java - MySQL 与 JDBC 的连接

Question

我正在尝试连接到我计算机上的数据库。数据库的密码是root，用户也是如此。我的项目库中有连接器 jar 文件，我有 7.0 jre 和 jdk，数据库“Testing1”中的表“clients”存在，并且有 1 个条目和 3 个字段，

+--------------------+
| Tables_in_Testing1 |
+--------------------+
| clients            |
| esk                |
| files              |
+--------------------+

客户表:

+----+------------------+-----------------------+
| id | PublicKey        | Email                 |
+----+------------------+-----------------------+
|  1 | PublicKeyNumber1 | FirstClient@email.com |
+----+------------------+-----------------------+

这是代码:

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

class JDBCTest {
    private static Connection con=null;
    private static Statement st=null;
    private static ResultSet rs=null;

    public static void main(String args[])throws Exception {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/Testing1?user=root&password=root");
        }catch (Exception e) {
            System.out.println("e");
        }
        try{
            st=con.createStatement();
            rs=st.executeQuery("select * from clients");
            while(rs.next()) {
                System.out.println(rs.getString("Key"));
            }
        }
        finally {
            try {
                if (rs != null) {
                        rs.close();
                }
                if (st != null) {
                    st.close();
                }
                if (con != null) {
                con.close();
                }
            }catch (SQLException e) {
                System.out.println(e);
            }
        }
    }
}

所有这些都会返回一个错误(在 Eclipse 中)

e

Exception in thread "main" java.lang.NullPointerException at JDBCTest.main(JDBCTest.java:22)

我认为这是因为没有与数据库的连接，所以con为空......但为什么？

Answer 1

我已经尝试过你的代码，它对我有用，只是对网址做了一些更改。获取连接时也会打印异常。

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

public class JDBCTest{
    private static Connection con = null;
    private static Statement  st  = null;
    private static ResultSet  rs  = null;

    public static void main(String args[]) throws Exception {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/Testing1?user=root&password=root");
        } catch (Exception e) {
            System.out.println(e);
        }
        try {
            st = con.createStatement();
            rs = st.executeQuery("select * from users");
            while (rs.next()) {
                System.out.println(rs.getString("Key"));
            }
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (st != null) {
                    st.close();
                }
                if (con != null) {
                    con.close();
                }
            } catch (SQLException e) {
                System.out.println(e);
            }
        }
    }
}