Java 后台发送请求

在Java后台（通常指服务器端）发送HTTP请求，可以使用多种库来处理，其中最常用的是`java.net`包下的`HttpURLConnection`或更现代的第三方库如`Apache HttpClient`、`OkHttp`或`Spring Web Client`。

1. 使用`HttpURLConnection`:

URL url = new URL("http://example.com/api");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET"); // 或POST、PUT等
connection.setDoOutput(true); // 如果有数据需要发送

try (BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()))) {
    String inputLine;
    StringBuilder response = new StringBuilder();
    while ((inputLine = in.readLine()) != null) {
        response.append(inputLine);
    }
    System.out.println(response.toString());
}

// 关闭连接
connection.disconnect();

2. 使用`Apache HttpClient`:

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpGet httpGet = new HttpGet("http://example.com/api");
try {
    HttpResponse response = httpClient.execute(httpGet);
    StatusLine statusLine = response.getStatusLine();
    if (statusLine.getStatusCode() == HttpStatus.SC_OK) {
        System.out.println(response.getStatusLine().toString());
        try (BufferedReader in = new BufferedReader(
                new InputStreamReader(response.getEntity().getContent()))) {
            String inputLine;
            StringBuffer content = new StringBuffer();
            while ((inputLine = in.readLine()) != null) {
                content.append(inputLine);
            }
            System.out.println(content.toString());
        } finally {
            response.close();
        }
    }
} catch (IOException e) {
    e.printStackTrace();
} finally {
    httpClient.close();
}

3. 使用`Spring WebClient` (Spring Boot项目):

RestTemplate restTemplate = new RestTemplate(); // 如果配置了Spring自动注入，则不需要创建
ResponseEntity<String> response = restTemplate.getForEntity("http://example.com/api", String.class);
String responseBody = response.getBody();
System.out.println(responseBody);