利用Java实现调用http请求

以下是利用Java实现调用HTTP请求的完整攻略。

简介

在Java中，我们可以使用HttpURLConnection或者Apache HttpClient来实现HTTP请求。两者区别在于HttpURLConnection是内置于Java SDK中的，而Apache HttpClient是第三方库。以下分别讲解这两种方式的使用方法。

使用HttpURLConnection实现HTTP请求

1. 创建URL对象，指定请求的URL地址。

URL url = new URL("http://example.com/api");

1. 使用url.openConnection()方法打开一个连接并得到HttpURLConnection对象。

HttpURLConnection connection = (HttpURLConnection) url.openConnection();

1. 设置请求方法和请求头信息。

connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/json; charset=UTF-8");

1. 添加请求参数，并发送请求。

connection.setDoOutput(true);
OutputStream os = connection.getOutputStream();
os.write(json.getBytes("UTF-8"));
os.flush();
os.close();

1. 接收响应。

int status = connection.getResponseCode();

InputStream inputStream;
if (status >= 400) {
    inputStream = connection.getErrorStream();
} else {
    inputStream = connection.getInputStream();
}

BufferedReader in = new BufferedReader(new InputStreamReader(inputStream));
String inputLine;
StringBuilder response = new StringBuilder();
while ((inputLine = in.readLine()) != null) {
    response.append(inputLine);
}
in.close();

完整代码示例：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class HttpURLConnectionExample {

    private static final String USER_AGENT = "Mozilla/5.0";
    private static final String POST_URL = "http://example.com/api";
    private static final String POST_PARAMS = "{\n" + "\"name\": \"John\",\r\n" + "\"age\": 31,\r\n" + "\"city\": \"New York\"\r\n" + "}";

    public static void main(String[] args) throws Exception {

        URL obj = new URL(POST_URL);
        HttpURLConnection con = (HttpURLConnection) obj.openConnection();

        con.setRequestMethod("POST");
        con.setRequestProperty("User-Agent", USER_AGENT);
        con.setRequestProperty("Accept-Language", "en-US,en;q=0.5");

        // For POST only - START
        con.setDoOutput(true);
        OutputStream os = con.getOutputStream();
        os.write(POST_PARAMS.getBytes());
        os.flush();
        os.close();
        // For POST only - END

        int responseCode = con.getResponseCode();
        System.out.println("POST Response Code :: " + responseCode);

        if (responseCode == HttpURLConnection.HTTP_OK) { //success
            BufferedReader in = new BufferedReader(new InputStreamReader(
                    con.getInputStream()));
            String inputLine;
            StringBuffer response = new StringBuffer();

            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();

            // print result
            System.out.println(response.toString());
        } else {
            System.out.println("POST request not worked");
        }

    }
}

使用Apache HttpClient实现HTTP请求

1. 导入httpclient和httpcore两个依赖包。

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.13</version>
</dependency>
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpcore</artifactId>
    <version>4.4.14</version>
</dependency>

1. 创建CloseableHttpClient对象。

CloseableHttpClient httpclient = HttpClients.createDefault();

1. 创建HttpPost对象，并设置请求头信息。

HttpPost httppost = new HttpPost("http://example.com/api");

httppost.setHeader("Content-type", "application/json; charset=UTF-8");

1. 添加请求参数，并执行请求。

StringEntity params = new StringEntity(json.toString(),"UTF-8");
httppost.setEntity(params);
HttpResponse response = httpclient.execute(httppost);

1. 接收响应。

HttpEntity entity = response.getEntity();
String responseString = EntityUtils.toString(entity, "UTF-8");

完整代码示例：

import java.io.IOException;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
import org.json.JSONObject;

public class HttpClientExample {

    public static void main(String[] args) throws ClientProtocolException, IOException {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpPost httppost = new HttpPost("http://example.com/api");

        httppost.setHeader("Content-type", "application/json; charset=UTF-8");

        JSONObject json = new JSONObject();
        json.put("name", "John");
        json.put("age", 31);
        json.put("city", "New York");

        StringEntity params = new StringEntity(json.toString(),"UTF-8");
        httppost.setEntity(params);

        HttpResponse response = httpclient.execute(httppost);

        HttpEntity entity = response.getEntity();
        String responseString = EntityUtils.toString(entity, "UTF-8");
        System.out.println(responseString);
    }
}

以上就是利用Java实现调用HTTP请求的完整攻略，两种方式均已详细讲解，并且分别提供了一个完整代码示例。