主方法：ResponseEntity<Student> responseEntity = restTemplate.exchange(url, HttpMethod.POST, entity, Student.class, aa);
说明：
0. url 请求路径；
1. exchange可以发送HttpMethod.POST,DELE,GET,PUT请求；
2. entity包含请求的对象和消息头；
3. Student.class返回的信息的对象；
4. aa是url中出现的路径变量。
示例：
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);
String url = "http://localhost:8080/test/testadd/{id}";
Map<String, String> aa = new HashMap<String, String>();
aa.put("id", "123");
Student student = new Student();
student.setAa("大智慧aa");
student.setBb("bb");
student.setCc("cc");
RestTemplate restTemplate = new RestTemplate();
HttpEntity<Student> entity = new HttpEntity<Student>(student, headers);
// ResponseEntity<Student> responseEntity = restTemplate.exchange(url, HttpMethod.POST, entity, Student.class, aa);
ResponseEntity<Student> responseEntity = restTemplate.postForEntity(url, entity, Student.class, aa);
Student studentResult = responseEntity.getBody();
System.out.println(studentResult.getAa());
System.out.println(studentResult.getBb());
System.out.println(studentResult.getCc());