java - 为什么我得到一个从打开的 MySQL 连接关闭的 java.sql.PreparedStatement？

Question

为什么我得到一个 java.sql.PreparedStatement 从打开的 MySQL 连接关闭？

这是我的代码:

import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.ResultSet;



public class MySqlTest1 
{
    Connection connection = null;
    PreparedStatement stmt = null;
    public MySqlTest1() 
    {
        System.out.println("Loading driver...");

        try {
            Class.forName("com.mysql.jdbc.Driver");
            System.out.println("Driver loaded!");
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException("Cannot find the driver in the classpath!", e);
        }
        String url = "jdbc:mysql://localhost:3306/world?autoReconnect=true&useSSL=false";
        String username = "jee";
        String password = "????????";

        System.out.println("Connecting database...");

        try (Connection connection =  DriverManager.getConnection(url, username, password)) 
        {
            if (connection.isClosed())
            {
                System.out.println("Returned connection is closed");
                return;
            }
            System.out.println("Database connected!");
            System.out.println("create statement ...");
            **stmt = connection.prepareStatement("select * from city");**
        } catch (SQLException e) {
            throw new IllegalStateException("Cannot connect the database!", e);
        }

        System.out.println("Selecting data ...");
        ResultSet rs = null;
        try {
            System.out.println("execute query ...");
            rs = stmt.executeQuery();
            if (rs != null)
            {
                System.out.println("Data selected");
            }
        }
        catch (SQLException ex)
        {
            System.err.println("SQLException: " + ex.getMessage());
            System.err.println("SQLState: " + ex.getSQLState());
            System.err.println("VendorError: " + ex.getErrorCode());
            return;
        }

该代码的结果是

Loading driver...
Driver loaded!
Connecting database...
Database connected!
create statement ...
Selecting data ...
execute query ...
****SQLException: No operations allowed after statement closed.****
SQLState: S1009
VendorError: 0

我也尝试过使用 Statement 并查看它的值，发现“isClosed”是正确的。我查看了 MySQL 日志，但一无所获。

Answer 1

您正在 try-with-resource block 中打开连接。 block 终止后，连接将关闭，并隐式地关闭从它创建的所有语句。只需扩展此 block 以包括该语句的用法，您就可以了。