无法从结果集中检索值 (jdbc)

Question

在 ResultSet rs = stmt.executeQuery(query); 之后，代码不会执行。因此，变量不会从结果集中获取值。有人能帮我解决这个问题吗？

public User storeTempDetails(String s1){ 
Statement stmt = null;
String query = "select username, account_no,name from accounts where username = ?";
try {
  connection = DriverManager.getConnection(DBurl, DBusername, DBpassword);
  System.out.println("Database connected!");
  stmt = connection.createStatement();
  ResultSet rs = stmt.executeQuery(query);
  System.out.println("this line is not printed");
  while (rs.next()) {
    userName = rs.getString("username");
    accountNo = rs.getString("account_no");
    name = rs.getString("name");System.out.println("ss"+accountNo);
  }
} catch (SQLException e ) {
  throw new RuntimeException("Cannot connect the database!", e);
} finally {
   User userObj=new User(userName,accountNo,name);
        System.out.println("Closing the connection.");
        if (connection != null) try { connection.close(); }
        catch (SQLException ignore) {}
        return userObj;
}

}

Answer 1

从这样的事情开始。这些实用程序类将使您在开始使用 JDBC 时变得更加轻松：

package persistence;

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

/**
 * util.DatabaseUtils
 * User: Michael
 * Date: Aug 17, 2010
 * Time: 7:58:02 PM
 */
public class DatabaseUtils
{
    private static final String DEFAULT_DRIVER = "oracle.jdbc.driver.OracleDriver";
    private static final String DEFAULT_URL = "jdbc:oracle:thin:@host:1521:database";
    private static final String DEFAULT_USERNAME = "username";
    private static final String DEFAULT_PASSWORD = "password";
/*
    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;
    }
}

考虑到这些实用程序，我将如何编写它。我可能会将 User 放在 model 包中，而 UserDaoImpl 将是其自己的 .java 文件中的公共类。我只是懒：

package persistence;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

/**
 * UserDao
 * @author Michael
 * @since 10/2/11
 */
public interface UserDao
{
    User find(String username);
}

class UserDaoImpl implements UserDao {

    public static final String SELECT_USER_BY_USERNAME = "select username, account_no,name from accounts where username = ?";

    private Connection connection;

    UserDaoImpl(Connection connection)
    {
        this.connection = connection;
    }

    public User find(String username)
    {
        User user = null;
        PreparedStatement ps = null;
        ResultSet rs = null;

        try
        {
            ps = this.connection.prepareStatement(SELECT_USER_BY_USERNAME);
            ps.setString(1, username);
            rs = ps.executeQuery();
            while (rs.next()) {
                String account = rs.getString("account_no");
                String name = rs.getString("name");
                user = new User(name, username, account);
            }
        }
        catch (SQLException e)
        {
            e.printStackTrace(); 
        }
        finally
        {
            DatabaseUtils.close(rs);
            DatabaseUtils.close(ps);
        }

        return user;
    }
}

class User {
    private final String name;
    private final String username;
    private final String account;

    User(String name, String username, String account)
    {
        this.name = name;
        this.username = username;
        this.account = account;
    }

    public String getName()
    {
        return name;
    }

    public String getUsername()
    {
        return username;
    }

    public String getAccount()
    {
        return account;
    }

    @Override
    public String toString()
    {
        final StringBuilder sb = new StringBuilder();
        sb.append("User");
        sb.append("{name='").append(name).append('\'');
        sb.append(", username='").append(username).append('\'');
        sb.append(", account='").append(account).append('\'');
        sb.append('}');
        return sb.toString();
    }
}

Answer 2

有一个异常抛出。但由于 finally 块中的 return，该异常永远不会发生。谷歌搜索“java”、“return”和“finally”，你会感觉到这有多糟糕。

修复代码并向我们展示堆栈跟踪。