在Spring Boot应用中使用RestTemplate类发送不同的HTTP GET请求（带请求头、不带请求头）

原文链接：https://www.cnblogs.com/windyWu/p/16872871.html

在本文中，你将学会在Spring Boot应用中使用RestTemplate类发送不同的HTTP GET请求。

简单GET请求

发送GET HTTP请求，可以使用getForObject()或getForEntity()方法。
如下示例，使用getForObject()方法获取JSON字符串形式的用户信息：

// request url
String url = "https://jsonplaceholder.typicode.com/posts/1";

// create an instance of RestTemplate
RestTemplate restTemplate = new RestTemplate();

// make an HTTP GET request
String json = restTemplate.getForObject(url, String.class);

// print json
System.out.println(json);

带参数的GET请求

发送请求参数时，可以把参数直接追加到URL尾部，或者使用占位符。
示例如下，将请求参数追加到URL尾部的GET请求：

// request url
String url = "https://google.com/search?q=java";

// create an instance of RestTemplate
RestTemplate restTemplate = new RestTemplate();

// make an HTTP GET request
String html = restTemplate.getForObject(url, String.class);

相似的，在URL里增加占位符的方式携带请求参数：

// request url
String url = "https://google.com/search?q={q}";

// create an instance of RestTemplate
RestTemplate restTemplate = new RestTemplate();

// make an HTTP GET request
String html = restTemplate.getForObject(url, String.class, "java");

带参数和头的GET请求

在HTTP GET请求中携带自定义的请求头，应该使用RestTemplate类提供的通用的exchange()方法。
如下GET请求携带了请求参数和请求头：

// request url
String url = "https://jsonplaceholder.typicode.com/posts/{id}";

// create an instance of RestTemplate
RestTemplate restTemplate = new RestTemplate();

// create headers
HttpHeaders headers = new HttpHeaders();

// set `Content-Type` and `Accept` headers
headers.setContentType(MediaType.APPLICATION_JSON);
headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));

// example of custom header
headers.set("X-Request-Source", "Desktop");

// build the request
HttpEntity request = new HttpEntity(headers);

// make an HTTP GET request with headers
ResponseEntity<String> response = restTemplate.exchange(
        url,
        HttpMethod.GET,
        request,
        String.class,
        1
);

// check response
if (response.getStatusCode() == HttpStatus.OK) {
    System.out.println("Request Successful.");
    System.out.println(response.getBody());
} else {
    System.out.println("Request Failed");
    System.out.println(response.getStatusCode());
}

}

使用Basic Authentication的GET请求

如下示例，在RestTemplate GET请求中携带basic authentication。

// request url
String url = "https://jsonplaceholder.typicode.com/posts";

// create an instance of RestTemplate
RestTemplate restTemplate = new RestTemplate();

// create headers
HttpHeaders headers = new HttpHeaders();

// add basic authentication header
headers.setBasicAuth("username", "password");

// build the request
HttpEntity request = new HttpEntity(headers);

// make an HTTP GET request with headers
ResponseEntity<String> response = restTemplate.exchange(
        url,
        HttpMethod.GET,
        request,
        String.class
);

// check response
if (response.getStatusCode() == HttpStatus.OK) {
    System.out.println("Request Successful.");
    System.out.println(response.getBody());
} else {
    System.out.println("Request Failed");
    System.out.println(response.getStatusCode());
}

将响应映射到Java对象的GET请求

使用RestTemplate，可以将JSON形式的响应直接映射为Java对象。首先创建一个简单的实体类：
Post.java

public class Post implements Serializable {

    private int userId;
    private int id;
    private String title;
    private String body;

    public Post() {
    }

    public Post(int userId, int id, String title, String body) {
        this.userId = userId;
        this.id = id;
        this.title = title;
        this.body = body;
    }

    // getters and setters, equals(), toString() .... (omitted for brevity)
}

现在，exchange()方法可以使用Post类作为响应类型了。

// request url
String url = "https://jsonplaceholder.typicode.com/posts/1";

// create an instance of RestTemplate
RestTemplate restTemplate = new RestTemplate();

// create headers
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);
headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));

// build the request
HttpEntity request = new HttpEntity(headers);

// make an HTTP GET request with headers
ResponseEntity<Post> response = restTemplate.exchange(
        url,
        HttpMethod.GET,
        request,
        Post.class
);

// check response
if (response.getStatusCode() == HttpStatus.OK) {
    System.out.println("Request Successful.");
    System.out.println(response.getBody());
} else {
    System.out.println("Request Failed");
    System.out.println(response.getStatusCode());
}