我的 Java 程序没有打印我对 MySQL 的查询的结果

Question

我制作了一个程序，为登录表单制作了一个简单的 GUI。只是为了测试目的，我这样做了，所以当您单击“登录”按钮时，Java 应该只打印出 MySQL 数据库中的查询结果。在 MySQL 中，我有一个名为“test”的数据库模式和一个名为“login”的表。登录表只有 1 行：“1, angelo, password”，列下：loginID、Username 和 Password。

MYSQL 用户名是 root。主机是 localhost 端口 3306。MySQL 服务器当前正在运行。

我在 DrJava 的“Extra Classpath”文件夹中有“mysql-connector-java-5.1.25-bin”（这是我用来编程的软件）。

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import java.sql.SQLException;

（GUI的代码在这里）

                Connection connection = null;

try {
    // Load the JDBC driver
    String driverName = "com.mysql.jdbc.Driver"; // MySQL MM JDBC driver
    Class.forName(driverName);

    // Create a connection to the database
    String serverName = "localhost:3306";
    String mydatabase = "test";
    String url = "jdbc:mysql://" + serverName +  "/" + mydatabase; // a JDBC url
    String username = "root";
    String password = "";
    connection = DriverManager.getConnection(url, username, password);

                    Statement st = connection.createStatement();
                ResultSet rs = st.executeQuery("select * from login");

                while(rs.next()) {
                    System.out.println( rs.getString("username"));
                }
                st.close();
                rs.close();
                connection.close(); 


} catch (ClassNotFoundException e) {
    // Could not find the database driver
} catch (SQLException e) {
    // Could not connect to the database
}

每当我运行它时都没有错误，只是没有打印出来。

编辑：当我添加 e.printStackTrace 时出现此错误：

java.lang.ClassNotFoundException
at edu.rice.cs.plt.reflect.PathClassLoader.findClass(PathClassLoader.java:148)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at swing_sample$handler.actionPerformed(swing_sample.java:108)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

根据我发布的内容，您能告诉我我做错了什么吗？谢谢。

Answer 1

您似乎在以下语句中使用了行数据来打印：

 System.out.println( rs.getString("angelo"));

您需要将列名或列索引提供给rs.getString()

例如：

 System.out.println( rs.getString("user_name"));

或

 System.out.println( rs.getString(1));// assuming what you are lookin for is found in the first column

Answer 2

1. 设置变量url后尝试打印url 它看起来像：jdbc:mysql://localhost:3306/dbName

2. 正如 codeMan 所指出的，请使用列名而不是数据，在您的情况下将 angelo 替换为用户名。

Answer 3

问题在于加载 JDBC 驱动程序... 当我不将mysql-connector.jar 添加到我的build path 时，我的应用程序中会出现相同的error。 看来您的classpath 设置不正确.. 您是否将mysql-connector.jar 添加到build path？

Answer 4

您的代码看起来是正确的，我认为您的代码会尝试加载一些不在您的类中的类，请确保所有依赖类都应该在您的类路径中。

clean code and build your project .

它可能会解决你的问题解决你的问题。