在做mybatis案例的時候發現了一個問題，報錯如下：

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

首先一看到錯，對于英語不好的我來說，腦海里瞬間閃過一句話：TMD，這是什么鬼東西。。。然后仔細的看了看，看到了個關鍵字collection ，于是想想是不是沒有連接上數據庫，錯誤原因要么在數據庫資源文件里面，要么就在UserMapper.xml里面。回去看了看資源文件：

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

沒毛病，那就是在UserMapper.xml里面，于是跳轉到UserMapper.xml里面看了看，還真在這里面，注意看下面的內容

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<?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"><!-- 查詢用戶表的記錄數 --><select id="count" resultType="int">select count(*) from user</select> </mapper>

注意看著一行代碼：

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

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

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<?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"><!-- 查詢用戶表的記錄數 --><select id="count" resultType="int">select count(*) from user</select> </mapper>

問題完美解決！

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