对象关系映射(Object Relational Mapping)，是一种程序技术，用于实现面向对象编程语言里不同类型系统的数据之间的转换。从效果上说，它其实是创建了一个可在编程语言里使用的“虚拟对象数据库”。
面向对象是从软件工程的基本原则（如耦合、聚合、封装）的基础上发展起来的，而关系数据库则是从数学理论发展而来的，两套理论存在显著的区别。为了解决这个不匹配的现象，对象关系映射技术应运而生。
MySQL可以根据建表语句获得相应的字段和字段类型
-- ----------------------------
-- Table structure for comment
-- ----------------------------
DROP TABLE IF EXISTS `comment`;
CREATE TABLE `comment` (
`id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '评论id',
`customer_id` bigint(20) NOT NULL COMMENT '客户id',
`seller_id` bigint(20) NOT NULL COMMENT ' 卖家id',
`deliver_id` bigint(20) DEFAULT NULL COMMENT '外卖员id',
`food_id` bigint(20) NOT NULL COMMENT '食品id',
`food_comment` varchar(100) DEFAULT NULL COMMENT '对商品的评论内容',
`seller_comment` varchar(100) DEFAULT NULL COMMENT '对商家的评论内容',
`deliver_comment` varchar(100) DEFAULT NULL COMMENT '对快递员评论内容',
`food_score` tinyint(1) DEFAULT '5' COMMENT '商品评分：1,2,3,4,5，数字越大评分越高',
`deliver_score` tinyint(1) DEFAULT '5' COMMENT '快递员评分：1,2,3,4,5，数字越大评分越高',
`seller_score` tinyint(1) DEFAULT '5' COMMENT '卖家评分：1,2,3,4,5，数字越大评分越高',
`comment_date` datetime DEFAULT NULL,
`named` tinyint(1) DEFAULT '1' COMMENT '是否匿名：0匿名，1不匿名',
`deleted` tinyint(1) DEFAULT '0' COMMENT '评论删除状态0：未删除，1已删除',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='评价表';
开发时往往需要根据字段的类型得到基本的Pojo类：
package files;
import com.shgx.order.enums.StatusEnum;
import com.shgx.order.enums.ScoreEnum;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import javax.persistence.*;
import java.util.Date;
@Entity
@Data
@Builder
@AllArgsConstructor
@Table(name = "comment")
public class Comment {
/**
* 主键自增id
*/
@Id
@Column(name = "id")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
/**
* 客户id
*/
@Column(name = "customer_id")
private Long customerId;
/**
* 卖家id
*/
@Column(name = "seller_id")
private Long sellerId;
/**
* 外卖员id
*/
@Column(name = "deliver_id")
private Long deliverId;
/**
* 食品id
*/
@Column(name = "food_id")
private Long foodId;
/**
* 对商品的评论内容
*/
@Column(name = "food_comment")
private String foodComment;
/**
* 对卖家的评论内容
*/
@Column(name = "seller_comment")
private String sellerComment;
/**
* 对外卖员的评论内容
*/
@Column(name = "deliver_comment")
private String deliverComment;
/**
* 商品评分
*/
@Column(name = "food_score")
@Convert(converter = ScoreEnum.Converter.class)
private ScoreEnum foodScore;
/**
* 外卖员评分
*/
@Column(name = "deliver_score")
@Convert(converter = ScoreEnum.Converter.class)
private ScoreEnum deliverScore;
/**
* 卖家评分
*/
@Column(name = "seller_score")
@Convert(converter = ScoreEnum.Converter.class)
private ScoreEnum sellerScore;
/**
* 评分时间
*/
@Column(name = "comment_date")
private Date commentDate;
/**
* 是否匿名
*/
@Column(name = "named")
@Convert(converter = StatusEnum.Converter.class)
private StatusEnum named;
/**
* 是否删除
*/
@Column(name = "deleted")
@Convert(converter = StatusEnum.Converter.class)
private StatusEnum deleted;
}
这里想根据数据库建表语句生成一个基本的Pojo类(事例代码比较简单，很多细节没有考虑，仅供参考，欢迎补充完善)
public class Sql2Pojo {
/**
* 按行读文件
* @param fin
* @throws IOException
* @return
*/
public static List<String> readFile(File fin) throws IOException {
FileInputStream fis = new FileInputStream(fin);
BufferedReader br = new BufferedReader(new InputStreamReader(fis));
String line = null;
List<String> lines = new ArrayList<>();
while ((line = br.readLine()) != null) {
lines.add(line);
}
br.close();
return lines;
}
/**
* 按行匹配得到想要的结果
* @param line
* @return
*/
