我正在尝试连接到我大学的 MySQL 数据库，但连接已挂起。

import java.sql.*;

public class ConnectToDB {
        public static void main(String args[]){
                try {
                        Class.forName("com.mysql.jdbc.Driver").newInstance(); 
                        String url = "jdbc:mysql://db.cs.myUniversity.com/dbName"; 
                        System.out.println("BEFORE"); 
                        Connection con = DriverManager.getConnection(url,"me", "password");
                        System.out.println("AFTER");
                        ...

这个调用:time java ConnectToDB 打印(在我最终杀死它之后):

Copyright 2004, R.G.Baldwin
BEFORE
AFTER

real    3m9.343s
user    0m0.316s
sys     0m0.027s

我刚刚从 here 下载了 MySQL Connector/J .我不确定这是否是问题的一部分。我相当准确地遵循了指示。

我也可以像这样在命令行上连接到 mysql:

$ mysql -u me -h db.cs.myUniversity.com -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 882328
Server version: 5.0.77 Source distribution

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> use dbName;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> SHOW tables;
+-------------------+
| Tables_in_dbName  |
+-------------------+
| classics          | 
+-------------------+
1 row in set (0.00 sec)

可能出现的问题:

• 我写的Java代码
• 我是如何安装 MySQL Connector/J 的
• 某种网络问题阻止了连接

问题:我应该怎么做才能解决这个问题？为什么 getConnection 调用挂起？

我正在关注 this tutorial

最佳答案

您提供的输出没有帮助。

我看到打印之前和之后，所以建立了连接。代码没有显示这些时间包含的内容，所以我无法说出它们的含义。

如果您建议您的代码因为从未建立连接而不得不终止，这可能是因为您的用户名、密码和客户端 IP 尚未获得所需的 GRANTed 权限。

可能是:

1. 你的大学网络；找网络工程师询问防火墙。
2. MySQL数据库权限；找到 DBA 并询问。
3. 你的代码；你没有张贴足够告诉。发布全类。

版权是怎么回事？我会失去它。

此代码有效。修改它，使相关参数符合您的问题。 (我的使用 MySQL 5.1.51 和一个名为 Party 的表。)当我在本地机器上运行它时，我得到了 641 毫秒的挂起时间。

package persistence;

import java.sql.*;
import java.util.*;

/**
 * DatabaseUtils
 * User: Michael
 * Date: Aug 17, 2010
 * Time: 7:58:02 PM
 */
public class DatabaseUtils
{
/*
    private static final String DEFAULT_DRIVER = "org.postgresql.Driver";
    private static final String DEFAULT_URL = "jdbc:postgresql://localhost:5432/party";
    private static final String DEFAULT_USERNAME = "pgsuper";
    private static final String DEFAULT_PASSWORD = "pgsuper";
*/
    private static final String DEFAULT_DRIVER = "com.mysql.jdbc.Driver";
    private static final String DEFAULT_URL = "jdbc:mysql://localhost:3306/party";
    private static final String DEFAULT_USERNAME = "party";
    private static final String DEFAULT_PASSWORD = "party";

    public static void main(String[] args)
    {
        long begTime = System.currentTimeMillis();

        String driver = ((args.length > 0) ? args[0] : DEFAULT_DRIVER);
        String url = ((args.length > 1) ? args[1] : DEFAULT_URL);
        String username = ((args.length > 2) ? args[2] : DEFAULT_USERNAME);
        String password = ((args.length > 3) ? args[3] : DEFAULT_PASSWORD);

        Connection connection = null;

        try
        {
            connection = createConnection(driver, url, username, password);
            DatabaseMetaData meta = connection.getMetaData();
            System.out.println(meta.getDatabaseProductName());
            System.out.println(meta.getDatabaseProductVersion());

