RestTemplate詳解

1.概要

RestTemplate是Spring提供的用於訪問的Rest服務的客戶端，提供用於訪問Rest服務的客戶端，RestTemplate提供了多種便捷訪問遠程Http服務的方法，能夠大大提高客戶端的編寫效率。

2.應用

常規配置

 public MyRestClientService(RestTemplateBuilder restTemplateBuilder) {  
        this.restTemplate = restTemplateBuilder  
            .basicAuthorization("username", "password")  
            .setConnectTimeout(3000)  
            .setReadTimeout(5000)  
            .rootUri("http://api.example.com/")  
            .build();  
    }

ClientHttpRequestInterceptor

學習使用帶有Spring RestTemplate的ClientHttpRequestInterceptor，以Spring AOP風格記錄請求和響應頭和主體。

攔截器記錄請求和響應

import org.slf4j.Logger;import org.slf4j.LoggerFactory;
import org.springframework.http.HttpRequest;
import org.springframework.http.client.ClientHttpRequestExecution;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.http.client.ClientHttpResponse;
import org.springframework.util.StreamUtils;
  
import java.io.IOException;
import java.nio.charset.Charset;
 
public class RequestResponseLoggingInterceptor implements ClientHttpRequestInterceptor {
      
    private final Logger log = LoggerFactory.getLogger(this.getClass());
  
    @Override
    public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException
    {
        logRequest(request, body);
        ClientHttpResponse response = execution.execute(request, body);
        logResponse(response);
 
        //Add optional additional headers
        response.getHeaders().add("headerName", "VALUE");
 
        return response;
    }
  
    private void logRequest(HttpRequest request, byte[] body) throws IOException
    {
        if (log.isDebugEnabled())
        {
            log.debug("===========================request begin================================================");
            log.debug("URI         : {}", request.getURI());
            log.debug("Method      : {}", request.getMethod());
            log.debug("Headers     : {}", request.getHeaders());
            log.debug("Request body: {}", new String(body, "UTF-8"));
            log.debug("==========================request end================================================");
        }
    }
  
    private void logResponse(ClientHttpResponse response) throws IOException
    {
        if (log.isDebugEnabled())
        {
            log.debug("============================response begin==========================================");
            log.debug("Status code  : {}", response.getStatusCode());
            log.debug("Status text  : {}", response.getStatusText());
            log.debug("Headers      : {}", response.getHeaders());
            log.debug("Response body: {}", StreamUtils.copyToString(response.getBody(), Charset.defaultCharset()));
            log.debug("=======================response end=================================================");
        }
    }
}

注冊ClientHttpRequestInterceptor

@Bean
public RestTemplate restTemplate(){
    RestTemplate restTemplate = new RestTemplate();
 
    restTemplate.setRequestFactory(newBufferingClientHttpRequestFactory(clientHttpRequestFactory()));
    restTemplate.setMessageConverters(Collections.singletonList(mappingJacksonHttpMessageConverter()));
 
    restTemplate.setInterceptors( Collections.singletonList(newRequestResponseLoggingInterceptor()) );
 
    return restTemplate;
}

 

三、請求示例

GET

private static void getEmployees(){
    final String uri = "http://localhost:8080/springrestexample/employees";
     
    RestTemplate restTemplate = new RestTemplate();
    String result = restTemplate.getForObject(uri, String.class);
     
    System.out.println(result);
}

使用RestTemplate定制HTTP頭文件

private static void getEmployees(){
    final String uri = "http://localhost:8080/springrestexample/employees";
     
    RestTemplate restTemplate = new RestTemplate();
     
    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(Arrays.asList(MediaType.APPLICATION_JSON));
    HttpEntity<String> entity = new HttpEntity<String>("parameters", headers);
     
    ResponseEntity<String> result = restTemplate.exchange(uri, HttpMethod.GET, entity, String.class);
     
    System.out.println(result);
}

Get請求獲取響應為一個對象

private static void getEmployees(){
    final String uri = "http://localhost:8080/springrestexample/employees";
    RestTemplate restTemplate = new RestTemplate();
     
