Closing jdbc resultset

It is important to always close JDBC ResultSet objects to free up resources and prevent memory leaks. You can close a ResultSet by calling the `close()` method on it. For example:

ResultSet rs = statement.executeQuery(sql);
// process the ResultSet
rs.close();

Alternatively, you can use a try-with-resources block to automatically close the ResultSet when you're done with it:

try (ResultSet rs = statement.executeQuery(sql)) {
    // process the ResultSet
}

相关问题

用jdbc以老师表为操作对象，实现查询id，name，subject，class

在Java中，使用JDBC (Java Database Connectivity) 进行数据库操作时，如果你想要查询老师表（假设名为Teacher）中的id、name、subject和class字段，你需要按照以下步骤编写代码：

1. 首先，确保已经导入了所需的JDBC库：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

2. 获取到数据库连接：

String url = "jdbc:mysql://localhost:3306/your_database_name";
String username = "your_username";
String password = "your_password";

Connection connection = null;
try {
    connection = DriverManager.getConnection(url, username, password);
} catch (SQLException e) {
    System.err.println("Error connecting to the database: " + e.getMessage());
}

3. 创建SQL查询语句，并准备Statement对象：

String sqlQuery = "SELECT id, name, subject, class FROM Teacher";
Statement statement = connection.createStatement();
ResultSet resultSet = null;

4. 执行查询并获取结果集：

try {
    resultSet = statement.executeQuery(sqlQuery);
} catch (SQLException e) {
    System.err.println("Error executing query: " + e.getMessage());
}

5. 处理查询结果：

while (resultSet.next()) {
    int id = resultSet.getInt("id");
    String name = resultSet.getString("name");
    String subject = resultSet.getString("subject");
    String classField = resultSet.getString("class");

    // 打印或做其他操作
    System.out.println("ID: " + id + ", Name: " + name + ", Subject: " + subject + ", Class: " + classField);
}

6. 关闭资源：

finally {
    try {
        if (resultSet != null) {
            resultSet.close();
        }
        if (statement != null) {
            statement.close();
        }
        if (connection != null && !connection.isClosed()) {
            connection.close();
        }
    } catch (SQLException e) {
        System.err.println("Error closing resources: " + e.getMessage());
    }
}

根据以下要求：Instead of using a binary file to save the arraylist of points, change the savaData method and the constructor of the Model class to use a database to write / read the coordinates of all the points. Use XAMPP and phpMyAdmin to create a database called "java" with a table called "points" that has two integer columns x and y (in addition to the ID primary key). Hint: make sure you delete all the old point coordinates from the database before inserting new ones. Hint: use phpMyAdmin to check what is stored in the database. Use the Test class to run all the tests for the software and check that all the tests still work. Use the Start class to run the software and check that closing the software correctly saves the point coordinates in the database (use phpMyAdmin to check the content of the database). Run the software again and check that all the points from the previous run are correctly displayed，修改下述代码：public class Model implements Serializable { private ArrayList<Point> points; private ArrayList<ModelListener> listeners; private static final String FILE_NAME = "points.bin"; public Model() { points = new ArrayList<Point>(); listeners = new ArrayList<ModelListener>(); // Read points from file if it exists File file = new File(FILE_NAME); if (file.exists()) { try { ObjectInputStream in = new ObjectInputStream(new FileInputStream(file)); points = (ArrayList<Point>) in.readObject(); in.close(); } catch (IOException e) { e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } } } public void addListener(ModelListener l) { listeners.add(l); } public ArrayList<Point> getPoints() { return points; } public void addPoint(Point p) { points.add(p); notifyListeners(); // points changed so notify the listeners. saveData(); // save point to file } public void clearAllPoints() { points.clear(); notifyListeners(); // points changed so notify the listeners. saveData(); // save empty list to file } public void deleteLastPoint() { if (points.size() > 0) { points.remove(points.size() - 1); notifyListeners(); // points changed so notify the listeners. saveData(); // save updated list to file } } private void notifyListeners() { for (ModelListener l : listeners) { l.update(); // Tell the listener that something changed. } } public int numberOfPoints() { return points.size(); } public void saveData() { try { ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(FILE_NAME)); out.writeObject(points); out.close(); } catch (IOException e) { e.printStackTrace(); } }

public class Model {
private ArrayList<Point> points;
private ArrayList<ModelListener> listeners;
private static final String DB_URL = "jdbc:mysql://localhost/java";
private static final String DB_USER = "root";
private static final String DB_PASSWORD = "";

public Model() {
points = new ArrayList<Point>();
listeners = new ArrayList<ModelListener>();
// Read points from database
try {
Connection conn = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
Statement stmt = conn.createStatement();
String sql = "SELECT * FROM points";
ResultSet rs = stmt.executeQuery(sql);
while (rs.next()) {
int x = rs.getInt("x");
int y = rs.getInt("y");
Point p = new Point(x, y);
points.add(p);
}
rs.close();
stmt.close();
conn.close();
} catch (SQLException e) {
e.printStackTrace();
}
}

public void addListener(ModelListener l) {
listeners.add(l);
}

public ArrayList<Point> getPoints() {
return points;
}

public void addPoint(Point p) {
points.add(p);
notifyListeners(); // points changed so notify the listeners.
saveData(); // save point to database
}

public void clearAllPoints() {
points.clear();
notifyListeners(); // points changed so notify the listeners.
saveData(); // save empty list to database
}

public void deleteLastPoint() {
if (points.size() > 0) {
points.remove(points.size() - 1);
notifyListeners(); // points changed so notify the listeners.
saveData(); // save updated list to database
}
}

private void notifyListeners() {
for (ModelListener l : listeners) {
l.update(); // Tell the listener that something changed.
}
}

public int numberOfPoints() {
return points.size();
}

public void saveData() {
try {
Connection conn = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
Statement stmt = conn.createStatement();
String sql = "DELETE FROM points";
stmt.executeUpdate(sql);
for (Point p : points) {
sql = "INSERT INTO points (x, y) VALUES (" + p.x + ", " + p.y + ")";
stmt.executeUpdate(sql);
}
stmt.close();
conn.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}