builder.BuilderException: Error evaluating expression 'month.length>0 and year.length>0'. 
    Cause: org.apache.ibatis.ognl.OgnlException: source is null for getProperty(null, "length")
	at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:96)
	at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:441)
	at com.sun.proxy.$Proxy91.selectList(Unknown Source)
	at org.mybatis.spring.SqlSessionTemplate.selectList(SqlSessionTemplate.java:224)
	at com.baomidou.mybatisplus.core.override.MybatisMapperMethod.executeForIPage(MybatisMapperMethod.java:121)
	at com.baomidou.mybatisplus.core.override.MybatisMapperMethod.execute(MybatisMapperMethod.java:85)
	at com.baomidou.mybatisplus.core.override.MybatisMapperProxy$PlainMethodInvoker.invoke(MybatisMapperProxy.java:148)
	at com.baomidou.mybatisplus.core.override.MybatisMapperProxy.invoke(MybatisMapperProxy.java:89)
	at com.sun.proxy.$Proxy93.listArticle(Unknown Source)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:198)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
	at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:215)
	at com.sun.proxy.$Proxy94.listArticle(Unknown Source)
	at top.ambtwill.blog.service.impl.ArticleServiceImpl.listArticle(ArticleServiceImpl.java:59)

解决问题

首先来看这个例子

    @org.junit.Test
    public void tt(){
        String a= null;
        System.out.println(a.length());
    }

控制台报错：java.lang.NullPointerException

也就是说：为Null的字符串调用length()函数会报空指针异常

所以：我们要先判断字符串是否为空

     Cause: org.apache.ibatis.ognl.OgnlException: source is null for getProperty(null, "length")

getProperty(null, "length") 是反射，相当于

    String s=null;
    int ans = s.length();

所以就报错啦，只需要在动态SQL里面判断条件多一个判断非空即可

即

     <if test="month != null and month.length>0 and year != null and year.length>0">