Duplicate entry '0' for key 'PRIMARY'错误

自动增量问题

JPA的注解来定义实体的时候，使用@Id来注解主键属性即可。如果数据库主键是自增长的，需要在增加一个注解@GeneratedValue，即：

1     @GeneratedValue(strategy=GenerationType.IDENTITY)
2     @Id
3     private String id;

PS：@GeneratedValue注解的strategy属性提供四种值：

–AUTO：主键由程序控制，是默认选项，不设置即此项。

–IDENTITY：主键由数据库自动生成，即采用数据库ID自增长的方式，Oracle不支持这种方式。

–SEQUENCE：通过数据库的序列产生主键，通过@SequenceGenerator 注解指定序列名，mysql不支持这种方式。

–TABLE：通过特定的数据库表产生主键，使用该策略可以使应用更易于数据库移植。

也可以手动设置主键自动增长

mvn spring-boot:run运行spring-boot

{
"username":"hjc",
"creadate":"234",
"foods":
[
{"name": "xilanhua ","weight" :"3 ","flag":"1"},
{ "name":"doujiang ","weight": "1 ","flag":"2"}
]

}

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json数据传输数组的格式，json对空格很敏感，比如“creadate ”这种加空格传输后，获取jsonObject.getString("creadate")方式是不行的

JAVA中快速解析JSON对象里包含的JSON数组

例如现在有这样一个Json

String Value = {"data":[{"school_name":"西北农林科技大学","school_id":"8"},{"school_name":"西北大学","school_id":"6"},

{"school_name":"西北工业大学","school_id":"5"},{"school_name":"西北政法大学","school_id":"2"},{"school_name":"西安交通大学","school_id":"4"},{"school_name":"西安建筑科技大学","school_id":"10"},{"school_name":"西安电子科技大学","school_id":"3"},{"school_name":"西安邮电大学","school_id":"1"},{"school_name":"长安大学","school_id":"9"},{"school_name":"陕西师范大学","school_id":"7"},{"school_name":"陕西科技大学","school_id":"11"}],"msg":"列表获取成功","status":0}

用2个数组接收school_name和school_id

首先

public String[] schoolID;
public String[] schoolName;

JSONObject jsonObj_school；//用来接收解析的JSON字符串

JSONArray jsonArr_school ；//用来接收JSON对象里的数组

try{
jsonObj_school = new JSONObject ( Value ); //解析JSON字符串
jsonArr_school =jsonObj_school.getJSONArray("data");//接收JSON对象里的数组
   int jsonSize_school = jsonArr_school.length();.//获取数组长度
   schoolID = new String[ jsonSize_school ];//初始化数组
  schoolName = new String[ jsonSize_school];//初始化数组
  for(int i = 0; i < jsonSize_school; i++ )//通过循环取出数组里的值
  {
JSONObject jsonTemp = (JSONObject)jsonArr_school.getJSONObject(i);
   schoolID[i] = jsonTemp.getString("school_id");
   schoolName[i] = jsonTemp.getString("school_name");
   }

}

catch(Exception e){
e.printStackTrace();
}

数据库笔记

Duplicate entry '0' for key 'PRIMARY'错误

JAVA中快速解析JSON对象里包含的JSON数组

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