    EmployeeListVO result = restTemplate.getForObject(uri, EmployeeListVO.class);
     
    System.out.println(result);
}

URL 參數

private static void getEmployeeById(){
    final String uri = "http://localhost:8080/springrestexample/employees/{id}";
     
    Map<String, String> params = new HashMap<String, String>();
    params.put("id", "1");
     
    RestTemplate restTemplate = new RestTemplate();
    EmployeeVO result = restTemplate.getForObject(uri, EmployeeVO.class, params);
     
    System.out.println(result);
}

POST

private static void createEmployee(){
    final String uri = "http://localhost:8080/springrestexample/employees";
 
    EmployeeVO newEmployee = new EmployeeVO(-1, "Adam", "Gilly", "[e-mail]");
 
    RestTemplate restTemplate = new RestTemplate();
    EmployeeVO result = restTemplate.postForObject( uri, newEmployee, EmployeeVO.class);
 
    System.out.println(result);
}

Submit Form Data

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 
MultiValueMap<String, String> map= new LinkedMultiValueMap<>();
map.add("id", "1");
 
HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(map, headers);
 
RestTemplate restTemplate = new RestTemplate();
EmployeeVO result = restTemplate.postForObject( uri, request, EmployeeVO.class);
System.out.println(result);

PUT

private static void updateEmployee(){
    final String uri = "http://localhost:8080/springrestexample/employees/{id}";
     
    Map<String, String> params = new HashMap<String, String>();
    params.put("id", "2");
     
    EmployeeVO updatedEmployee = new EmployeeVO(2, "New Name", "Gilly", "[e-mail]");
     
    RestTemplate restTemplate = new RestTemplate();
    restTemplate.put ( uri, updatedEmployee, params);
}

Simple PUT 

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

使用.exchange和請求回調

RequestCallback requestCallback(final Foo 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());
    };
}

DELETE

private static void deleteEmployee(){
    final String uri = "http://localhost:8080/springrestexample/employees/{id}";
     
    Map<String, String> params = new HashMap<String, String>();
    params.put("id", "2");
     
    RestTemplate restTemplate = new RestTemplate();
    restTemplate.delete ( uri,  params );
}

3.簡述

RestTemplate是Spring用於同步client端的核心類，簡化了與hhtp服務通信，並滿足RestFul原則，程序代碼可以給它提供URL，並提取結果。默認情況下。RestTemplate默認以來jdk的http連接工具。也可以通過setRequestFactory屬性切換到不同的HTTP源，比如Apache，HttpComponents、Netty和OkHttp。

3.1 該類的入口主要根據HTTP六個方法制定：

Http method	RestTemplate methods
DELETE	delete
GET	getForObject
	getFOrEntity
HEAD	headForHeaders
OPTIONS	optionsForAllow
POST	postForLoation
	postForObject
PUT	put
any	exchange
	execute

在內部， RestTemplate默認使用 HttpMessageConverter實例將 HTTP消息轉換成 POJO或者從 POJO轉換成 HTTP消息。默認情況下會注冊主mime類型的轉換器，但也可以通過 setMessageConverters注冊其他的轉換器。(其實這點在使用的時候是察覺不到的，很多方法有一個responseType 參數，它讓你傳入一個響應體所映射成的對象，然后底層用HttpMessageConverter將其做映射)

HttpMessageConverterExtractor<T> responseExtractor =
                new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);

HttpMessageConverter.java源碼：

public interface HttpMessageConverter<T> {
        //指示此轉換器是否可以讀取給定的類。
    boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);

        //指示此轉換器是否可以寫給定的類。
    boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);

        //返回List<MediaType>
    List<MediaType> getSupportedMediaTypes();

        //讀取一個inputMessage
    T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
            throws IOException, HttpMessageNotReadableException;

        //往output message寫一個Object
    void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
            throws IOException, HttpMessageNotWritableException;

}

在內部， RestTemplate默認使用 SimpleClientHttpRequestFactory和 DefaultResponseErrorHandler來分別處理 HTTP的創建和錯誤，但也可以通過 setRequestFactory和 setErrorHandler來覆蓋。

