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

发现问题

                  select * from ms_article                            1=1                                    and category_id=#{categoryId}                                                    and id in (select article_id from ms_article_tag where tag_id = #{tagId})                                                    and year(FROM_UNIXTIME(create_date/1000)) = #{year}                    and month(FROM_UNIXTIME(create_date/1000)) = #{month}                                        order by weight,create_date desc;        

    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���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