什麽是RestTemplate

RestTemplate是Spring提供的用於訪問Rest服務的客戶端，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。

項目中註入RestTemplate

首先在項目中添加依賴：

<!-- Jackson對自動解析JSON和XML格式的支持 -->
<dependency>
    <groupId>com.fasterxml.jackson.jaxrs</groupId>
    <artifactId>jackson-jaxrs-json-provider</artifactId
 
>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-xml</artifactId>
</dependency>
 
<!-- HttpClient -->
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <
 
artifactId>httpclient</artifactId>
</dependency>

在註入RestTemplate的bean的時候，可以通過ClientHtppRequestFactory指定RestTemplate發起HTTP請求的底層實現所采用的類庫。對此，ClientHttpRequestFactory接口主要提供了以下兩種方法：
一種是SimpleClientHttpRequestFactory，使用J2SE提供的方式(即java.net包提供的方式)創建底層的HTTP請求連接。
另一種是使用HttpComponentsClientHttpRequestFactory方式，底層使用HttpClient訪問遠程的http服務，使用HttpClient可以配置連接池和證書等信息。
以下的兩個方法都采用線程安全的單例(懶漢模式)
(1)SimpleClientHttpRequestFactory

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.Lazy;
import org.springframework.http.client.SimpleClientHttpRequestFactory;
import org.springframework.http.converter.FormHttpMessageConverter;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.StringHttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter;
import org.springframework.stereotype.Component;
import org.springframework.web.client.DefaultResponseErrorHandler;
import org.springframework.web.client.RestTemplate;

import javax.annotation.PostConstruct;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;

@Component
@Lazy(false)
public class SimpleRestClient {

    private static final Logger LOGGER = LoggerFactory.getLogger(SimpleRestClient.class);

    private static RestTemplate restTemplate;

    static {
        SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
        requestFactory.setReadTimeout(5000);
        requestFactory.setConnectTimeout(5000);

        // 添加轉換器
        List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
        messageConverters.add(new StringHttpMessageConverter(Charset.forName("UTF-8")));
        messageConverters.add(new FormHttpMessageConverter());
        messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
        messageConverters.add(new MappingJackson2HttpMessageConverter());

        restTemplate = new RestTemplate(messageConverters);
        restTemplate.setRequestFactory(requestFactory);
        restTemplate.setErrorHandler(new DefaultResponseErrorHandler());

        LOGGER.info("SimpleRestClient初始化完成");
    }

    private SimpleRestClient() {

    }

    @PostConstruct
    public static RestTemplate getClient() {
        return restTemplate;
    }

}

(2）HttpComponentsClientHttpRequestFactory（推薦使用）

import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;
 
import org.apache.http.Header;
import org.apache.http.client.HttpClient;
import org.apache.http.impl.client.DefaultConnectionKeepAliveStrategy;
import org.apache.http.impl.client.DefaultHttpRequestRetryHandler;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.message.BasicHeader;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestFactory;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.http.converter.FormHttpMessageConverter;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.StringHttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter;
import org.springframework.web.client.RestTemplate;
 
@Configuration
public class RestTemplateConfig {
 
    /**
     * 返回RestTemplate
     * @param factory
     * @return
     */
    @Bean
    public RestTemplate restTemplate(ClientHttpRequestFactory factory){
        //消息轉換器，Spring Boot環境可省略，只需要添加相關依賴即可
//        List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
//        messageConverters.add(new StringHttpMessageConverter(Charset.forName("UTF-8")));
//        messageConverters.add(new FormHttpMessageConverter());
//        messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
//        messageConverters.add(new MappingJackson2HttpMessageConverter());
        
        RestTemplate restTemplate = new RestTemplate(factory);
//        restTemplate.setMessageConverters(messageConverters);
        
        return restTemplate;
    }
    
    /**
     * ClientHttpRequestFactory接口的另一種實現方式（推薦使用），即：
     * HttpComponentsClientHttpRequestFactory：底層使用Httpclient連接池的方式創建Http連接請求
     * @return
     */
    @Bean
    public HttpComponentsClientHttpRequestFactory httpComponentsClientHttpRequestFactory(){
        //Httpclient連接池，長連接保持30秒
        PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager(30, TimeUnit.SECONDS);
        
        //設置總連接數
        connectionManager.setMaxTotal(1000);
        //設置同路由的並發數
        connectionManager.setDefaultMaxPerRoute(1000);
        
