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.Testpublic 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">
)
