如何使用java.net.URLConnection触发和处理HTTP请求？

在这里，经常会问到的用法是什么，而且对它的描述过于简洁

该教程基本上只显示了如何触发GET请求并读取响应。它没有解释如何使用它来执行POST请求、设置请求头、读取响应头、处理cookie、提交HTML表单、上载文件等

那么，我如何使用

java.net.URLConnection

来触发和处理“高级”HTTP请求呢？

首先要声明：发布的代码片段都是基本示例。您需要处理琐碎的

IOException

s和

RuntimeException

s，如

NullPointerException

，

ArrayIndexOutOfBoundsException

，并与您自己进行关联

如果您是为Android而不是Java开发的，请注意，自从引入API级别28以来，明文HTTP请求就不存在了。我们鼓励您使用

HttpsURLConnection

，但如果确实需要，可以在应用程序清单中启用明文

---

准备 我们首先需要至少知道URL和字符集。参数是可选的，取决于功能要求

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));

查询参数必须采用

name=value

格式，并由

&

连接。您通常还可以使用指定的字符集来指定查询参数

字符串#format（）

只是为了方便起见。当我需要两次以上的字符串连接操作符

+

时，我更喜欢它

---

使用（可选）查询参数触发请求 这是一项微不足道的任务。这是默认的请求方法

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

任何查询字符串都应该使用

？

连接到URL。标头可能会提示服务器参数的编码方式。如果不发送任何查询字符串，则可以将

Accept字符集

标题保留。如果您不需要设置任何标题，那么您甚至可以使用快捷方式方法

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

无论哪种方式，如果另一端是，则将调用其方法，并且参数将由提供

出于测试目的，您可以将响应正文打印到stdout，如下所示：

try (Scanner scanner = new Scanner(response)) {
    String responseBody = scanner.useDelimiter("\\A").next();
    System.out.println(responseBody);
}

---

使用查询参数触发请求 将设置为

true

会隐式地将请求方法设置为POST。web表单的标准HTTP POST类型为

application/x-www-form-urlencoded

，其中查询字符串被写入请求主体

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();
// ...

注意：当您希望以编程方式提交HTML表单时，不要忘记将任何

元素的

name=value

对放入查询字符串，当然也要将您希望以编程方式“按”的

元素的

name=value

对放入查询字符串（因为这通常在服务器端用于区分是否按下了按钮，如果按下了按钮，则是哪个按钮）

您也可以将获取的连接强制转换为并使用其。但是，如果您试图将该连接用于输出，则仍需要设置为

true

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

无论哪种方式，如果另一端是，则将调用其方法，并且参数将由提供

---

实际触发HTTP请求 您可以使用显式激发HTTP请求，但当您想要获取有关HTTP响应的任何信息（例如使用的响应正文等）时，请求将根据需要自动激发。上面的示例正是这样做的，因此

connect（）

调用实际上是多余的

---

收集HTTP响应信息

:

在这里你需要一个角色，必要时先施展

    int status = httpConnection.getResponseCode();

:

---

维持会议 服务器端会话通常由cookie支持。某些web表单要求您登录和/或由会话跟踪。您可以使用API维护cookie。在发送所有HTTP请求之前，您需要准备一个带有的

// 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]);
}
// ...

请注意，这并非在所有情况下都能正常工作。如果失败，那么最好是手动收集和设置cookie头。您基本上需要从登录响应或第一个

GET

请求中获取所有

set cookie

头，然后将其传递给后续请求

// 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]);
}
// ...

但是，如果事先确实不知道内容长度，那么您可以通过相应地设置来使用分块流模式。这将把HTTP头设置为

chunked

，这将强制以分块的形式发送请求正文。下面的示例将以1KB的分块发送正文

httpConnection.setChunkedStreamingMode(1024);

---

用户代理 可能会发生这种情况。服务器端可能正在根据请求头阻止请求。默认情况下，

URLConnection

会将其设置为

Java/1.6.0_19

，其中最后一部分显然是JRE版本。您可以按如下方式覆盖此设置：

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"); // Do as if you're using Chrome 41 on Windows 7.

从中使用用户代理字符串

---

错误处理 如果HTTP响应代码为

4nn

（客户端错误）或

5nn

（服务器错误），则您可能需要读取

HttpURLConnection#getErrorStream（）

，以查看服务器是否发送了任何有用的错误信息

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

如果HTTP响应代码为-1，则连接和响应处理出现问题。

HttpURLConnection

实现在较旧的JRE中，在保持连接活动方面有些问题。您可能希望通过将

HTTP.keepAlive

系统属性设置为

false

来关闭它。您可以执行此程序在您的申请开始时：

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

---

上传文件 您通常会对混合发布内容（二进制和字符数据）使用编码。中详细介绍了编码

如果另一端是，则将调用它的方法，并且部件将由（注意，因此不是

getParameter（）

等等！）提供

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

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();
}

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);
    }
}

int responseCode = httpURLConnection.getResponseCode();

