java如何调用http接口

Java可以使用HttpURLConnection或者HttpClient来调用http接口。其中HttpURLConnection是Java自带的类，HttpClient是Apache的开源项目。以下是使用HttpURLConnection调用http接口的示例代码：

import java.net.HttpURLConnection;
import java.net.URL;
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class HttpUrlConnectionExample {

    public static void main(String[] args) {
        try {
            URL url = new URL("http://example.com/api");
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Accept", "application/json");

            if (conn.getResponseCode() != 200) {
                throw new RuntimeException("Failed : HTTP error code : " + conn.getResponseCode());
            }

            BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream())));

            String output;
            System.out.println("Output from Server .... \n");
            while ((output = br.readLine()) != null) {
                System.out.println(output);
            }

            conn.disconnect();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

以上代码使用GET方法请求一个http接口，并且设置了Accept头部为application/json。如果返回的状态码不是200，则抛出异常。最后输出接口返回的内容。

如果你想使用HttpClient来调用http接口，可以参考以下代码：

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.HttpClientBuilder;
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class HttpClientExample {

    public static void main(String[] args) {
        try {
            HttpClient client = HttpClientBuilder.create().build();
            HttpGet request = new HttpGet("http://example.com/api");
            request.addHeader("Accept", "application/json");

            HttpResponse response = client.execute(request);

            BufferedReader rd = new BufferedReader(
                    new InputStreamReader(response.getEntity().getContent()));

            StringBuffer result = new StringBuffer();
            String line = "";
            while ((line = rd.readLine()) != null) {
                result.append(line);
            }

            System.out.println(result.toString());

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

以上代码使用HttpClient来调用http接口，并且设置了Accept头部为application/json。最后输出接口返回的内容。