【Stackoverflow问题精选】如何使用java.net.URLConnection收发HTTP请求

String url = "http://example.com";
String charset = "UTF-8";
String param1 = "value1";
String param2 = "value2";
// ...
String query = String.format("param1=%s¶m2=%s", 
URLEncoder.encode(param1, charset), 
URLEncoder.encode(param2, charset));

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

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();
// ...

HttpURLConnection httpConnection = (HttpURLConnection) new URL(url).openConnection();
httpConnection.setRequestMethod("POST");
// ...

int status = httpConnection.getResponseCode();

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

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.
}

// 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();
// ...
connection = new URL(url).openConnection();
// ...
connection = new URL(url).openConnection();
// ...

// Gather all cookies on the first request.
URLConnection connection = new URL(url).openConnection();
List<String> cookies = connection.getHeaderFields().get("Set-Cookie");
// ...

// Then use the same cookies on all subsequent requests.
connection = new URL(url).openConnection();
for (String cookie : cookies) {
    connection.addRequestProperty("Cookie", cookie.split(";", 2)[0]);
}
// ...

httpConnection.setFixedLengthStreamingMode(contentLength);

httpConnection.setChunkedStreamingMode(1024);

connection.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.3) Gecko/20100401"); // Do as if you're using Firefox 3.6.3.

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

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();
}

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