[mybatis]org.apache.ibatis.exceptions.PersistenceException: ### Error querying database. 异常解决

跟着狂神学mybatis 建立第一个程序就遇到了一直解决不了的问题。。。不停百度查找，也一直没解决问题，今天终于在csdn找到了解决方法
参考链接：https://blog.csdn.net/zks_1998/article/details/103327519?spm=1001.2014.3001.5501

遇到的问题：
**org.apache.ibatis.exceptions.PersistenceException: **
### Error querying database. Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet successfully received from the server was 513 milliseconds ago. The last packet sent successfully to the server was 509 milliseconds ago.
### The error may exist in com/young/dao/UserMapper.xml
### The error may involve com.young.dao.UserDao.getUserList
### The error occurred while executing a query
### Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet successfully received from the server was 525 milliseconds ago. The last packet sent successfully to the server was 521 milliseconds ago.
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:149)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140)
at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:147)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:80)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:57)
at com.sun.proxy.$Proxy4.getUserList(Unknown Source)
at com.young.dao.UserDaoTest.test(UserDaoTest.java:16)
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.inteal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.inteal.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$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:220)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:53)
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet successfully received from the server was 525 milliseconds ago. The last packet sent successfully to the server was 521 milliseconds ago.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:990)
at com.mysql.jdbc.ExportControlled.transformSocketToSSLSocket(ExportControlled.java:201)
at com.mysql.jdbc.MysqlIO.negotiateSSLConnection(MysqlIO.java:4914)
at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1663)
at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1224)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2199)
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2230)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2025)
at com.mysql.jdbc.ConnectionImpl. (ConnectionImpl.java:778)
at com.mysql.jdbc.JDBC4Connection.(JDBC4Connection.java:47)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:386)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:330)
at java.sql.DriverManager.getConnection(DriverManager.java:664)
at java.sql.DriverManager.getConnection(DriverManager.java:208)
at org.apache.ibatis.datasource.unpooled.UnpooledDataSource.doGetConnection(UnpooledDataSource.java:221)
at org.apache.ibatis.datasource.unpooled.UnpooledDataSource.doGetConnection(UnpooledDataSource.java:216)
at org.apache.ibatis.datasource.unpooled.UnpooledDataSource.getConnection(UnpooledDataSource.java:95)
at org.apache.ibatis.datasource.pooled.PooledDataSource.popConnection(PooledDataSource.java:422)
at org.apache.ibatis.datasource.pooled.PooledDataSource.getConnection(PooledDataSource.java:89)
at org.apache.ibatis.transaction.jdbc.JdbcTransaction.openConnection(JdbcTransaction.java:138)
at org.apache.ibatis.transaction.jdbc.JdbcTransaction.getConnection(JdbcTransaction.java:60)
at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:336)
at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:85)
at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:62)
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324)
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147)
... 28 more
Caused by: javax.net.ssl.SSLHandshakeException: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Path does not chain with any of the trust anchors
at sun.security.ssl.Alert.createSSLException(Alert.java:131)
at sun.security.ssl.TransportContext.fatal(TransportContext.java:353)
at sun.security.ssl.TransportContext.fatal(TransportContext.java:296)
at sun.security.ssl.TransportContext.fatal(TransportContext.java:291)
at sun.security.ssl.CertificateMessage$T12CertificateConsumer.checkServerCerts(CertificateMessage.java:652)
at sun.security.ssl.CertificateMessage$T12CertificateConsumer.onCertificate(CertificateMessage.java:471)
at sun.security.ssl.CertificateMessage$T12CertificateConsumer.consume(CertificateMessage.java:367)
at sun.security.ssl.SSLHandshake.consume(SSLHandshake.java:376)
at sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:444)
at sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:422)
at sun.security.ssl.TransportContext.dispatch(TransportContext.java:183)
at sun.security.ssl.SSLTransport.decode(SSLTransport.java:154)
at sun.security.ssl.SSLSocketImpl.decode(SSLSocketImpl.java:1279)
at sun.security.ssl.SSLSocketImpl.readHandshakeRecord(SSLSocketImpl.java:1188)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:401)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:373)
at com.mysql.jdbc.ExportControlled.transformSocketToSSLSocket(ExportControlled.java:186)
... 60 more
Caused by: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Path does not chain with any of the trust anchors
at com.mysql.jdbc.ExportControlled$X509TrustManagerWrapper.checkServerTrusted(ExportControlled.java:302)
at sun.security.ssl.AbstractTrustManagerWrapper.checkServerTrusted(SSLContextImpl.java:1255)
at sun.security.ssl.CertificateMessage$T12CertificateConsumer.checkServerCerts(CertificateMessage.java:636)
... 72 more
Caused by: java.security.cert.CertPathValidatorException: Path does not chain with any of the trust anchors
at sun.security.provider.certpath.PKIXCertPathValidator.validate(PKIXCertPathValidator.java:159)
at sun.security.provider.certpath.PKIXCertPathValidator.engineValidate(PKIXCertPathValidator.java:85)
at java.security.cert.CertPathValidator.validate(CertPathValidator.java:292)
at com.mysql.jdbc.ExportControlled$X509TrustManagerWrapper.checkServerTrusted(ExportControlled.java:295)
... 74 more

Process finished with exit code -1

Mybaitis核心配置文件mybatis-config.xml 中的数据库url的value只保留这一部分<propertyname="url"value="jdbc:mysql://localhost:3306/mybatis?>
然后就可以显示数据库的数据了!!!可是它还是有一个警告:

**Mon Apr 19 14:39:39 CST 2021 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification. **
翻译：不建议在没有服务器身份验证的情况下建立SSL连接。根据MySQL 5.5.45+、5.6.26+和5.7.6+的要求，如果不设置显式选项，则必须建立默认的SSL连接。需要通过设置useSSL=false来显式禁用SSL，或者设置useSSL=true并为服务器证书验证提供信任存储。

出现这个警告的原因：mysql在高版本中需要指明是否进行SSL连接
解决方案如下：
在mysql连接字符串url中加入ssl=true或者false即可，如下所示。
<property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&amp;userUnicode=true&amp;characterEncoding=UTF-8"/>

SSL协议提供服务主要：
1）认证用户服务器，确保数据发送到正确的服务器； 　　 .
2）加密数据，防止数据传输途中被窃取使用；
3）维护数据完整性，验证数据在传输过程中是否丢失；
当前支持SSL协议两层：
SSL记录协议（SSL Record Protocol）：建立靠传输协议（TCP）高层协议提供数据封装、压缩、加密等基本功能支持
SSL握手协议（SSL Handshake Protocol）：建立SSL记录协议用于实际数据传输始前通讯双进行身份认证、协商加密
算法、 交换加密密钥等。

所以建议设置useSSL为false，有时遇到的问题可以这样来考虑