java - connection.getMetaData 似乎没有返回表信息

Question

我正在使用一个简单的应用程序来帮助学习 Apache.Derby 数据库并在 Eclipse 中工作。以下代码运行正常，但 conn.getMetaData() 不会返回任何与表相关的有意义的内容 - 例如 colnameslist.size 为 0。但是我添加了 meta.getDatabaseProductName() 来查看发生了什么，并返回“Apache.Derby”，所以我猜测其中存在某种联系。

The connection url is "jdbc:derby:C:/Users/RonLaptop/MyDB".
The string passed into getTableContents() is "MYENERGYAPP.ENERGYTABLE7".

因为它没有错误，所以我有点不知所措。

package com.energy;

import javax.swing.*; 
import javax.swing.table.*; 
import java.sql.*; 
import java.util.*;
/** an immutable table model built from getting 
    metadata about a table in a jdbc database 
*/ 
public class JDBCTableModel extends AbstractTableModel {
    Object[][] contents;
    String[] columnNames;
    Class[] columnClasses;

    public JDBCTableModel (Connection conn,
               String string)
        throws SQLException {
        super();
        getTableContents (conn, string);

    }
    protected void getTableContents (Connection conn,
                 String string)
        throws SQLException {

    // get metadata: what columns exist and what
    // types (classes) are they?
    DatabaseMetaData meta = conn.getMetaData();
    String productName = meta.getDatabaseProductName();
    String[] types = null;

    System.out.println ("got meta = " + meta);
    ResultSet results =
        meta.getColumns (null, null, string, null) ;
    System.out.println ("got column results");
    ArrayList colNamesList = new ArrayList();
    ArrayList colClassesList = new ArrayList();
    while (results.next()) {
        colNamesList.add (results.getString ("COLUMN_NAME")); 
        System.out.println ("name: " + 
            results.getString ("COLUMN_NAME"));
        int dbType = results.getInt ("DATA_TYPE");
        switch (dbType) {
        case Types.INTEGER:
    colClassesList.add (Integer.class); break; 
        case Types.FLOAT:
    colClassesList.add (Float.class); break; 
        case Types.DOUBLE: 
        case Types.REAL:
    colClassesList.add (Double.class); break; 
        case Types.DATE: 
        case Types.TIME: 
        case Types.TIMESTAMP:
    colClassesList.add (java.sql.Date.class); break; 
        default:
    colClassesList.add (String.class); break; 
        }; 
        System.out.println ("type: " +
            results.getInt ("DATA_TYPE"));
        }
        columnNames = new String [colNamesList.size()];
        colNamesList.toArray (columnNames);
        columnClasses = new Class [colClassesList.size()];
        colClassesList.toArray (columnClasses);

        // get all data from table and put into
        // contents array

        Statement statement =
    conn.createStatement ();
        results = statement.executeQuery ("SELECT * FROM " +
                      string);

        ArrayList rowList = new ArrayList();
        while (results.next()) {
    ArrayList cellList = new ArrayList(); 
    for (int i = 0; i<columnClasses.length; i++) { 
        Object cellValue = null;


        if (columnClasses[i] == String.class) 
    cellValue = results.getString (columnNames[i]); 
        else if (columnClasses[i] == Integer.class) 
    cellValue = new Integer ( 
            results.getInt (columnNames[i])); 
        else if (columnClasses[i] == Float.class) 
    cellValue = new Float ( 
            results.getInt (columnNames[i])); 
        else if (columnClasses[i] == Double.class) 
    cellValue = new Double ( 
            results.getDouble (columnNames[i]));
        else if (columnClasses[i] == java.sql.Date.class) 
    cellValue = results.getDate (columnNames[i]); 
        else 
    System.out.println ("Can't assign " + 
            columnNames[i]);
        cellList.add (cellValue);
    }// for
    Object[] cells = cellList.toArray();
    rowList.add (cells);

} // while
// finally create contents two-dim array
contents = new Object[rowList.size()] [];
for (int i=0; i<contents.length; i++)

    contents[i] = (Object []) rowList.get (i);
System.out.println ("Created model with " +
           contents.length + " rows");

// close stuff
results.close();
statement.close();

}
// AbstractTableModel methods
public int getRowCount() {
    return contents.length;
}

public int getColumnCount() {
    if (contents.length == 0)
        return 0;
    else
        return contents[0].length;
    }

    public Object getValueAt (int row, int column) {
        return contents [row][column];
    }

    // overrides methods for which AbstractTableModel
    // has trivial implementations

    public Class getColumnClass (int col) {
        return columnClasses [col];
    }

    public String getColumnName (int col) { 
        return columnNames [col]; 
    } 
}

Answer 1

尝试仅使用表的名称；当我这样做时，我不需要将“应用程序”级别的名称放在表名称前面(即只是 EVERYGYTABLE7)。并确保表名具有大写字母；可以创建一个混合大小写名称的表，我认为 Derby 对此很严格。