java.sql.SQLException : Before start of result set

Question

我已尝试使用以下代码检索存储在数据库中的图像。我创建了一个名为 image_db 的数据库，其中包含一个名为 image_details 的表。该表有两个字段，id 和 image_path，两者都是 mediumblob 类型。我在 image_path< 中存储了一些图像 字段为二进制。现在我想检索并显示它。

package cbir.imageAddition;
import java.awt.Image;
import java.awt.Toolkit;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.*;

enter code here

public class ImageRetrieve {

    public ImageRetrieve() throws SQLException, IOException, ClassNotFoundException
    {

        Connection con = null;
        Statement st = null;
        ResultSet rs = null;
        String url = "jdbc:mysql://localhost:3306/";
        String db = "image_db";
        String driver = "com.mysql.jdbc.Driver";
        String user = "root";
        String pass = "root";

            Class.forName(driver);
            con = DriverManager.getConnection(url + db, user, pass);
            //System.out.println("Connection url : "+url + db);

            st = con.createStatement();
            String sql = "select image_path from image_details where id=1";
            rs = st.executeQuery(sql);

    InputStream stream = rs.getBinaryStream(2);
    ByteArrayOutputStream output = new ByteArrayOutputStream();
    int a1 = stream.read();
    while (a1 >= 0) {
      output.write((char) a1);
      a1 = stream.read();
    }
    Image myImage = Toolkit.getDefaultToolkit().createImage(output.toByteArray());
    output.close();

    }
}

运行上述代码时出现以下异常:

awtJan 12, 2012 12:55:48 AM cbir.imageAddition.add_image_window jButton5ActionPerformed
SEVERE: null
java.sql.SQLException: Before start of result set
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1073)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:987)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:982)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:927)
    at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:841)
    at com.mysql.jdbc.ResultSetImpl.getStringInternal(ResultSetImpl.java:5650)
    at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5570)
    at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5610)
    at cbir.imageAddition.ImageRetrieve.<init>(ImageRetrieve.java:49)
    at cbir.imageAddition.add_image_window.jButton5ActionPerformed(add_image_window.java:280)
    at cbir.imageAddition.add_image_window.access$400(add_image_window.java:26)
    at cbir.imageAddition.add_image_window$5.actionPerformed(add_image_window.java:89)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6504)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6269)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4860)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4686)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2713)
    at java.awt.Component.dispatchEvent(Component.java:4686)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
    at java.awt.EventQueue.access$000(EventQueue.java:101)
    at java.awt.EventQueue$3.run(EventQueue.java:666)
    at java.awt.EventQueue$3.run(EventQueue.java:664)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:680)
    at java.awt.EventQueue$4.run(EventQueue.java:678)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
    at java..EventDispatchThread.run(EventDispatchThread.java:90)

这是怎么引起的，我该如何解决？

Answer 1

您必须调用 rs.next()(并检查它是否返回 true)才能访问结果集的第一行:

if (rs.next() {
    InputStream stream = rs.getBinaryStream(1);
    ...

也不是索引应该是 1，因为您的查询只选择一列。

我也不明白将 int 转换为 char 的意义。该方法将 int 作为参数。转换为字节至少是合乎逻辑的，但字节和字符在 Java 中不是一回事。