初学Mybatis 构建sqlSession 错误问题 Error building SqlSession Mybatis java.lang.NullPointerException

最新推荐文章于 2023-03-26 12:38:34 发布

原创最新推荐文章于 2023-03-26 12:38:34 发布 · 7.6k 阅读

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#Error building SqlSession. #The error may exist in mapper/ #mybatis java.lang.NullPointerE

这篇博客分享了作者在初学Mybatis时遇到的`Error building SqlSession`问题，错误可能存在于mapper/User.xml中。尝试了删除XML注释、检查resultType和添加别名等方法未果。最终发现是mapper标签缺少namespace属性导致，解决方案是在mapper标签中添加namespace，并在测试代码中相应调整，从而成功运行。

错误信息

org.apache.ibatis.exceptions.PersistenceException:
### Error building SqlSession.
### The error may exist in mapper/User.xml
### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.NullPointerException
   at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26)
   at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:82)
   at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:66)
   at com.mybatis.test.TestMybatis.testQueryById(TestMybatis.java:20)
   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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
   at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
   at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
   at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
   at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
   at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
   at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
   at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
   at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
   at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
   at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
   at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
   at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
   at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
   at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.NullPointerException
   at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:109)
   at org.apache.ibatis.builder.xml.XMLConfigBuilder.parse(XMLConfigBuilder.java:92)
   at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:80)
   ... 25 more
Caused by: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: java.lang.NullPointerException
   at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:120)
   at org.apache.ibatis.builder.xml.XMLMapperBuilder.parse(XMLMapperBuilder.java:92)
   at org.apache.ibatis.builder.xml.XMLConfigBuilder.mapperElement(XMLConfigBuilder.java:322)
   at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:107)
   ... 27 more
Caused by: java.lang.NullPointerException
   at org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XMLMapperBuilder.java:109)
   ... 30 more

初学 mybatis 时遇到的问题，今天拿出来与大家分享一下。错误提示：

### Error building SqlSession.

### The error may exist in mapper/User.xml

提示很清晰，构建sqlSession 失败，错误可能在映射文件 User.xml 中。下面是User.xml 配置，只是一个简单的 select id 查询配置。

Mybatis 核心配置文件：

pojo 类：

测试类：

起初百度查了一些解决方法，有说删除xml中所有注释的，然而没有什么卵用。有说是 resultType 返回类型错误的，但是返回类型写的是user.java 的全路径，没有问题；后来就在 SqlMapConfig.xml 中加了一个别名，修改完成后还是没有什么卵用。万般痛苦的折腾了两个多小时，试了所有网上能查到的方法，还是没用。

正在万般苦恼的时候，突然想起了映射文件中<mapper> 的namespace 属性，抱着试试的态度，给 mapper 加了一个 namespace 属性，在测试代码中，将User user=session.selectOne("queryById",1); 改成了

User user=session.selectOne("test.queryById",1);

结果成功运行。运行结果：