mysql有表了，但是要建實體類，都是重複性工作，比較浪費時間，於是有了自動生成實體類的想法。如果用mybatis generator，會生成很多沒必要的程式碼，而且很多，看起來更亂了，實際執行的sql，比普通的增刪查改要複雜，生成的sql也其實沒什麼用，實體類還比較有用，於是只要生成實體類。
本文參考改寫自：https://blog.csdn.net/zhum_sjz/article/details/80343549 非常感謝。加上了下劃線轉駝峰和lombok。
本文用的是lombok，如果不用lombok，稍微修改下程式碼即可。如果需要加入什麼包，設定就可以了。

package com.pydt.fgw.core;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 從資料庫表反射出實體類，自動生成實體類
 *
 * @author fangshetian
 *
 */
public class SqlHelper {
    //基本資料配置
    private String packageOutPath = "com.pydt.fgw.core.model";// 指定實體生成所在包的路徑
    private String authorName = "fangsht";// 作者名字
    private String tablename = "xxx";// 表名
    private String[] colnames; // 列名陣列
    private String[] colTypes; // 列名型別陣列
    private String version = "V0.01"; // 版本
    private int[] colSizes; // 列名大小陣列
    private boolean f_util = false; // 是否需要匯入包java.util.*
    private boolean f_sql = false; // 是否需要匯入包java.sql.*
    private boolean f_lang = false; // 是否需要匯入包java.sql.*
    private String defaultPath = "/src/main/java/";
    // 資料庫連線
    private static final String URL = "jdbc:mysql://192.168.1.xxx:3306/xxx";
    private static final String NAME = "user";
    private static final String PASS = "passwd";
    private static final String DRIVER = "com.mysql.jdbc.Driver";

    /*
     * 建構函式
     */
    public SqlHelper() {
        // 建立連線
        Connection con;
        // 查要生成實體類的表
        String sql = "select * from " + tablename;
        PreparedStatement pStemt = null;
        try {
            try {
                Class.forName(DRIVER);
            } catch (ClassNotFoundException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            con = DriverManager.getConnection(URL, NAME, PASS);
            pStemt = con.prepareStatement(sql);
            ResultSetMetaData rsmd = pStemt.getMetaData();
            int size = rsmd.getColumnCount(); // 統計列
            colnames = new String[size];
            colTypes = new String[size];
            colSizes = new int[size];
            for (int i = 0; i < size; i++) {

                colnames[i] = rsmd.getColumnName(i + 1);
                colTypes[i] = rsmd.getColumnTypeName(i + 1);
                //自動生成包配置


                // if (colTypes[i].equalsIgnoreCase("datetime")) {
                // f_util = true;
                // }
                if (colTypes[i].equalsIgnoreCase("image") || colTypes[i].equalsIgnoreCase("text")
                        || colTypes[i].equalsIgnoreCase("datetime") || colTypes[i].equalsIgnoreCase("time")
                        || colTypes[i].equalsIgnoreCase("date") || colTypes[i].equalsIgnoreCase("datetime2")) {
                    f_sql = true;
                }
                // if (colTypes[i].equalsIgnoreCase("int")) {
                // f_lang = true;
                // }
                colSizes[i] = rsmd.getColumnDisplaySize(i + 1);
            }

            String content = parse(colnames, colTypes, colSizes);

            try {
                File directory = new File("");

                String path = this.getClass().getResource("").getPath();

                System.out.println(path);

                String outputPath = directory.getAbsolutePath() + this.defaultPath
                        + this.packageOutPath.replace(".", "/") + "/" + initcap(underlineToHump(tablename)) + ".java";
                System.out.println("執行完畢，生成路徑為："+outputPath);
                FileWriter fw = new FileWriter(outputPath);
                PrintWriter pw = new PrintWriter(fw);
                pw.println(content);
                pw.flush();
                pw.close();
            } catch (IOException e) {
                e.printStackTrace();
            }

        } catch (SQLException e) {
            e.printStackTrace();
        } finally {

        }
    }

    /**
     * 功能：生成實體類主體程式碼
     *
     * @param colnames
     * @param colTypes
     * @param colSizes
     * @return
     */
    private String parse(String[] colnames, String[] colTypes, int[] colSizes) {
        StringBuffer sb = new StringBuffer();
        // 生成package包路徑
        sb.append("package " + this.packageOutPath + ";\r\n");
        // 判斷是否匯入工具包
        if (f_util) {
            sb.append("import java.util.Date;\r\n");
        }
        if (f_sql) {
            sb.append("import java.sql.*;\r\n");
        }
        if (f_lang) {
            sb.append("import java.lang.*;\r\n");
        }
        sb.append("import lombok.Data;\r\n");

        sb.append("\r\n");
        // 註釋部分
        sb.append("   /**\r\n");
        sb.append("    * @檔名稱：" + initcap(underlineToHump(this.tablename)) + ".java\r\n");
        sb.append("    * @建立時間：" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date()) + "\r\n");
        sb.append("    * @創  建  人：" + this.authorName + " \r\n");
        sb.append("    * @檔案描述：" + tablename + " 實體類\r\n");
        sb.append("    * @檔案版本：" + this.version + " \r\n");
        sb.append("    */ \r\n");
        sb.append("\n\r\
 
