Spring Cloud入门教程(番外篇一)：RestTempl

RestTemplate是Spring提供的用于访问Rest服务的客户端，RestTemplate提供了多种便捷访问远程Http服务的方法，能够大大提高客户端的编写效率。

1. 使用GET

1.1 获取JSON字符串

我们可以使用getForEntity()方法:

RestTemplate restTemplate = new RestTemplate();
String fooResourceUrl = "http://localhost:8080/spring-rest/foos";
ResponseEntity<String> response = restTemplate.getForEntity(fooResourceUrl + "/1", String.class);
assertThat(response.getStatusCode(), equalTo(HttpStatus.OK));

需要说明的是，通过getForEntity()我们可以获取到完整的HTTP response，因此我们可以通过检测状态码来判断请求是否真正执行成功。我们也可以通过getBody()方法获取返回的具体内容，如：

ObjectMapper mapper = new ObjectMapper();
JsonNode root = mapper.readTree(response.getBody());
JsonNode name = root.path("name");
assertThat(name.asText(), notNullValue());

1.2 获取POJO对象

我们也可以直接将请求直接映射为一个POJO对象，如：

public class User implements Serializable {
    private long id;
 
    private String name;
    // 这里省略了getters和setters
}

获取User对象:

User user = restTemplate.getForObject(userResourceUrl + "/1", User.class);
assertThat(user.getName(), notNullValue());
assertThat(user.getId(), is(1L));

2. 获取Headers

代码如下：

HttpHeaders httpHeaders = restTemplate.headForHeaders(fooResourceUrl);
assertTrue(httpHeaders.getContentType().includes(MediaType.APPLICATION_JSON));

3. 使用POST

RestTemplate提供了三个API用来创建资源，它们分别是postForLocation()、 postForObject()和postForEntity()。postForLocation()返回新创建资源的URI，postForObject()则返回新创建的资源本身。

3.1 postForObject方法

ClientHttpRequestFactory requestFactory = getClientHttpRequestFactory();
RestTemplate restTemplate = new RestTemplate(requestFactory);
 
HttpEntity<Foo> request = new HttpEntity<>(new Foo("bar"));
Foo foo = restTemplate.postForObject(fooResourceUrl, request, Foo.class);
assertThat(foo, notNullValue());
assertThat(foo.getName(), is("bar"));

3.2 postForLocation方法

HttpEntity<Foo> request = new HttpEntity<>(new Foo("bar"));
URI location = restTemplate.postForLocation(fooResourceUrl, request);
assertThat(location, notNullValue());

3.3 exchange方法

RestTemplate还提供了一个更加通用的方法：exchange，下面我们来看看如何使用该方法完成一个POST请求：:

RestTemplate restTemplate = new RestTemplate();
HttpEntity<User> request = new HttpEntity<>(new User("CD826"));
ResponseEntity<User> response = restTemplate.exchange(userResourceUrl, HttpMethod.POST, request, User.class);
  
assertThat(response.getStatusCode(), is(HttpStatus.CREATED));
  
User user = response.getBody();
  
assertThat(user, notNullValue());
assertThat(user.getName(), is("CD826"));  

4. 获取允许执行操作列表

optionsForAllow方法可以让我们获取给定URI中允许执行的操作列表:

Set<HttpMethod> optionsForAllow = restTemplate.optionsForAllow(fooResourceUrl);
HttpMethod[] supportedMethods = {HttpMethod.GET, HttpMethod.POST, HttpMethod.PUT, HttpMethod.DELETE};
assertTrue(optionsForAllow.containsAll(Arrays.asList(supportedMethods)));

5. 使用PUT

5.1 简单的PUT

我们先看一个简单的PUT示例，这里需要注意的时该请求并不会有任何返回:

User updatedInstance = new User("newName");
updatedInstance.setId(createResponse.getBody().getId());
String resourceUrl = userResourceUrl + '/' + createResponse.getBody().getId();
HttpEntity<User> requestUpdate = new HttpEntity<>(updatedInstance, headers);
template.exchange(resourceUrl, HttpMethod.PUT, requestUpdate, Void.class);

5.2 带回调的PUT

我们先定义一个回调函数:

RequestCallback requestCallback(final User updatedInstance) {
    return clientHttpRequest -> {
        ObjectMapper mapper = new ObjectMapper();
        mapper.writeValue(clientHttpRequest.getBody(), updatedInstance);
        clientHttpRequest.getHeaders().add(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE);
        clientHttpRequest.getHeaders().add(HttpHeaders.AUTHORIZATION, "Basic " + getBase64EncodedLogPass());
    };
}

然后，通过POST先创建需要更新的资源:

ResponseEntity<User> response = restTemplate.exchange(userResourceUrl, HttpMethod.POST, request, User.class);
assertThat(response.getStatusCode(), is(HttpStatus.CREATED));

最后，我们使用PUT进行更新:

User updatedInstance = new User("newName");
updatedInstance.setId(response.getBody().getId());
String resourceUrl = userResourceUrl + '/' + response.getBody().getId();
restTemplate.execute(resourceUrl, HttpMethod.PUT, 
                     requestCallback(updatedInstance), 
                     clientHttpResponse -> null);

6. 使用DELETE

示例如下:

String entityUrl = fooResourceUrl + "/" + existingResource.getId();
restTemplate.delete(entityUrl);