Java 网络编程基础与 HTTP 客户端开发 (全网最易懂教程)

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;

public class SimpleHttpClient {

    public static void main(String[] args) {
        try {
            // GET 请求示例
            URL getUrl = new URL("https://jsonplaceholder.typicode.com/todos/1");
            HttpURLConnection getConn = (HttpURLConnection) getUrl.openConnection();
            getConn.setRequestMethod("GET");
            getConn.setConnectTimeout(5000); // 5秒连接超时
            getConn.setReadTimeout(5000);    // 5秒读取超时

            int getResponseCode = getConn.getResponseCode();
            System.out.println("GET Response Code: " + getResponseCode);

            if (getResponseCode == HttpURLConnection.HTTP_OK) {
                BufferedReader in = new BufferedReader(new InputStreamReader(getConn.getInputStream()));
                String inputLine;
                StringBuilder content = new StringBuilder();
                while ((inputLine = in.readLine()) != null) {
                    content.append(inputLine);
                }
                in.close();
                System.out.println("GET Response Body: " + content.toString());
            } else {
                System.out.println("GET Request Failed.");
            }
            getConn.disconnect();

            System.out.println("\n---");

            // POST 请求示例
            URL postUrl = new URL("https://jsonplaceholder.typicode.com/posts");
            HttpURLConnection postConn = (HttpURLConnection) postUrl.openConnection();
            postConn.setRequestMethod("POST");
            postConn.setDoOutput(true); // 允许写入数据
            postConn.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
            postConn.setConnectTimeout(5000);
            postConn.setReadTimeout(5000);

            String jsonInputString = "{\"title\": \"foo\", \"body\": \"bar\", \"userId\": 1}";
            try (OutputStream os = postConn.getOutputStream()) {
                byte[] input = jsonInputString.getBytes(StandardCharsets.UTF_8);
                os.write(input, 0, input.length);
            }

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

            if (postResponseCode >= 200 && postResponseCode < 300) { // 2xx 表示成功
                BufferedReader in = new BufferedReader(new InputStreamReader(postConn.getInputStream(), StandardCharsets.UTF_8));
                String inputLine;
                StringBuilder content = new StringBuilder();
                while ((inputLine = in.readLine()) != null) {
                    content.append(inputLine);
                }
                in.close();
                System.out.println("POST Response Body: " + content.toString());
            } else {
                System.out.println("POST Request Failed. Error Stream:");
                // 错误响应可能在ErrorStream中
                try (BufferedReader errorReader = new BufferedReader(new InputStreamReader(postConn.getErrorStream(), StandardCharsets.UTF_8))) {
                    String errorLine;
                    StringBuilder errorContent = new StringBuilder();
                    while ((errorLine = errorReader.readLine()) != null) {
                        errorContent.append(errorLine);
                    }
                    System.out.println(errorContent.toString());
                }
            }
            postConn.disconnect();

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

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

public class ModernHttpClient {

    public static void main(String[] args) {
        HttpClient client = HttpClient.newBuilder()
                .version(HttpClient.Version.HTTP_2) // 默认HTTP/2，也可以指定HTTP/1.1
                .connectTimeout(java.time.Duration.ofSeconds(5)) // 连接超时
                .build();

        // 同步 GET 请求示例
        HttpRequest getRequest = HttpRequest.newBuilder()
                .uri(URI.create("https://jsonplaceholder.typicode.com/todos/1"))
                .GET() // 默认就是GET，也可以不写
                .build();

        try {
            HttpResponse<String> getResponse = client.send(getRequest, HttpResponse.BodyHandlers.ofString());
            System.out.println("GET Response Status Code: " + getResponse.statusCode());
            System.out.println("GET Response Body: " + getResponse.body());
        } catch (Exception e) {
            System.err.println("GET Request failed: " + e.getMessage());
            e.printStackTrace();
        }

        System.out.println("\n---");

        // 异步 POST 请求示例
        String jsonBody = "{\"title\": \"async foo\", \"body\": \"async bar\", \"userId\": 10}";
        HttpRequest postRequest = HttpRequest.newBuilder()
                .uri(URI.create("https://jsonplaceholder.typicode.com/posts"))
                .header("Content-Type", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString(jsonBody))
                .build();

        CompletableFuture<HttpResponse<String>> postFuture = client.sendAsync(postRequest, HttpResponse.BodyHandlers.ofString());

        // 异步处理响应
        postFuture.thenApply(HttpResponse::statusCode)
                .thenAccept(statusCode -> System.out.println("Async POST Response Status Code: " + statusCode))
                .thenCompose(v -> postFuture.thenApply(HttpResponse::body)) // 获取body
                .thenAccept(body -> System.out.println("Async POST Response Body: " + body))
                .exceptionally(e -> { // 异常处理
                    System.err.println("Async POST Request failed: " + e.getMessage());
                    e.printStackTrace();
                    return null;
                });

        // 为了让主线程等待异步请求完成
        try {
            TimeUnit.SECONDS.sleep(2); // 简单等待，实际应用中会用更健壮的同步机制
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            System.err.println("Main thread interrupted.");
        }
    }
}