java连接到mysql数据库失败

Question

I am creating a vaadin web application using tomcat and having an external database on another server. I am not able to connect to the external database no matter what. I have even created a new pure java project using j2se jre 1.6 and trying to connect to the database but the connection still fails. I downloaded the mysql driver and copied it into the ~/Library/Java/Extensions and even added/removed from build path and still nothing changed. I have my firewall off and I dont know what else to do. I believe that my code is correct, but anyway:

String host = "jdbc:mysql://db.host.sk:3306/myDB";
    String name = "name";
    String pass = "pass";
    Connection connection;

    try {
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        System.out.println("Connecting to database...");
        connection = DriverManager.getConnection(host, name,
                pass);
        System.out.println("SUCCESSFUL!!!");

    } catch (SQLException e) {
        System.out.println("Connecting to database failed");
        System.out.println(e.getMessage());
        e.printStackTrace();
    }
    catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

Could you please help me with this? Thank you.

EDIT

The stack trace:

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:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1121)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:355)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2479)
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2516)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2301)
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:834)
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at     sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:416)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:317)
at java.sql.DriverManager.getConnection(DriverManager.java:579)
at java.sql.DriverManager.getConnection(DriverManager.java:221)
at DatabaseHelper.main(DatabaseHelper.java:17)
Caused by: java.net.ConnectException: Operation timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:391)
at java.net.Socket.connect(Socket.java:579)
at java.net.Socket.connect(Socket.java:528)
at java.net.Socket.<init>(Socket.java:425)
at java.net.Socket.<init>(Socket.java:241)
at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:259)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:305)
... 15 more

Answer 1

检查您的数据库服务已启动并在专用服务器上运行。

Answer 2

Never faced this before however, I read few articles and found the possible reasons could be there's something in between Java and database that is blocking the connection, eg a firewall or server is down/does not support tcp/ip or hostname in jdbc url is not recognized by local dns server, etc etc. Well you can try the following advice(worth giving a shot):

1. Go to Windows services in the control panel and start the MySQL service.
2. Check your wait timeout set on the DB server and update it to, say, 10000

   SET GLOBAL wait_timeout = 10000;

You can also check a similiar kind of Question having a good answer, posted by Soheil here: Solving a "communications link failure" with JDBC and MySQL

Answer 3

I (un)fortunately ended up using mysql on my machine with eclipse and everything is running smoothly now. It's a pity I didnt make it work like I wanted in the first place but the current way suites me better. I dont have to make any changes and configuration and can finally start coding. I appreciate all your help.