MySQL通信链路故障

Question

这是错误。

  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:408)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1117)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:355)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2461)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2498)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2283)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:822)
    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:408)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:404)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:317)
    at java.sql.DriverManager.getConnection(DriverManager.java:664)
    at java.sql.DriverManager.getConnection(DriverManager.java:247)
    at eventmanagementsystem.infinity.dbHandler.buildConnection(dbHandler.java:43)
    at eventmanagementsystem.infinity.dbHandler.getParticipantResultSet(dbHandler.java:395)
    at eventmanagementsystem.infinity.ParticipantList.populateList(ParticipantList.java:96)
    at eventmanagementsystem.infinity.EMS.<init>(EMS.java:31)
    at eventmanagementsystem.infinity.EventManagementSystemInfinity.main(EventManagementSystemInfinity.java:25)
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:345)
    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:589)
    at java.net.Socket.connect(Socket.java:538)
    at java.net.Socket.<init>(Socket.java:434)
    at java.net.Socket.<init>(Socket.java:244)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:259)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:305)
    ... 19 more
May 24, 2015 3:48:58 PM eventmanagementsystem.infinity.EMS <init>
SEVERE: null
java.lang.NullPointerException
    at eventmanagementsystem.infinity.dbHandler.getParticipantResultSet(dbHandler.java:396)
    at eventmanagementsystem.infinity.ParticipantList.populateList(ParticipantList.java:96)
    at eventmanagementsystem.infinity.EMS.<init>(EMS.java:31)
    at eventmanagementsystem.infinity.EventManagementSystemInfinity.main(EventManagementSystemInfinity.java:25)

BUILD SUCCESSFUL (total time: 21 seconds)

这是代码。

public static Connection buildConnection()
{
    try{
    //Class.forName("org.apache.derby.jdbc.ClientDriver");
    //Class.forName("oracle.jdbc.driver.OracleDriver");
        Class.forName("com.mysql.jdbc.Driver");



    //String connString = "jdbc:derby://localhost:4040/EventManagementSystem";
    //String username = "Event";
    //String password = "event";

    //String username = "system";
    //String password = "a1234";
    String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    String DB_URL = "jdbc:mysql://localhost:3306/user";
    String USER = "root";
    String PASS = "a1234";
    //Connection conn = DriverManager.getConnection(connString,username,password);
    //Connection conn = DriverManager.getConnection("jdbc:oracle:thin:@oracle-prod:1521:OPROD",username,password);
    Connection conn = DriverManager.getConnection(DB_URL,USER,PASS);
    return conn;
    }
    catch(Exception e)
    {

        e.printStackTrace();
    }
    return null;
}

我认为问题出在DB_URL上，但我不太确定。我也添加了mysql-connector jar。它无法访问我创建的数据库。错误列表还显示连接被拒绝但我不知道为什么会这样。