借鉴大佬纠正方法,贴出来防止以后找不到了,转载自:https://blog.csdn.net/u012972315/article/details/51725646

 

 

具体错误:

Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: 
### Error opening session.  Cause: java.lang.NullPointerException
### Cause: java.lang.NullPointerException
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
    at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(DefaultSqlSessionFactory.java:100)
    at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession(DefaultSqlSessionFactory.java:47)
    at com.mybatis.test.UserDao.init(UserDao.java:32)
    at com.mybatis.test.UserDao.main(UserDao.java:41)
Caused by: java.lang.NullPointerException
    at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(DefaultSqlSessionFactory.java:95)
    ... 3 more

 解决办法:< environment default=”“> 该值为下面配置的< environment id=”“>中需要使用的环境的id即可    此代码放在mybatis的xml文件中  具体代码:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration
    PUBLIC "-//mybatis.org/DTD Config 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <environments default="database1">  <!-- default属性选择此次开发时需要的数据库环境的id值即可 (database1、database2、database3) -->
        <environment id="database1">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost/mybatis_test1"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
        <environment id="database2">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost/mybatis_test2"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
        <environment id="database3">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost/mybatis_test3"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/mybatis/map/User.xml"/>
    </mappers>
</configuration>