            String sqlQuery = "SELECT PERSON_ID, FIRST_NAME, LAST_NAME FROM PERSON ORDER BY LAST_NAME";
            System.out.println("before insert: " + query(connection, sqlQuery, Collections.EMPTY_LIST));

            connection.setAutoCommit(false);
            String sqlUpdate = "INSERT INTO PERSON(FIRST_NAME, LAST_NAME) VALUES(?,?)";
            List parameters = Arrays.asList( "Foo", "Bar" );
            int numRowsUpdated = update(connection, sqlUpdate, parameters);
            connection.commit();

            System.out.println("# rows inserted: " + numRowsUpdated);
            System.out.println("after insert: " + query(connection, sqlQuery, Collections.EMPTY_LIST));
        }
        catch (Exception e)
        {
            rollback(connection);
            e.printStackTrace();
        }
        finally
        {
            close(connection);
            long endTime = System.currentTimeMillis();
            System.out.println("wall time: " + (endTime - begTime) + " ms");
        }
    }

    public static Connection createConnection(String driver, String url, String username, String password) throws ClassNotFoundException, SQLException
    {
        Class.forName(driver);

        if ((username == null) || (password == null) || (username.trim().length() == 0) || (password.trim().length() == 0))
        {
            return DriverManager.getConnection(url);
        }
        else
        {
            return DriverManager.getConnection(url, username, password);
        }
    }

    public static void close(Connection connection)
    {
        try
        {
            if (connection != null)
            {
                connection.close();
            }
        }
        catch (SQLException e)
        {
            e.printStackTrace();
        }
    }


    public static void close(Statement st)
    {
        try
        {
            if (st != null)
            {
                st.close();
            }
        }
        catch (SQLException e)
        {
            e.printStackTrace();
        }
    }

    public static void close(ResultSet rs)
    {
        try
        {
            if (rs != null)
            {
                rs.close();
            }
        }
        catch (SQLException e)
        {
            e.printStackTrace();
        }
    }

    public static void rollback(Connection connection)
    {
        try
        {
            if (connection != null)
            {
                connection.rollback();
            }
        }
        catch (SQLException e)
        {
            e.printStackTrace();
        }
    }

    public static List<Map<String, Object>> map(ResultSet rs) throws SQLException
    {
        List<Map<String, Object>> results = new ArrayList<Map<String, Object>>();

        try
        {
            if (rs != null)
            {
                ResultSetMetaData meta = rs.getMetaData();
                int numColumns = meta.getColumnCount();
                while (rs.next())
                {
                    Map<String, Object> row = new HashMap<String, Object>();
                    for (int i = 1; i <= numColumns; ++i)
                    {
                        String name = meta.getColumnName(i);
                        Object value = rs.getObject(i);
                        row.put(name, value);
                    }
                    results.add(row);
                }
            }
        }
        finally
        {
            close(rs);
        }

        return results;
    }

    public static List<Map<String, Object>> query(Connection connection, String sql, List<Object> parameters) throws SQLException
    {
        List<Map<String, Object>> results = null;

        PreparedStatement ps = null;
        ResultSet rs = null;

        try
        {
            ps = connection.prepareStatement(sql);

            int i = 0;
            for (Object parameter : parameters)
            {
                ps.setObject(++i, parameter);
            }

            rs = ps.executeQuery();
            results = map(rs);
        }
        finally
        {
            close(rs);
            close(ps);
        }

        return results;
    }

    public static int update(Connection connection, String sql, List<Object> parameters) throws SQLException
    {
        int numRowsUpdated = 0;

        PreparedStatement ps = null;

        try
        {
            ps = connection.prepareStatement(sql);

            int i = 0;
            for (Object parameter : parameters)
            {
                ps.setObject(++i, parameter);
            }

            numRowsUpdated = ps.executeUpdate();
        }
        finally
        {
            close(ps);
        }

        return numRowsUpdated;
    }
}

关于java - 为什么 java.sql.DriverManager.getConnection(...) 挂起？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4059004/