java 如何发送请求

发送HTTP请求的方法

Java中发送HTTP请求可以通过多种方式实现，包括使用原生HttpURLConnection、第三方库如Apache HttpClient或OkHttp，以及Java 11引入的HttpClient。

使用HttpURLConnection

HttpURLConnection是Java标准库提供的HTTP客户端，适合简单的请求场景。

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

public class HttpUrlConnectionExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://example.com/api");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");

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

        BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
        String inputLine;
        StringBuilder response = new StringBuilder();

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

        System.out.println(response.toString());
    }
}

使用Apache HttpClient

Apache HttpClient功能更强大，适合复杂的HTTP操作。

import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

public class ApacheHttpClientExample {
    public static void main(String[] args) throws Exception {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpGet request = new HttpGet("https://example.com/api");

        CloseableHttpResponse response = httpClient.execute(request);
        String result = EntityUtils.toString(response.getEntity());
        System.out.println(result);

        response.close();
        httpClient.close();
    }
}

使用OkHttp

OkHttp是现代化的HTTP客户端，支持HTTP/2和连接池。

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class OkHttpExample {
    public static void main(String[] args) throws Exception {
        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder()
                .url("https://example.com/api")
                .build();

        try (Response response = client.newCall(request).execute()) {
            System.out.println(response.body().string());
        }
    }
}

使用Java 11 HttpClient

Java 11引入的HttpClient支持同步和异步请求。

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class Java11HttpClientExample {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api"))
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.body());
    }
}

发送POST请求示例

以Java 11 HttpClient为例，发送POST请求需要设置请求体和Content-Type头。

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class PostRequestExample {
    public static void main(String[] args) throws Exception {
        String jsonBody = "{\"key\":\"value\"}";

        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api"))
                .header("Content-Type", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString(jsonBody))
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.body());
    }
}

处理HTTPS请求

对于HTTPS请求，Java默认会验证证书。如果需要绕过证书验证，可以自定义SSLContext。

import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
import java.security.cert.X509Certificate;
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class HttpsRequestExample {
    public static void main(String[] args) throws Exception {
        TrustManager[] trustAllCerts = new TrustManager[]{
            new X509TrustManager() {
                public X509Certificate[] getAcceptedIssuers() { return null; }
                public void checkClientTrusted(X509Certificate[] certs, String authType) {}
                public void checkServerTrusted(X509Certificate[] certs, String authType) {}
            }
        };

        SSLContext sslContext = SSLContext.getInstance("SSL");
        sslContext.init(null, trustAllCerts, new java.security.SecureRandom());

        HttpClient client = HttpClient.newBuilder()
                .sslContext(sslContext)
                .build();

        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api"))
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.body());
    }
}

异步请求处理

Java 11 HttpClient支持异步请求，不会阻塞当前线程。

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.util.concurrent.CompletableFuture;

public class AsyncRequestExample {
    public static void main(String[] args) {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://example.com/api"))
                .build();

        CompletableFuture<HttpResponse<String>> future = 
            client.sendAsync(request, HttpResponse.BodyHandlers.ofString());

        future.thenApply(HttpResponse::body)
              .thenAccept(System.out::println)
              .join();
    }
}