java.sql。SQLException:在结果集开始之前[复制]

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ResultSet exception - before start of result set 6 answers
ResultSet异常——在开始结果集6个答案之前

I have tried the following code for retrieving an image stored in a database. I have created a database called image_db that contains an table called image_details. The table has two fields, id and image_path and both are of type mediumblob.I have stored a few images in the image_path field as binary. Now I want to retrieve & display it.

我尝试了以下代码来检索存储在数据库中的映像。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，它们都是mediumblob类型。我在image_path字段中存储了一些图像作为二进制的。现在我想要检索和显示它。

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();

    }
}

I get the following exception when running the code above:

我在运行上述代码时遇到以下异常:

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)

How is this caused and how can I solve it?

这是怎么引起的，我怎么解决呢?

2 个解决方案

#1

You must call rs.next() (and check that it returns true) to access the first row of the result set:

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

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

Also not that the index should be 1, since your query only selects one column.

也不是索引应该是1，因为您的查询只选择一个列。

I also don't understand the point in casting the int to a char. The method takes an int as argument. A cast to byte would at least be logical, but bytes and char are not the same thing in Java.

我也不理解将int类型转换为char的意义。该方法以int作为参数。对字节的转换至少是合乎逻辑的，但是在Java中字节和char是不一样的。

#2

Once if you execute the select query will get ResultSet object then iterate it you won't get this exception. ResultSet rs = null;

一旦执行了select查询，就会得到ResultSet对象，然后遍历它，就不会得到这个异常。ResultSet rs =零;

rs = statement.executeQuery("select UUID_BINARY()");

rs =声明。executeQuery(“选择UUID_BINARY()");

        if(rs.next()){

            newTripUUID = rs.getBytes(1);

        }

#1