public static String match(String line){
String[] words = line.trim().split(" ");
String result = null;
switch (words[0]){
case "--":
case "DROP": {
break;
}
case "CREATE":{
result = words[2].substring(1,words[2].length()-1);
//首字母大写
result = result.substring(0,1).toUpperCase()+result.substring(1);
result = "public class "+ result + "{\n";
break;
}
default:{
String lastWord = words[words.length-1];
/**
* 主键自增id
*/
String notes = "\n /**\n * "+ lastWord.substring(1, lastWord.length()-2)+"\n */";
char lastChar = lastWord.charAt(lastWord.length()-1);
switch (lastChar){
case ',':{
//转换为对象的属性
//@Column(name = "customer_id")
String temp = words[0].substring(1, words[0].length()-1);
String annotate = "\n @Column(name = \""+temp+"\")\n";
String[] items = temp.split("_");
int index = 0;
for(String item:items){
if(index==0){
result = item;
index ++;
continue;
}
item = item.substring(0,1).toUpperCase()+item.substring(1);
result = result+item;
}
//转换属性的类型
String qualifiers = words[1];
result = notes+annotate + " " + recQualifiers(qualifiers)+" "+result+";\n";
break;
}default:{
result = null;
}
}
}
}
return result;
}
/**
* 修饰符判断
* @param qualifiers
* @return
*/
private static String recQualifiers(String qualifiers){
String result = null;
if(qualifiers==null){
return result;
}
int index = (qualifiers.indexOf('(')!=-1?qualifiers.indexOf('('):qualifiers.length()-1);
qualifiers = qualifiers.substring(0, index);
switch (qualifiers){
case "bigint":{
result = "Long";
break;
}case "varchar":{
result = "String";
break;
}case "tinyint":{
result = "Integer";
break;
}case "datetime":{
result = "Date";
break;
}
default:{
result = "String";
}
}
return result;
}
/**
* 转换语句
* @param lines
* @return
*/
public static List<String> convert(List<String> lines){
List<String> results = new ArrayList<>();
for(String line:lines){
String result = match(line);
if(result!=null){
results.add(result);
}
}
return results;
}
/**
* 把实验结果写入到文件中
* @param result
*/
public static void writeToFile(List<String> result){
Path fpath= Paths.get("src/files/comment_result.txt");
//创建文件
if(!Files.exists(fpath)) {
try {
Files.createFile(fpath);
} catch (IOException e) {
e.printStackTrace();
}
}
//创建BufferedWriter
try {
BufferedWriter bfw=Files.newBufferedWriter(fpath);
int count = result.size();
for(String line : result){
bfw.write(line);
if(--count==0){
bfw.write("}");
}
}
bfw.flush();
bfw.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
public class Comment{
/**
* 评论id
*/
@Column(name = "id")
Long id;
/**
* 客户id
*/
@Column(name = "customer_id")
Long customerId;
/**
* 家id
*/
@Column(name = "seller_id")
Long sellerId;
/**
* 外卖员id
*/
@Column(name = "deliver_id")
Long deliverId;
/**
* 食品id
*/
@Column(name = "food_id")
Long foodId;
/**
* 对商品的评论内容
*/
@Column(name = "food_comment")
String foodComment;
/**
* 对商家的评论内容
*/
@Column(name = "seller_comment")
String sellerComment;
/**
* 对快递员评论内容
*/
@Column(name = "deliver_comment")
String deliverComment;
/**
* 商品评分：1,2,3,4,5，数字越大评分越高
*/
@Column(name = "food_score")
Integer foodScore;
/**
* 快递员评分：1,2,3,4,5，数字越大评分越高
*/
@Column(name = "deliver_score")
Integer deliverScore;
/**
* 卖家评分：1,2,3,4,5，数字越大评分越高
*/
@Column(name = "seller_score")
Integer sellerScore;
/**
* UL
*/
@Column(name = "comment_date")
String commentDate;
/**
* 是否匿名：0匿名，1不匿名
*/
@Column(name = "named")
Integer named;
/**
* 评论删除状态0：未删除，1已删除
*/
@Column(name = "deleted")
Integer deleted;
}
ORM