4.get請求實踐

4.1 getForObject()方法

public <T> T getForObject(String url, Class<T> responseType, Object... uriVariables){}
public <T> T getForObject(String url, Class<T> responseType, Map<String, ?> uriVariables)
public <T> T getForObject(URI url, Class<T> responseType)

getForObject()其實比 getForEntity()多包含了將HTTP轉成POJO的功能，但是 getForObject沒有處理 response的能力。因為它拿到手的就是成型的 pojo。省略了很多 response的信息。

4.1.1 pojo

public class Notice {
    private int status;
    private Object msg;
    private List<DataBean> data;
}
public  class DataBean {
  private int noticeId;
  private String noticeTitle;
  private Object noticeImg;
  private long noticeCreateTime;
  private long noticeUpdateTime;
  private String noticeContent;
}

4.1.2 不帶get請求

    /**
     * 不帶參的get請求
     */
    @Test
    public void restTemplateGetTest(){
        RestTemplate restTemplate = new RestTemplate();
        Notice notice = restTemplate.getForObject("http://xxx.top/notice/list/1/5"
                , Notice.class);
        System.out.println(notice);
    }

控制台打印：

INFO 19076 --- [           main] c.w.s.c.w.c.HelloControllerTest          
: Started HelloControllerTest in 5.532 seconds (JVM running for 7.233)

Notice{status=200, msg=null, data=[DataBean{noticeId=21, noticeTitle='aaa', noticeImg=null, 
noticeCreateTime=1525292723000, noticeUpdateTime=1525292723000, noticeContent='<p>aaa</p>'}, 
DataBean{noticeId=20, noticeTitle='ahaha', noticeImg=null, noticeCreateTime=1525291492000, 
noticeUpdateTime=1525291492000, noticeContent='<p>ah.......'

示例：4.1.3 帶參數的get請求1

Notice notice = restTemplate.getForObject("http://fantj.top/notice/list/{1}/{2}"
                , Notice.class,1,5);

明眼人一眼能看出是用了占位符{1}。

示例：4.1.4 帶參數的get請求2

        Map<String,String> map = new HashMap();
        map.put("start","1");
        map.put("page","5");
        Notice notice = restTemplate.getForObject("http://fantj.top/notice/list/"
                , Notice.class,map);

明眼人一看就是利用map裝載參數，不過它默認解析的是PathVariable的url形式。

4.2 getForEntity()方法

public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Object... uriVariables){}
public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Map<String, ?> uriVariables){}
public <T> ResponseEntity<T> getForEntity(URI url, Class<T> responseType){}

與getForObject()方法不同的是返回的是 ResponseEntity對象，如果需要轉換成pojo，還需要json工具類的引入，這個按個人喜好用。不會解析json的可以百度 FastJson或者 Jackson等工具類。然后我們就研究一下 ResponseEntity下面有啥方法。

ResponseEntity、HttpStatus、BodyBuilder結構

ResponseEntity.java

public HttpStatus getStatusCode(){}
public int getStatusCodeValue(){}
public boolean equals(@Nullable Object other) {}
public String toString() {}
public static BodyBuilder status(HttpStatus status) {}
public static BodyBuilder ok() {}
public static <T> ResponseEntity<T> ok(T body) {}
public static BodyBuilder created(URI location) {}
...

HttpStatus.java

public enum HttpStatus {
public boolean is1xxInformational() {}
public boolean is2xxSuccessful() {}
public boolean is3xxRedirection() {}
public boolean is4xxClientError() {}
public boolean is5xxServerError() {}
public boolean isError() {}
}

BodyBuilder.java

public interface BodyBuilder extends HeadersBuilder<BodyBuilder> {
    //設置正文的長度，以字節為單位，由Content-Length標頭
      BodyBuilder contentLength(long contentLength);
    //設置body的MediaType 類型
      BodyBuilder contentType(MediaType contentType);
    //設置響應實體的主體並返回它。
      <T> ResponseEntity<T> body(@Nullable T body);
｝

可以看出來，ResponseEntity包含了HttpStatus和BodyBuilder的這些信息，這更方便我們處理response原生的東西。