如何使用java.net.URLConnection发起和处理HTTP请求

如何使用java.net.URLConnection发起和处理HTTP请求

技术背景

在Java开发中，经常需要与网络上的资源进行交互，发起HTTP请求是常见的操作。java.net.URLConnection 是Java标准库中用于处理URL连接的抽象类，它提供了一种方便的方式来发起和处理HTTP请求。不过从Java 11开始，也有了 java.net.http.HttpClient 这一相对简洁的API用于处理HTTP请求。

实现步骤

准备工作

首先需要知道请求的URL和字符集，参数是可选的，取决于功能需求。

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String url = "http://example.com";
String charset = "UTF-8";  // Or in Java 7 and later, use the constant: java.nio.charset.StandardCharsets.UTF_8.name()
String param1 = "value1";
String param2 = "value2";
// ...

String query = String.format("param1=%s&param2=%s",
    URLEncoder.encode(param1, charset),
    URLEncoder.encode(param2, charset));

发起HTTP GET请求

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URLConnection connection = new URL(url + "?" + query).openConnection();
connection.setRequestProperty("Accept-Charset", charset);
InputStream response = connection.getInputStream();
// ...

如果不需要设置任何头信息，还可以使用 URL#openStream() 快捷方法：

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InputStream response = new URL(url).openStream();
// ...

发起HTTP POST请求

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URLConnection connection = new URL(url).openConnection();
connection.setDoOutput(true); // Triggers POST.
connection.setRequestProperty("Accept-Charset", charset);
connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=" + charset);

try (OutputStream output = connection.getOutputStream()) {
    output.write(query.getBytes(charset));
}

InputStream response = connection.getInputStream();
// ...

设置超时时间

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HttpURLConnection httpConnection = (HttpURLConnection) new URL(url).openConnection();
httpConnection.setConnectTimeout(3000); // 3s
httpConnection.setReadTimeout(6000); // 6s

在使用基于Sun/Oracle的JRE时，对于POST请求，需要关闭读取超时重试机制：

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System.setProperty("sun.net.http.retryPost", "false");

在Android开发中，需要使用以下变通方法：

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httpConnection.setChunkedStreamingMode(0);

收集HTTP响应信息

获取HTTP响应状态

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int status = httpConnection.getResponseCode();

获取HTTP响应头

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for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
    System.out.println(header.getKey() + "=" + header.getValue());
}

获取HTTP响应编码

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String contentType = connection.getHeaderField("Content-Type");
String charset = null;

for (String param : contentType.replace(" ", "").split(";")) {
    if (param.startsWith("charset=")) {
        charset = param.split("=", 2)[1];
        break;
    }
}

if (charset != null) {
    try (BufferedReader reader = new BufferedReader(new InputStreamReader(response, charset))) {
        for (String line; (line = reader.readLine()) != null;) {
            // ... System.out.println(line)?
        }
    }
} else {
    // It's likely binary content, use InputStream/OutputStream.
}

维护会话

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// First set the default cookie manager.
CookieHandler.setDefault(new CookieManager(null, CookiePolicy.ACCEPT_ALL));

// All the following subsequent URLConnections will use the same cookie manager.
URLConnection connection = new URL(url).openConnection();
// ...

设置流式传输模式

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httpConnection.setFixedLengthStreamingMode(contentLength);

如果事先不知道内容长度，可以使用分块流式传输模式：

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httpConnection.setChunkedStreamingMode(1024);

设置User-Agent

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connection.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36");

错误处理

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InputStream error = ((HttpURLConnection) connection).getErrorStream();

如果HTTP响应代码为 -1，可以关闭连接保持机制：

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System.setProperty("http.keepAlive", "false");

上传文件

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String param = "value";
File textFile = new File("/path/to/file.txt");
File binaryFile = new File("/path/to/file.bin");
String boundary = Long.toHexString(System.currentTimeMillis()); // Just generate some unique random value.
String CRLF = "\r\n"; // Line separator required by multipart/form-data.
URLConnection connection = new URL(url).openConnection();
connection.setDoOutput(true);
connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);

try (
    OutputStream output = connection.getOutputStream();
    PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, charset), true);
) {
    // Send normal param.
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"param\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
    writer.append(CRLF).append(param).append(CRLF).flush();

    // Send text file.
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"textFile\"; filename=\"" + textFile.getName() + "\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF); // Text file itself must be saved in this charset!
    writer.append(CRLF).flush();
    Files.copy(textFile.toPath(), output);
    output.flush(); // Important before continuing with writer!
    writer.append(CRLF).flush(); // CRLF is important! It indicates end of boundary.

