post参数需要json类型

大神们，求助
java怎么post json参数的请求
我使用java调用创建会话报 post参数需要json类型，下面是我的代码。
url是https://oapi.dingtalk.com/chat/create?access_token=xxxxxx

                        URL realUrl = new URL(url);
            // 打开和URL之间的连接
            URLConnection conn = realUrl.openConnection();
            // 设置通用的请求属性
            conn.setRequestProperty("accept", "*/*");
            conn.setRequestProperty("connection", "Keep-Alive");
            conn.setRequestProperty("user-agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1)");
            conn.setRequestProperty("Content-Type", "application/json;charset=UTF-8");
            // 发送POST请求必须设置如下两行
            conn.setDoOutput(true);
            conn.setDoInput(true);
            // 获取URLConnection对象对应的输出流
            out = new PrintWriter(conn.getOutputStream());
            // 发送请求参数
            // flush输出流的缓冲
            out.print("{\"name\":\"测试\",\"owner\":\"张三\",\"useridlist\":\"[\"张三\",\"李四\"]\"}");
            out.flush();