mapper method attempted to return null from a method with a primitive return type (int)
一、场景和背景
工具：mybatis + pgsql
dao代码：
int selectMaxAgeBySex(String sex); 查询性别是女生的最大年龄
sql:
<select id = "selectMaxAgeBySex" resultType="int">select max(age) from user where sex = '女'；
</select>
二、错误原因
如果数据表user中没有性别是女的用户数据，那么pgsql返回的函数max值是null，此时把null赋值给了resultType=“int”，报错信息：
mapper method attempted to return null from a method with a primitive return type (int)
三、解决
pgsql数据库的max返回值有两种，null和数值。
当返回的数据是nu l l空对象时,即意味着把null赋值给int。如果用程序实现将会报空指针异常，例如：
package com.test.boke;public class TestJava {public static void main(String[] args) {Integer integer = null;int num = integer;System.out.println("num = " + num);}
}
console输出：
Exception in thread "main" java.lang.NullPointerException
at com.test.boke.TestJava.main(TestJava.java:6)
所以不可以使用resultType="int"作为返回类型。正确的代码是：
dao代码：
Integer selectMaxAgeBySex(String sex); 查询性别是女生的最大年龄
sql:
<select id = "selectMaxAgeBySex" resultType="java.lang.Integer">select max(age) from user where sex = '女'；
</select>
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