[email protected]");
        // 實體部分
        sb.append("\npublic class " + initcap(underlineToHump(tablename)) + "{\r\n");
        processAllAttrs(sb);// 屬性
//        processAllMethod(sb);// get set方法
        sb.append("}\r\n");

        // System.out.println(sb.toString());
        return sb.toString();
    }

    /**
     * 功能：生成所有屬性
     *
     * @param sb
     */
    private void processAllAttrs(StringBuffer sb) {

        for (int i = 0; i < colnames.length; i++) {
            sb.append("\tprivate " + sqlType2JavaType(colTypes[i]) + " " + this.underlineToHump(colnames[i]) + ";\r\n");
        }

    }

    /**
     * 功能：生成所有方法
     *
     * @param sb
     */
//    private void processAllMethod(StringBuffer sb) {
//
//        for (int i = 0; i < colnames.length; i++) {
//            sb.append("\tpublic void set" + initcap(colnames[i]) + "(" + sqlType2JavaType(colTypes[i]) + " "
//                    + colnames[i] + "){\r\n");
//            sb.append("\tthis." + colnames[i] + "=" + colnames[i] + ";\r\n");
//            sb.append("\t}\r\n");
//            sb.append("\tpublic " + sqlType2JavaType(colTypes[i]) + " get" + initcap(colnames[i]) + "(){\r\n");
//            sb.append("\t\treturn " + colnames[i] + ";\r\n");
//            sb.append("\t}\r\n");
//        }
//
//    }

    /**
     * 功能：將輸入字串的首字母改成大寫
     *
     * @param str
     * @return
     */
    private String initcap(String str) {

        char[] ch = str.toCharArray();
        if (ch[0] >= 'a' && ch[0] <= 'z') {
            ch[0] = (char) (ch[0] - 32);
        }

        return new String(ch);
    }

    /**
     * 功能：獲得列的資料型別
     *
     * @param sqlType
     * @return
     */
    private String sqlType2JavaType(String sqlType) {

        if (sqlType.equalsIgnoreCase("bit")) {
            return "Boolean";
        } else if (sqlType.equalsIgnoreCase("decimal") || sqlType.equalsIgnoreCase("money")
                || sqlType.equalsIgnoreCase("smallmoney") || sqlType.equalsIgnoreCase("numeric")
                || sqlType.equalsIgnoreCase("bigint")) {
            return "Long";
        } else if (sqlType.equalsIgnoreCase("float")) {
            return "Double";
        } else if (sqlType.equalsIgnoreCase("int") || sqlType.equalsIgnoreCase("int identity")) {
            return "Integer";
        } else if (sqlType.equalsIgnoreCase("image") || sqlType.equalsIgnoreCase("varbinary(max)")
                || sqlType.equalsIgnoreCase("varbinary") || sqlType.equalsIgnoreCase("udt")
                || sqlType.equalsIgnoreCase("binary")) {
            return "Byte[]";
        } else if (sqlType.equalsIgnoreCase("nchar") || sqlType.equalsIgnoreCase("nvarchar(max)")
                || sqlType.equalsIgnoreCase("nvarchar") || sqlType.equalsIgnoreCase("nvarchar(ntext)")
                || sqlType.equalsIgnoreCase("uniqueidentifier") || sqlType.equalsIgnoreCase("xml")
                || sqlType.equalsIgnoreCase("char") || sqlType.equalsIgnoreCase("varchar(max)")
                || sqlType.equalsIgnoreCase("text") || sqlType.equalsIgnoreCase("varchar")) {
            return "String";
        } else if (sqlType.equalsIgnoreCase("real")) {
            return "Float";
        } else if (sqlType.equalsIgnoreCase("smallint") || sqlType.equalsIgnoreCase("tinyint")) {
            return "Short";
        } else if (sqlType.equalsIgnoreCase("date") || sqlType.equalsIgnoreCase("datetime")
                || sqlType.equalsIgnoreCase("time") || sqlType.equalsIgnoreCase("datetime2")
                || sqlType.equalsIgnoreCase("timestamp") )  {
            return "Date";
        } else {
            System.out.println("資料型別異常，型別為：" + sqlType);
        }

        return null;
    }
        /**
     * 下劃線轉駝峰
     *
     * @String para
     */
    private  String underlineToHump(String para){
        StringBuilder result=new StringBuilder();
        String a[]=para.split("_");
        for(String s:a){
            if(result.length()==0){
                result.append(s.toLowerCase());
            }else{
                result.append(s.substring(0, 1).toUpperCase());
                result.append(s.substring(1).toLowerCase());
            }
        }
        return result.toString();
    }


    /**
     * 出口 TODO
     *
     * @param args
     */
    public static void main(String[] args) {

        new SqlHelper();

    }
}