在做mybatis案例的时候发现了一个问题，报错如下：

org.apache.ibatis.exceptions.PersistenceException:
### Error querying database.  Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for org.dao.UserMapper.count
### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for org.dao.UserMapper.count
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:23)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:107)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:98)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:62)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:57)
    at org.test.UserMapperTest.test(UserMapperTest.java:55)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.junit.runners.BlockJUnit4ClassRunner.runNotIgnored(BlockJUnit4ClassRunner.java:79)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:71)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for org.dao.UserMapper.count
    at org.apache.ibatis.session.Configuration$StrictMap.get(Configuration.java:775)
    at org.apache.ibatis.session.Configuration.getMappedStatement(Configuration.java:615)
    at org.apache.ibatis.session.Configuration.getMappedStatement(Configuration.java:608)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:103)
    ... 28 more

首先一看到错，对于英语不好的我来说，脑海里瞬间闪过一句话：TMD，这是什么鬼东西。。。然后仔细的看了看，看到了个关键字collection ，于是想想是不是没有连接上数据库，错误原因要么在数据库资源文件里面，要么就在UserMapper.xml里面。回去看了看资源文件：

driver=com.mysql.jdbc.Driver
url=jdbc\:mysql\://localhost\:3306/smbms?useUnicode\=true&characterEncoding\=utf-8
user=root
password=123

没毛病，那就是在UserMapper.xml里面，于是跳转到UserMapper.xml里面看了看，还真在这里面，注意看下面的内容

 

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="org.dao.UserMapper.xml"><!-- 查询用户表的记录数 --><select id="count" resultType="int">select count(*) from user</select>
</mapper>

注意看着一行代码：

 

 

<mapper namespace="org.dao.UserMapper.xml">

我迷糊的居然在后面加了个.xml,于是乎把它去掉就可以了！，正确的映射文件如下：

 

 

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="org.dao.UserMapper"><!-- 查询用户表的记录数 --><select id="count" resultType="int">select count(*) from user</select>
</mapper>

问题完美解决！