if (responseCode == HttpURLConnection.HTTP_OK) {

package org.boon.utils;


import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.util.Map;

import static org.boon.utils.IO.read;

public class HTTP {

public static String get(
        final String url) {

    Exceptions.tryIt(() -> {
        URLConnection connection;
        connection = doGet(url, null, null, null);
        return extractResponseString(connection);
    });
    return null;
}

public static String getWithHeaders(
        final String url,
        final Map<String, ? extends Object> headers) {
    URLConnection connection;
    try {
        connection = doGet(url, headers, null, null);
        return extractResponseString(connection);
    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }
}

public static String getWithContentType(
        final String url,
        final Map<String, ? extends Object> headers,
        String contentType) {
    URLConnection connection;
    try {
        connection = doGet(url, headers, contentType, null);
        return extractResponseString(connection);
    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }
}
public static String getWithCharSet(
        final String url,
        final Map<String, ? extends Object> headers,
        String contentType,
        String charSet) {
    URLConnection connection;
    try {
        connection = doGet(url, headers, contentType, charSet);
        return extractResponseString(connection);
    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }
}

public static String postBody(
        final String url,
        final String body) {
    URLConnection connection;
    try {
        connection = doPost(url, null, "text/plain", null, body);
        return extractResponseString(connection);
    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }
}

public static String postBodyWithHeaders(
        final String url,
        final Map<String, ? extends Object> headers,
        final String body) {
    URLConnection connection;
    try {
        connection = doPost(url, headers, "text/plain", null, body);
        return extractResponseString(connection);
    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }
}



public static String postBodyWithContentType(
        final String url,
        final Map<String, ? extends Object> headers,
        final String contentType,
        final String body) {

    URLConnection connection;
    try {
        connection = doPost(url, headers, contentType, null, body);


        return extractResponseString(connection);


    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }


}


public static String postBodyWithCharset(
        final String url,
        final Map<String, ? extends Object> headers,
        final String contentType,
        final String charSet,
        final String body) {

    URLConnection connection;
    try {
        connection = doPost(url, headers, contentType, charSet, body);


        return extractResponseString(connection);


    } catch (Exception ex) {
        Exceptions.handle(ex);
        return null;
    }


}

private static URLConnection doPost(String url, Map<String, ? extends Object> headers,
                                    String contentType, String charset, String body
                                    ) throws IOException {
    URLConnection connection;/* Handle output. */
    connection = new URL(url).openConnection();
    connection.setDoOutput(true);
    manageContentTypeHeaders(contentType, charset, connection);

    manageHeaders(headers, connection);


    IO.write(connection.getOutputStream(), body, IO.CHARSET);
    return connection;
}

private static void manageHeaders(Map<String, ? extends Object> headers, URLConnection connection) {
    if (headers != null) {
        for (Map.Entry<String, ? extends Object> entry : headers.entrySet()) {
            connection.setRequestProperty(entry.getKey(), entry.getValue().toString());
        }
    }
}

private static void manageContentTypeHeaders(String contentType, String charset, URLConnection connection) {
    connection.setRequestProperty("Accept-Charset", charset == null ? IO.CHARSET : charset);
    if (contentType!=null && !contentType.isEmpty()) {
        connection.setRequestProperty("Content-Type", contentType);
    }
}

private static URLConnection doGet(String url, Map<String, ? extends Object> headers,
                                    String contentType, String charset) throws IOException {
    URLConnection connection;/* Handle output. */
    connection = new URL(url).openConnection();
    manageContentTypeHeaders(contentType, charset, connection);

    manageHeaders(headers, connection);

    return connection;
}

private static String extractResponseString(URLConnection connection) throws IOException {
/* Handle input. */
    HttpURLConnection http = (HttpURLConnection)connection;
    int status = http.getResponseCode();
    String charset = getCharset(connection.getHeaderField("Content-Type"));

    if (status==200) {
        return readResponseBody(http, charset);
    } else {
        return readErrorResponseBody(http, status, charset);
    }
}

private static String readErrorResponseBody(HttpURLConnection http, int status, String charset) {
    InputStream errorStream = http.getErrorStream();
    if ( errorStream!=null ) {
        String error = charset== null ? read( errorStream ) :
            read( errorStream, charset );
        throw new RuntimeException("STATUS CODE =" + status + "\n\n" + error);
    } else {
        throw new RuntimeException("STATUS CODE =" + status);
    }
}

private static String readResponseBody(HttpURLConnection http, String charset) throws IOException {
    if (charset != null) {
        return read(http.getInputStream(), charset);
    } else {
        return read(http.getInputStream());
    }
}

private static String getCharset(String contentType) {
    if (contentType==null)  {
        return null;
    }
    String charset = null;
    for (String param : contentType.replace(" ", "").split(";")) {
        if (param.startsWith("charset=")) {
            charset = param.split("=", 2)[1];
            break;
        }
    }
    charset = charset == null ?  IO.CHARSET : charset;

    return charset;
}

static class MyHandler implements HttpHandler {
    public void handle(HttpExchange t) throws IOException {

