java操作JSON

一、Jackson概念

1、作用

​ 它是用来前后端的交互功能，属于SpringMVC

二、ObjectMapper

2.1、作用

​ 是用来后端接收的json数据转换成各种格式。也可以转换各种格式

2.1.1、第一步，初始化

ObjectMapper mapper = new ObjectMapper();

2.1.2、写到文件中

User user = new User("243582", "h2435823336");
//mapper.writeValue(new File("test.txt"), user); // 写到文件中
//mapper.writeValue(System.out, user); //写到控制台

2.1.3、对象转为字符串

//对象转为字符串
String json_String = mapper.writeValueAsString(user);
System.out.println("对象转为字符串：" + json_String);

2.1.4、json字符串转为对象

User jsonStr_Class = mapper.readValue(json_String, User.class);
System.out.println("json字符串转为对象：" + jsonStr_Class);

2.1.5、对象转为byte数组

byte[] class_ByteArr = mapper.writeValueAsBytes(user);
System.out.println("对象转为byte数组：" + Arrays.toString(class_ByteArr));

2.1.6、byte数组转为对象

User byteArr_Class = mapper.readValue(class_ByteArr, User.class);
System.out.println("byte数组转为对象：" + byteArr_Class);

字符串转map

ObjectMapper om = new ObjectMapper();
om.readValue(loginCheck, Map.class);

2.2、导入依赖

<!--https://mvnrepository.com/artifact/org.codehaus.jackson/jackson-mapper-asl -->
<dependency><groupId>org.codehaus.jackson</groupId><artifactId>jackson-mapper-asl</artifactId><version>1.9.13</version>
</dependency>

<!-- https://mvnrepository.com/artifact/org.codehaus.jackson/jackson-core-asl -->
<dependency><groupId>org.codehaus.jackson</groupId><artifactId>jackson-core-asl</artifactId><version>1.9.13</version>
</dependency>

2.3、具体使用方法

2.3.1、HTML中

​ 使用了JQuery方式，得到的返回值放在span标签中

<script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.5.1/jquery.js"></script>
<script>function submit(){var username = $("#username").val();var password = $("#password").val();var user = {username: username,password: password};$.ajax({url:"SubmitServlet",// url:"/login",data:{"user":JSON.stringify(user),"username":username,"password":password},// data:JSON.stringify(user),cache:false,type:"POST",dataType:"json",// contentType: 'application/json;charset=utf-8',success: function (resultUser){$("#span1").text(resultUser.username);$("#span2").text(resultUser.password);}})}
</script>

2.3.2、Servlet中

1. ​ 首先测试能否到达此类，输出"成功到达后台"
2. ​ 接着开始类型转换
3. ​ 最后把结果对象返回到前端

@WebServlet("/SubmitServlet")
public class SubmitServlet extends HttpServlet {@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {request.setCharacterEncoding("UTF-8");response.setContentType("application/json;charset=utf-8");System.out.println("成功到达后台");//System.out.println("username: "+request.getParameter("username")+"    "+"password: "+request.getParameter("password"));String user =  request.getParameter("user");ObjectMapper mapper = new ObjectMapper();User userBean = mapper.readValue(user,User.class);System.out.println("userBean:   "+userBean);//把userBean对象返回到前端页面的成功回调函数mapper.writeValue(response.getWriter(),userBean);}
}

三、JSONObject

JSONObject只是一种数据结构，可以理解为JSON格式的数据结构（key-value 结构），可以使用put方法给json对象添加元素。JSONObject可以很方便的转换成字符串，也可以很方便的把其他对象转换成JSONObject对象。

1、依赖fastjson

<!-- https://mvnrepository.com/artifact/com.alibaba/fastjson -->
<dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.2.75</version>
</dependency>

1.2、JSONObject

1.2.1、初始化

JSONObject jsonObject = new JSONObject();

实体对象 转 JSON字符串

//对象转json字符串
String strs = JSONObject.toJSONString(user);

JSON字符串 转 JSON对象

//JSOn字符串转JSON对象
JSONObject infoJson = JSONObject.parseObject(strs);//get值    //不能set值
infoJson.getString("securityName");

1.3、JSONArrays数组

JSONArray jsonArray = new JSONArray();JSONObject liJson = new JSONObject();
liJson.put("materialUniqueFieldType", "idType");
liJson.put("materialUniqueFieldValue", materialId);
liJson.put("resourceType", "document");jsonArray.add(liJson);

显示效果：

[{"materialNo": 1,"playTime": 20,"resourceValue": "Station"
}]