org.apache.ibatis.exceptions.PersistenceException: ### Error querying database.问题的解决

org.apache.ibatis.exceptions.PersistenceException: 
### Error querying database.  Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
### The error may exist in com/byzhang/dao/IUserDao.xml
### The error may involve com.byzhang.dao.IUserDao.findAll
### The error occurred while executing a query
### Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
  at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
  at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:150)
  at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:141)
  at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:139)
  at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:76)
  at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:59)
  at com.sun.proxy.$Proxy7.findAll(Unknown Source)
  at com.byzhang.test.mybatisTest.findAllTest(mybatisTest.java:43)
  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.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
  at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
  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$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
  at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
  at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
  at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
  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.MysqlIO.<init>(MysqlIO.java:335)
  at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2187)
  at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2220)
  at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2015)
  at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:768)
  at com.mysql.jdbc.JDBC4Connection.<init>(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:385)
  at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:323)
  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:201)
  at org.apache.ibatis.datasource.unpooled.UnpooledDataSource.doGetConnection(UnpooledDataSource.java:196)
  at org.apache.ibatis.datasource.unpooled.UnpooledDataSource.getConnection(UnpooledDataSource.java:93)
  at org.apache.ibatis.datasource.pooled.PooledDataSource.popConnection(PooledDataSource.java:404)
  at org.apache.ibatis.datasource.pooled.PooledDataSource.getConnection(PooledDataSource.java:90)
  at org.apache.ibatis.transaction.jdbc.JdbcTransaction.openConnection(JdbcTransaction.java:139)
  at org.apache.ibatis.transaction.jdbc.JdbcTransaction.getConnection(JdbcTransaction.java:61)
  at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:338)
  at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:84)
  at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:62)
  at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:326)
  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:148)
  ... 30 more
Caused by: java.net.ConnectException: Connection refused: connect
  at java.net.DualStackPlainSocketImpl.connect0(Native Method)
  at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
  at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
  at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
  at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
  at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
  at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
  at java.net.Socket.connect(Socket.java:606)
  at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:211)
  at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:299)
  ... 59 more