添加了mysql驱动和连接池

.idea/libraries/mysql_connector_j_8_0_33.xml

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+<component name="libraryTable">
+  <library name="mysql-connector-j-8.0.33">
+    <CLASSES>
+      <root url="jar://$PROJECT_DIR$/src/database/mysql-connector-j-8.0.33.jar!/" />
+    </CLASSES>
+    <JAVADOC />
+    <SOURCES />
+  </library>
+</component>

src/backend/utils/DBConnectionPool.java

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+package backend.utils;
+
+import java.sql.Connection;
+import java.util.*;
+
+public class DBConnectionPool {
+    private static final int MAX_CONNECTION = 10;
+    private static Queue<Connection> connectionQueue;
+    static{
+        connectionQueue = new LinkedList<>();
+        for(int i=0;i<MAX_CONNECTION;++i)
+            connectionQueue.add(backend.utils.DBHelper.getInstance().getConnection());
+    }
+    public static Connection getConnection(){
+        if(connectionQueue.isEmpty())
+            return null;
+        return connectionQueue.poll();
+    }
+
+    public static void releaseConnection(Connection conn){
+        if(conn==null)
+            return;
+        connectionQueue.add(conn);
+    }
+}

src/backend/utils/DBHelper.java

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+package backend.utils;
+
+import java.sql.*;
+
+public class DBHelper {
+    // 数据库驱动名
+    private static final String JDBC_DRIVER = "com.mysql.cj.jdbc.Driver";
+    /***  以下请根据实际情况修改  ****/
+    private static final String IP = "kronecker.cc"; // ip地址
+    private static final String PORT = "3306"; // 端口
+    private static final String DB_NAME = "CET4"; // 数据库名
+    private static final String USER = "kronecker";
+    private static final String PASSWORD = "20060825fhy.";
+    /***  以上请根据实际情况修改  ****/
+    private static final String DB_URL = String.format("jdbc:mysql://%s:%s/%s", IP, PORT, DB_NAME);
+    // 单例模式写法之一
+    private final static DBHelper instance;
+
+    static {
+        try {
+            instance = new DBHelper();
+        } catch (SQLException e) {
+            throw new RuntimeException(e);
+        }
+    }
+    // 私有无参构造
+    private DBHelper() throws SQLException {}
+    public static DBHelper getInstance(){
+        return instance;
+    }
+    // 1. 注册驱动
+    static {
+        try {
+            Class.forName(JDBC_DRIVER);
+            System.out.println("数据库加载成功");
+        } catch (ClassNotFoundException e) {
+            System.out.println("驱动加载失败: " + e.getMessage());
+        }
+    }
+    // 2. 创建连接
+    public Connection getConnection(){
+        Connection conn = null;
+        try {
+            conn = DriverManager.getConnection(DB_URL, USER, PASSWORD);
+        } catch (SQLException e) {
+            System.out.println("获取连接失败: " + e.getMessage());
+        }
+        return conn;
+    }
+    // 3. 关闭连接
+    public void close(Connection conn, Statement st, ResultSet rs) {
+        try {
+            if (rs != null) {
+                rs.close();
+            }
+            if (st != null) {
+                st.close();
+            }
+            if (conn != null) {
+                conn.close();
+            }
+        } catch (SQLException e) {
+            System.err.println("资源关闭失败:" + e.getMessage());
+        }
+    }
+    public static void main(String[] args) {
+        DBHelper.getInstance().getConnection();
+    }
+}