        //設置header
        List<Header> headers = new ArrayList<Header>();
        headers.add(new BasicHeader("User-Agent", "Mozilla/5.0 (Windows NT 6.3; rv:36.0) Gecko/20100101 Firefox/36.04"));
        headers.add(new BasicHeader("Accept-Encoding", "gzip, deflate"));
        headers.add(new BasicHeader("Accept-Language", "zh-CN,zh;q=0.8,en-US;q=0.5,en;q=0.3"));
        headers.add(new BasicHeader("Connection", "keep-alive"));
        
        //創建HttpClient
        HttpClient httpClient = HttpClientBuilder.create()
                .setConnectionManager(connectionManager)
                .setDefaultHeaders(headers)
                .setRetryHandler(new DefaultHttpRequestRetryHandler(3, true)) //設置重試次數
                .setKeepAliveStrategy(new DefaultConnectionKeepAliveStrategy()) //設置保持長連接
                .build();
        
        //創建HttpComponentsClientHttpRequestFactory實例
        HttpComponentsClientHttpRequestFactory requestFactory = 
                new HttpComponentsClientHttpRequestFactory(httpClient);
        
        //設置客戶端和服務端建立連接的超時時間
        requestFactory.setConnectTimeout(5000);
        //設置客戶端從服務端讀取數據的超時時間
        requestFactory.setReadTimeout(5000);
        //設置從連接池獲取連接的超時時間，不宜過長
        requestFactory.setConnectionRequestTimeout(200);
        //緩沖請求數據，默認為true。通過POST或者PUT大量發送數據時，建議將此更改為false，以免耗盡內存
        requestFactory.setBufferRequestBody(false);
        
        return requestFactory;
    }
    
}

RestTemplate提供了很多方法，可以與HTTP方法對應。

接下來講一下RestTemplate的這幾個方法如何使用。

GET請求方式的兩種方法
1. getForEntity()
三個getForEntity()方法的簽名：

<T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Object... uriVariables) throws RestClientException;
 
<T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException;
 
<T> ResponseEntity<T> getForEntity(URI url, Class<T> responseType) throws RestClientException;

示例代碼：

    /**
     * 測試Get請求返回詳細信息，包括：響應正文、響應狀態碼、響應Header等
     */
    @Test
    public void testGetMethod3(){
        //第一個參數為要調用的服務的地址，第二個參數為返回值的類型，第三和第四個參數時url中的傳參
        ResponseEntity<DemoObj> responseEntity = restTemplate.getForEntity("http://127.0.0.1:9090/rest/testJson2?id={1}&name={2}"
                , DemoObj.class
                , 1,"Tom");
        
        DemoObj body = responseEntity.getBody();
        int statusCodeValue = responseEntity.getStatusCodeValue();
        HttpHeaders headers = responseEntity.getHeaders();
        
        System.out.println("responseEntity.getBody()：" + body);
        System.out.println("responseEntity.getStatusCodeValue()：" + statusCodeValue);
        System.out.println("responseEntity.getHeaders()：" + headers);
    }

結果：

responseEntity.getBody()：DemoObj [id=2, name=Tom Ret]
responseEntity.getStatusCodeValue()：200
responseEntity.getHeaders()：{Date=[Fri, 09 Feb 2018 06:22:28 GMT], Content-Type=[application/json;charset=utf-8], Transfer-Encoding=[chunked]}

再來看一個Map傳參的例子：

@RequestMapping("/sayhello2")
public String sayHello2() {
    Map<String, String> map = new HashMap<>();
    map.put("name", "李四");
    ResponseEntity<String> responseEntity = restTemplate.getForEntity("http://HELLO-SERVICE/sayhello?name={name}", String.class, map);
    return responseEntity.getBody();
}

當然，第一個參數也可以是URI而不是字符串，可以通過Spring中的UriComponents來構建uri即可。

@RequestMapping("/sayhello3")
public String sayHello3() {
    UriComponents uriComponents = UriComponentsBuilder.fromUriString("http://HELLO-SERVICE/sayhello?name={name}").build().expand("王五").encode();
    URI uri = uriComponents.toUri();
    ResponseEntity<String> responseEntity = restTemplate.getForEntity(uri, String.class);
    return responseEntity.getBody();
}

