public class POSTJson2 {
    public static void main(String[] args) {
        try {

            String defURL = "https://api.apiopen.top/api/login";
            URL url = new URL(defURL);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            con.setRequestMethod("POST");//请求POST方式
            con.setRequestProperty("Content-Type","application/json;charset=UTF-8");
            con.setDoOutput(true);// 设置是否使用HttpURLConnection进行输出，默认值为 false
            
            OutputStreamWriter writer = new OutputStreamWriter(con.getOutputStream(), "UTF-8");
            String body = "{\"account\":\"309324904@qq.com\",\"password\":\"123456\"}";
            writer.write(body);
            writer.flush();
            writer.close();

            int code = con.getResponseCode();
            System.out.println("http状态码:" + code);
            if (code == HttpURLConnection.HTTP_OK) {
                System.out.println("测试成功");
            } else {
                System.out.println("测试失败：" + code);
            }

            // 获取服务端响应，通过输入流来读取URL的响应
            InputStream is = con.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"));
            StringBuffer sbf = new StringBuffer();
            String strRead = null;
            while ((strRead = reader.readLine()) != null) {
                sbf.append(strRead);
                sbf.append("\r\n");
            }
            reader.close();

            // 关闭连接
            con.disconnect();

            // 打印读到的响应结果
            System.out.println("运行结束：" + sbf.toString());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

运行结果：

http状态码:200
测试成功
运行结束：{"code":200,"message":"成功!","result":{"id":572,"createdAt":"2022-12-09 08:28:14","updatedAt":"2022-12-09 08:28:14","deletedAt":null,"account":"309324904@qq.com","level":0,"token":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1aWQiOjU3MiwiaWQiOjU3MiwiY3JlYXRlZEF0IjoiMjAyMi0xMi0wOSAwODoyODoxNCIsInVwZGF0ZWRBdCI6IjIwMjItMTItMDkgMDg6Mjg6MTQiLCJkZWxldGVkQXQiOm51bGwsImFjY291bnQiOiIzMDkzMjQ5MDRAcXEuY29tIiwibGV2ZWwiOjAsImV4cCI6MTcyMDk2MDU2NCwiaXNzIjoiYXBpX29wZW4iLCJuYmYiOjE3MjAzNTQ3NjR9.NaW12ztp0jy3auF3ZACc_OX6vI_ERe87uB6PWC4Ycn8"}}

返回的json数据整理下：

{
    "code": 200,
    "message": "成功!",
    "result": {
        "id": 572,
        "createdAt": "2022-12-09 08:28:14",
        "updatedAt": "2022-12-09 08:28:14",
        "deletedAt": null,
        "account": "309324904@qq.com",
        "level": 0,
        "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1aWQiOjU3MiwiaWQiOjU3MiwiY3JlYXRlZEF0IjoiMjAyMi0xMi0wOSAwODoyODoxNCIsInVwZGF0ZWRBdCI6IjIwMjItMTItMDkgMDg6Mjg6MTQiLCJkZWxldGVkQXQiOm51bGwsImFjY291bnQiOiIzMDkzMjQ5MDRAcXEuY29tIiwibGV2ZWwiOjAsImV4cCI6MTcyMDk2MDU2NCwiaXNzIjoiYXBpX29wZW4iLCJuYmYiOjE3MjAzNTQ3NjR9.NaW12ztp0jy3auF3ZACc_OX6vI_ERe87uB6PWC4Ycn8"
    }
}

上面是手写的json字符串，如果使用java官方的安装包javax.json，json字符串拼接如下：

JsonObject object= Json.createObjectBuilder()
        .add("account","309324904@qq.com")
        .add("password","123456")
        .build();
writer.write(object.toString());

完整代码如下：

import javax.json.Json;
import javax.json.JsonObject;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;

public class POSTJson2 {
    public static void main(String[] args) {
        try {
            String defURL = "https://api.apiopen.top/api/login";
            URL url = new URL(defURL);
            HttpURLConnection con = (HttpURLConnection) url.openConnection();
            con.setRequestMethod("POST");//请求POST方式
            con.setRequestProperty("Content-Type","application/json;charset=UTF-8");
            con.setDoOutput(true);// 设置是否使用HttpURLConnection进行输出，默认值为 false

            OutputStreamWriter writer = new OutputStreamWriter(con.getOutputStream(), "UTF-8");
            JsonObject object= Json.createObjectBuilder()
                    .add("account","309324904@qq.com")
                    .add("password","123456")
                    .build();
            writer.write(object.toString());
            writer.flush();
            writer.close();



            int code = con.getResponseCode();
            System.out.println("http状态码:" + code);
            if (code == HttpURLConnection.HTTP_OK) {
                System.out.println("测试成功");
            } else {
                System.out.println("测试失败：" + code);
            }

            // 获取服务端响应，通过输入流来读取URL的响应
            InputStream is = con.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"));
            StringBuffer sbf = new StringBuffer();
            String strRead = null;
            while ((strRead = reader.readLine()) != null) {
                sbf.append(strRead);
                sbf.append("\r\n");
            }
            reader.close();

            // 关闭连接
            con.disconnect();

            // 打印读到的响应结果
            System.out.println("运行结束：" + sbf.toString());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}