connection.getMetaData似乎没有返回表信息 [英] connection.getMetaData does not seem to return table info

问题描述

我正在使用一个简单的应用程序来帮助学习数据库与Apache.Derby并在Eclipse中工作。
以下代码运行正常，但conn.getMetaData（）不返回任何有意义的表 - colnameslist.size例如是0.但是我添加了meta.getDatabaseProductName（）来查看发生了什么，并返回Apache。 Derby'，所以我想有一些连接。

I am using a simple app to aid learning database with Apache.Derby and working in Eclipse. The following code runs ok but conn.getMetaData() does not return anything meaningful regarding the table - colnameslist.size for example is 0. However I added meta.getDatabaseProductName() to see what happened and that returns 'Apache.Derby' so I guess there is some sort of connection.

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

由于没有错误，我有一点损失。

As it does not error I am at a bit of a loss.

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]; 
    } 
}

推荐答案

尝试只使用表的名称;当我这样做时，我不需要将'app'级别的名称放在表名前（即只有EVERYGYTABLE7）。并确保表名称具有该大写字母;可以创建一个混合名称的表，我认为Derby是严格的。

Try using just the name of the table; when I do this, I don't need to put the name of the 'app' level in front of the table name (i.e. just EVERYGYTABLE7). And make sure the table name has that capitalization; it is possible to create a table with a mixed-case name, and I think Derby is strict about that.

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