    // Send binary file.
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"binaryFile\"; filename=\"" + binaryFile.getName() + "\"").append(CRLF);
    writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
    writer.append("Content-Transfer-Encoding: binary").append(CRLF);
    writer.append(CRLF).flush();
    Files.copy(binaryFile.toPath(), output);
    output.flush(); // Important before continuing with writer!
    writer.append(CRLF).flush(); // CRLF is important! It indicates end of boundary.

    // End of multipart/form-data.
    writer.append("--" + boundary + "--").append(CRLF).flush();
}

处理不受信任或配置错误的HTTPS站点

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static {
    TrustManager[] trustAllCertificates = new TrustManager[] {
        new X509TrustManager() {
            @Override
            public X509Certificate[] getAcceptedIssuers() {
                return null; // Not relevant.
            }
            @Override
            public void checkClientTrusted(X509Certificate[] certs, String authType) {
                // Do nothing. Just allow them all.
            }
            @Override
            public void checkServerTrusted(X509Certificate[] certs, String authType) {
                // Do nothing. Just allow them all.
            }
        }
    };

    HostnameVerifier trustAllHostnames = new HostnameVerifier() {
        @Override
        public boolean verify(String hostname, SSLSession session) {
            return true; // Just allow them all.
        }
    };

    try {
        System.setProperty("jsse.enableSNIExtension", "false");
        SSLContext sc = SSLContext.getInstance("SSL");
        sc.init(null, trustAllCertificates, new SecureRandom());
        HttpsURLConnection.setDefaultSSLSocketFactory(sc.getSocketFactory());
        HttpsURLConnection.setDefaultHostnameVerifier(trustAllHostnames);
    }
    catch (GeneralSecurityException e) {
        throw new ExceptionInInitializerError(e);
    }
}

核心代码

以下是一个完整的示例代码，展示了如何使用 java.net.URLConnection 发起HTTP GET和POST请求：

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import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;
import java.util.Scanner;

public class HttpExample {
    public static void main(String[] args) throws IOException {
        String url = "http://example.com";
        String charset = StandardCharsets.UTF_8.name();
        String param1 = "value1";
        String param2 = "value2";
        String query = String.format("param1=%s&param2=%s",
                URLEncoder.encode(param1, charset),
                URLEncoder.encode(param2, charset));

        // HTTP GET request
        URLConnection getConnection = new URL(url + "?" + query).openConnection();
        getConnection.setRequestProperty("Accept-Charset", charset);
        InputStream getResponse = getConnection.getInputStream();
        try (Scanner scanner = new Scanner(getResponse)) {
            String responseBody = scanner.useDelimiter("\\A").next();
            System.out.println("GET Response: " + responseBody);
        }

        // HTTP POST request
        URLConnection postConnection = new URL(url).openConnection();
        postConnection.setDoOutput(true);
        postConnection.setRequestProperty("Accept-Charset", charset);
        postConnection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=" + charset);
        try (OutputStream output = postConnection.getOutputStream()) {
            output.write(query.getBytes(charset));
        }
        InputStream postResponse = postConnection.getInputStream();
        try (Scanner scanner = new Scanner(postResponse)) {
            String responseBody = scanner.useDelimiter("\\A").next();
            System.out.println("POST Response: " + responseBody);
        }
    }
}

最佳实践

  • 对于Java 11及更高版本，优先考虑使用 java.net.http.HttpClient 进行HTTP请求，它提供了更简洁的API和更好的异步支持。
  • 在处理HTTP请求时，始终处理可能的异常，如 IOException 等。
  • 对于需要维护会话的场景，使用 CookieHandler 来管理cookie。
  • 为了避免内存溢出问题，对于大文件上传，使用流式传输模式。

常见问题

403 Forbidden错误

可能是服务器基于 User-Agent 请求头阻止了请求，可以通过设置 User-Agent 来解决。

连接超时或读取超时

可以通过设置 setConnectTimeout() 和 setReadTimeout() 来解决。

不受信任的服务器证书

可以使用上述的 TrustManager 和 HostnameVerifier 来处理不受信任的服务器证书。

后端开发 > Java网络编程
#后端开发 #Java #java.net.URLConnection #HTTP请求处理 #Java网络编程
如何使用java.net.URLConnection发起和处理HTTP请求
https://119291.xyz/posts/how-to-use-java-net-urlconnection-to-fire-and-handle-http-requests/
作者
ww
发布于
2025年5月21日
许可协议