2.getForObject()
getForObject()實際上是對getForEntity()的進一步封裝，用法類似，唯一的區別就是getForObject()方法只返回請求類型的對象，而getForEntity()會返回請求的對象以及響應的Header，響應狀態碼等額外信息。

@RequestMapping("/book2")
public Book book2() {
    Book book = restTemplate.getForObject("http://HELLO-SERVICE/getbook1", Book.class);
    return book;
}

    @Test
    public void testGetMethod2(){
        Map<String, String> uriVariables = new HashMap<String, String>();
        uriVariables.put("var_id", "1");
        uriVariables.put("var_name", "Tom");
        
        DemoObj obj = restTemplate.getForObject("http://127.0.0.1:9090/rest/testJson2?id={var_id}&name={var_name}"
                , DemoObj.class
                , uriVariables);
        
        System.out.println(obj);
    }

POST請求的三種方法
1. postForEntity()
POST請求和GET請求類似，也是三種方法：

<T> ResponseEntity<T> postForEntity(String url, Object request, Class<T> responseType, Object... uriVariables)
            throws RestClientException;
 
<T> ResponseEntity<T> postForEntity(String url, Object request, Class<T> responseType, Map<String, ?> uriVariables)
            throws RestClientException;
 
<T> ResponseEntity<T> postForEntity(URI url, Object request, Class<T> responseType) throws RestClientException;

看個示例：

    @Test
    public void testPostMethod2(){
        DemoObj request = new DemoObj(1l, "Tim");
        
        ResponseEntity<DemoObj> responseEntity = restTemplate.postForEntity("http://127.0.0.1:9090/rest/testJson1"
                , request, DemoObj.class);
        
        DemoObj body = responseEntity.getBody();
        int statusCodeValue = responseEntity.getStatusCodeValue();
        HttpHeaders headers = responseEntity.getHeaders();
        
        System.out.println("responseEntity.getBody()：" + body);
        System.out.println("responseEntity.getStatusCodeValue()：" + statusCodeValue);
        System.out.println("responseEntity.getHeaders()：" + headers);
    }

結果：

responseEntity.getBody()：DemoObj [id=2, name=Tim Ret]
responseEntity.getStatusCodeValue()：200
responseEntity.getHeaders()：{Date=[Fri, 09 Feb 2018 06:32:02 GMT], Content-Type=[application/json;charset=utf-8], Transfer-Encoding=[chunked]}

2.postForObject()

    @Test
    public void testPostMethod1(){
        DemoObj request = new DemoObj(1l, "Tim");
        
        DemoObj obj = restTemplate.postForObject("http://127.0.0.1:9090/rest/testJson1"
                , request, DemoObj.class);
        
        System.out.println(obj);
    }

3.postForLocation()
postForLocation()也是提交新資源，提交成功之後，返回新資源的URI，postForLocation的參數和前面兩種的參數基本一致，只不過該方法的返回值為Uri，這個只需要服務提供者返回一個Uri即可，該Uri表示新資源的位置。

PUT請求
在RestTemplate中，PUT請求可以通過put方法調用，put方法的參數和前面介紹的postForEntity方法的參數基本一致，只是put方法沒有返回值而已。

@RequestMapping("/put")
public void put() {
    Book book = new Book();
    book.setName("紅樓夢");
    restTemplate.put("http://HELLO-SERVICE/getbook3/{1}", book, 99);
}

DELETE請求

@RequestMapping("/delete")
public void delete() {
    restTemplate.delete("http://HELLO-SERVICE/getbook4/{1}", 100);
}

exchange()方法執行指定的HTTP請求
exchange()方法同上面的很多方法不同的是，它可以指定請求的HTTP方式。

    @Test
    public void testExchange(){
        //設置header
        HttpHeaders headers = new HttpHeaders();
        headers.add("Content-Type", "application/x-zifangsky");
        
        //設置參數
        String requestBody = "1#Converter";
        HttpEntity<String> requestEntity = new HttpEntity<String>(requestBody,headers);
 
        ResponseEntity<String> responseEntity = restTemplate.exchange("http://127.0.0.1:9090/convert"
                , HttpMethod.POST, requestEntity, String.class);
        
        System.out.println("responseEntity.getBody()：" + responseEntity.getBody());
        System.out.println("responseEntity.getHeaders()：" + responseEntity.getHeaders());
    }

手動指定轉換器
調用restful接口傳遞的數據是json格式的字符串，返回的響應也是json格式的字符串。

Spring restTemplate