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端口错误或者数据库服务器没有启动 
提示的错误信息如下： 
com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.net.ConnectException 
MESSAGE: Connection refused: connect 

STACKTRACE: 

java.net.ConnectException: Connection refused: connect 
at java.net.PlainSocketImpl.socketConnect(Native Method) 
at java.net.PlainSocketImpl.doConnect(Unknown Source) 
at java.net.PlainSocketImpl.connectToAddress(Unknown Source) 
at java.net.PlainSocketImpl.connect(Unknown Source) 
at java.net.SocksSocketImpl.connect(Unknown Source) 
at java.net.Socket.connect(Unknown Source) 
at java.net.Socket.connect(Unknown Source) 
at java.net.Socket.<init>(Unknown Source) 
at java.net.Socket.<init>(Unknown Source) 
at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:132) 
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:273) 
at com.mysql.jdbc.Connection.createNewIO(Connection.java:1639) 
at com.mysql.jdbc.Connection.<init>(Connection.java:393) 
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:262) 
at java.sql.DriverManager.getConnection(Unknown Source) 
at java.sql.DriverManager.getConnection(Unknown Source) 
at test.JDBCTest.oracleTest(JDBCTest.java:25) 
at test.JDBCTest.main(JDBCTest.java:12) 


** END NESTED EXCEPTION ** 
解决方法：查看端口是否正确，数据库服务器是否启动。 

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