JSONObject类未定义getBackMysql()方法的Java MySQL查询问题

Hey there, let's break down this error and get your MySQL-to-JSON code working right away!

What's Causing This Error? #

This message is telling you that you're trying to call a method named getBackMysql() on a JSONObject instance—but the org.json.JSONObject class doesn't have any such method. Chances are you mixed up your own class name with a method call, or there's a mix-up in how you're trying to convert your MySQL results to JSON.

Step-by-Step Fixes #

• Locate the bad method call: Search your code for lines that look like this:

  JSONObject someJson = new JSONObject();
  someJson.getBackMysql(); // This is the problematic line!
  

  Delete this line entirely—it's not how you populate a JSONObject with database data.

• Replace with correct JSON population logic: Instead of calling a non-existent method, you need to pull data from your ResultSet and add it to the JSONObject manually. Here's a corrected snippet to integrate into your code:

  public class GetBackMysql { // Note: Fixed class name to follow Java conventions (PascalCase)
      static final String JDBC_DRIVER = "com.mysql.cj.jdbc.Driver";
      static final String DB_URL = "jdbc:mysql://your-host:3306/your-database";
      static final String USER = "your-username";
      static final String PASS = "your-password";
  
      public static void main(String[] args) {
          Connection conn = null;
          Statement stmt = null;
          ResultSet rs = null;
  
          try {
              // Register JDBC driver
              Class.forName(JDBC_DRIVER);
              // Open database connection
              conn = DriverManager.getConnection(DB_URL, USER, PASS);
              // Create and execute query
              stmt = conn.createStatement();
              String sql = "SELECT id, name, age FROM your-table"; // Replace with your actual query
              rs = stmt.executeQuery(sql);
  
              // Convert ResultSet data to JSONObject
              while (rs.next()) {
                  JSONObject jsonObj = new JSONObject();
                  jsonObj.put("id", rs.getInt("id"));
                  jsonObj.put("name", rs.getString("name"));
                  jsonObj.put("age", rs.getInt("age"));
                  // Add more fields from your table as needed
  
                  // Do something with the JSONObject (print, send to client, etc.)
                  System.out.println(jsonObj.toString());
              }
  
          } catch (SQLException se) {
              // Handle SQL errors
              se.printStackTrace();
          } catch (Exception e) {
              // Handle other exceptions
              e.printStackTrace();
          } finally {
              // Clean up database resources to avoid leaks
              try {
                  if (rs != null) rs.close();
                  if (stmt != null) stmt.close();
                  if (conn != null) conn.close();
              } catch (SQLException se) {
                  se.printStackTrace();
              }
          }
      }
  }
  

• Follow Java naming conventions: Your class was named getBackMysql—renaming it to GetBackMysql (PascalCase) will avoid confusion between class names and method names, which is likely part of how this mix-up happened.

Quick Reminders #

• Make sure the column names you use in rs.getInt()/rs.getString() exactly match the column names in your MySQL table.
• Always close database resources (Connection, Statement, ResultSet) in a finally block to prevent memory leaks.
• Double-check that your org.json.jar is properly added to your project's build path (you mentioned you already did this, but it never hurts to confirm!).

内容的提问来源于stack exchange，提问作者Z.Richard