        InputStream requestBody = t.getRequestBody();
        String body = IO.read(requestBody);
        Headers requestHeaders = t.getRequestHeaders();
        body = body + "\n" + copy(requestHeaders).toString();
        t.sendResponseHeaders(200, body.length());
        OutputStream os = t.getResponseBody();
        os.write(body.getBytes());
        os.close();
    }
}


@Test
public void testHappy() throws Exception {

    HttpServer server = HttpServer.create(new InetSocketAddress(9212), 0);
    server.createContext("/test", new MyHandler());
    server.setExecutor(null); // creates a default executor
    server.start();

    Thread.sleep(10);


    Map<String,String> headers = map("foo", "bar", "fun", "sun");

    String response = HTTP.postBodyWithContentType("http://localhost:9212/test", headers, "text/plain", "hi mom");

    System.out.println(response);

    assertTrue(response.contains("hi mom"));
    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));


    response = HTTP.postBodyWithCharset("http://localhost:9212/test", headers, "text/plain", "UTF-8", "hi mom");

    System.out.println(response);

    assertTrue(response.contains("hi mom"));
    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));

    response = HTTP.postBodyWithHeaders("http://localhost:9212/test", headers, "hi mom");

    System.out.println(response);

    assertTrue(response.contains("hi mom"));
    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));


    response = HTTP.get("http://localhost:9212/test");

    System.out.println(response);


    response = HTTP.getWithHeaders("http://localhost:9212/test", headers);

    System.out.println(response);

    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));



    response = HTTP.getWithContentType("http://localhost:9212/test", headers, "text/plain");

    System.out.println(response);

    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));



    response = HTTP.getWithCharSet("http://localhost:9212/test", headers, "text/plain", "UTF-8");

    System.out.println(response);

    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));

    Thread.sleep(10);

    server.stop(0);


}

@Test
public void testPostBody() throws Exception {

    HttpServer server = HttpServer.create(new InetSocketAddress(9220), 0);
    server.createContext("/test", new MyHandler());
    server.setExecutor(null); // creates a default executor
    server.start();

    Thread.sleep(10);


    Map<String,String> headers = map("foo", "bar", "fun", "sun");

    String response = HTTP.postBody("http://localhost:9220/test", "hi mom");

    assertTrue(response.contains("hi mom"));


    Thread.sleep(10);

    server.stop(0);


}

@Test(expected = RuntimeException.class)
public void testSad() throws Exception {

    HttpServer server = HttpServer.create(new InetSocketAddress(9213), 0);
    server.createContext("/test", new MyHandler());
    server.setExecutor(null); // creates a default executor
    server.start();

    Thread.sleep(10);


    Map<String,String> headers = map("foo", "bar", "fun", "sun");

    String response = HTTP.postBodyWithContentType("http://localhost:9213/foo", headers, "text/plain", "hi mom");

    System.out.println(response);

    assertTrue(response.contains("hi mom"));
    assertTrue(response.contains("Fun=[sun], Foo=[bar]"));

    Thread.sleep(10);

    server.stop(0);


}

String html = new JdkRequest("http://www.google.com").fetch().body();

// GET http://google.com?q=baseball%20gloves&size=100
String response = HttpRequest.get("http://google.com", true, "q", "baseball gloves", "size", 100)
        .accept("application/json")
        .body();
System.out.println("Response was: " + response);

HttpURLConnection.setFollowRedirects(true); // defaults to true

String url = "https://name_of_the_url";
URL request_url = new URL(url);
HttpURLConnection http_conn = (HttpURLConnection)request_url.openConnection();
http_conn.setConnectTimeout(100000);
http_conn.setReadTimeout(100000);
http_conn.setInstanceFollowRedirects(true);
System.out.println(String.valueOf(http_conn.getResponseCode()));

HttpURLConnection.setFollowRedirects(true); // defaults to true

String url = "https://name_of_the_url"
URL request_url = new URL(url);
HttpURLConnection http_conn = (HttpURLConnection)request_url.openConnection();
http_conn.setConnectTimeout(100000);
http_conn.setReadTimeout(100000);
http_conn.setInstanceFollowRedirects(true);
http_conn.setDoOutput(true);
PrintWriter out = new PrintWriter(http_conn.getOutputStream());
if (urlparameter != null) {
   out.println(urlparameter);
}
out.close();
out = null;
System.out.println(String.valueOf(http_conn.getResponseCode()));

OkHttpClient client = new OkHttpClient();

Request request = new Request.Builder()
      .url(url)
      .build();

Response response = client.newCall(request).execute();

urlConnection.setDoOutput(true);

// GET
HttpResponse response = HttpRequest
    .create(new URI("http://www.stackoverflow.com"))
    .headers("Foo", "foovalue", "Bar", "barvalue")
    .GET()
    .response();

int statusCode = response.statusCode();
String responseBody = response.body(HttpResponse.asString());

module com.foo.bar {
    requires jdk.incubator.httpclient;
}