Java对象转为JSON

使用步骤： 导入jackson的相关jar包
• 创建Jackson核心对象 objectMapper
• 调用objectMapper的相关方法进行转换

基本使用方法：

Person对象：

package edu.xalead.vo;

public class Person {
private String name;
private int age;
private String gender;

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public String getGender() {
return gender;
}

public void setGender(String gender) {
this.gender = gender;
}

@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", age=" + age +
", gender='" + gender + '\'' +
'}';
}
}

测试：

package edu.xalead.test;

import com.fasterxml.jackson.databind.ObjectMapper;
import edu.xalead.vo.Person;
import org.junit.Test;

import java.io.File;
import java.io.FileWriter;

public class JacksonTest {
//Java对象转为Json
@Test
public void test1() throws Exception {
//1.创建Person对象
Person p = new Person();
p.setName("李四");
p.setAge(12);
p.setGender("男");

//2.创建Jackson的核心对象 objectMapper
ObjectMapper mapper = new ObjectMapper();
//3.转换
/**
* 转换方法 ：
*  writeValue:(参数1，obj)
*      参数1：
*      File:将obj对象转换为Json字符串，并保存到指定文件
*      Writer:将obj对象转换为Json字符串，并将json数据填充到字符输出流
*      outputStream:将obj对象转换为Json字符串，并将json数据填充到字节输出流
*  writeValueAsString:将对象转换为json字符串
*/
String json = mapper.writeValueAsString(p);
System.out.println(json);//{"name":"李四","age":12,"gender":"男"}

//writeValue方法，将数据写到H://a.txt
mapper.writeValue(new File("H://a.txt"),p);
//writeValue方法，关联到Writer中
mapper.writeValue(new FileWriter("H://b.txt"),p);
}
}

1.转换方法：

• writeValue:(参数1，obj)

• 参数1：
  File:将obj对象转换为Json字符串，并保存到指定文件
  Writer:将obj对象转换为Json字符串，并将json数据填充到字符输出流
  outputStream:将obj对象转换为Json字符串，并将json数据填充到字节输出流

• writeValueAsString:将对象转换为json字符串

1. 注解：

   1. @JsonIgnore:排除属性。
   2. @JsonFormat:属性值的格式化
   @JsonFormat(pattern = "yyyy-MM-dd")

   复杂Java对象的转换
   -List :数组
   -Map：和对象格式一致
   实例及结果：

@Test
public void test3() throws Exception {
//1.创建Person对象
Person p = new Person();
p.setName("李四");
p.setAge(12);
p.setGender("男");
p.setBirthday(new Date());

Person p1 = new Person();
p1.setName("李四");
p1.setAge(12);
p1.setGender("男");
p1.setBirthday(new Date());

Person p2 = new Person();
p2.setName("李四");
p2.setAge(12);
p2.setGender("男");
p2.setBirthday(new Date());

//创建list集合
List<Person> ps = new ArrayList<>();
ps.add(p);
ps.add(p1);
ps.add(p2);
//Map集合
Map<String,Object> map = new HashMap<>();
map.put("name","张三");
map.put("ag2",23);
map.put("gender","男");

ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsString(ps);
String json1 = mapper.writeValueAsString(map);
System.out.println(json);
System.out.println(json1);
}
[{"name":"李四","age":12,"gender":"男","birthday":"2020-03-03"},{"name":"李四","age":12,"gender":"男","birthday":"2020-03-03"},{"name":"李四","age":12,"gender":"男","birthday":"2020-03-03"}]
{"gender":"男","name":"张三","ag2":23}

案例

• 校验用户名是否存在

  服务器端：

  package edu.xalead;
  
  import com.fasterxml.jackson.databind.ObjectMapper;
  
  import javax.servlet.ServletException;
  import javax.servlet.annotation.WebServlet;
  import javax.servlet.http.HttpServlet;
  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpServletResponse;
  import java.io.IOException;
  import java.sql.*;
  import java.util.HashMap;
  import java.util.Map;
  
  @WebServlet(name = "FindUserServlet",urlPatterns = "/findUserServlet")
  public class FindUserServlet extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  //从表单获取用户名
  String username = request.getParameter("username");
  
  //从数据库获取用户名
  String driverClass = "com.mysql.jdbc.Driver";
  String url = "jdbc:mysql://localhost:3306/user_test";
  String userName = "root";
  String password = "root";
  //链接对象
  Connection conn = null;
  //结果集
  ResultSet rs = null;
  //语句准备对象
  PreparedStatement ps = null;
  
  String sql = "select * from user_json where username=?";
  try {
  //加载驱动
  Class.forName(driverClass);
  conn = DriverManager.getConnection(url,userName,password);
  ps = conn.prepareStatement(sql);
  ps.setString(1,username);
  rs = ps.executeQuery();
  //解决乱码
  response.setContentType("text/html;charset=utf-8");
  Map<String,Object> map = new HashMap<>();
  if(rs.next()){
  //已经存在该用户
  map.put("userExsit",true);
  map.put("mes","此用户太受欢迎，请更换一个！");
  }else{
  map.put("userExsit",true);
  map.put("mes","用户名可用！");
  }
  //map转换为json,并传递给客户端
  ObjectMapper mapper = new ObjectMapper();
  mapper.writeValue(response.getWriter(),map);
  } catch (Exception e) {
  e.printStackTrace();
  }
  
  }
  
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  this.doPost(request,response);
  }
  }

客户端：

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>注册页面</title>
<script src="js/jquery-3.4.1.min.js"></script>
</head>
<body>
<form>
<input type="text" id="username" name="username" placeholder="请输入用户名">
<span id="errspan"> </span>
<br>
<input type="text" id="pwd" name="password" placeholder="请输入密码"><br>
<input type="submit" value="注册">
</form>
</body>
<script>
//在页面加载完成后
$(function(){//入口函数
//给username绑定blur事件
$("#username").blur(function () {
//获取username文本输入框的值
var username = $(this).val();
//发送ajax请求
//期望服务器返回数据的格式，{"userExsit":true,"mes":"此用户太受欢迎，请换一个"}
//{"userExsit":false,"mes":"用户名可用"}
$.get("findUserServlet",{username:username},function (data) {
// alert(data);
//判断userExsit键是否为true
var span = $("#errspan");
if(data.userExsit){
//用户名存在
span.css("color","red")
span.html(data.mes);
}else{
span.css("color","green")
span.html(data.mes);
}
},"json");
})
});
</script>
</html>

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