First a disclaimer beforehand: the posted code snippets are all basic examples. You'll need to handle trivial IOExceptions and RuntimeExceptions like NullPointerException, ArrayIndexOutOfBoundsException and consorts yourself.
Preparing
We first need to know at least the URL and the charset. The parameters are optional and depend on the functional requirements.
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¶m2=%s",
URLEncoder.encode(param1, charset),
URLEncoder.encode(param2, charset));
The query parameters must be in
name=value format and be concatenated by
&. You would normally also
URL-encode the query parameters with the specified charset using
URLEncoder#encode().
The String#format() is just for convenience. I prefer it when I would need the String concatenation operator + more than twice.
Firing a HTTP GET request with (optionally) query parameters
It's a trivial task. It's the default request method.
URLConnection connection = new URL(url + "?" + query).openConnection();
connection.setRequestProperty("Accept-Charset", charset);
InputStream response = connection.getInputStream();
// ...
Any query string should be concatenated to the URL using
?. The
Accept-Charset header may hint the server what encoding the parameters are in. If you don't send any query string, then you can leave the
Accept-Charset header away. If you don't need to set any headers, then you can even use the
URL#openStream() shortcut method.
InputStream response = new URL(url).openStream();
// ...
For testing purposes, you can print the response body to stdout as below:
try (Scanner scanner = new Scanner(response)) {
String responseBody = scanner.useDelimiter("\\A").next();
System.out.println(responseBody);
}
Firing a HTTP POST request with query parameters
Setting the
URLConnection#setDoOutput() to
true implicitly sets the request method to POST. The standard HTTP POST as web forms do is of type
application/x-www-form-urlencoded wherein the query string is written to the request body.
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();
// ...
Note: whenever you'd like to submit a HTML form programmatically, don't forget to take the name=value pairs of any <input type="hidden"> elements into the query string and of course also the name=value pair of the <input type="submit"> element which you'd like to "press" programmatically (because that's usually been used in the server side to distinguish if a button was pressed and if so, which one).
HttpURLConnection httpConnection = (HttpURLConnection) new URL(url).openConnection();
httpConnection.setRequestMethod("POST");
// ...
Actually firing the HTTP request
You can fire the HTTP request explicitly with
URLConnection#connect(), but the request will automatically be fired on demand when you want to get any information about the HTTP response, such as the response body using
URLConnection#getInputStream() and so on. The above examples does exactly that, so the
connect() call is in fact superfluous.
Gathering HTTP response information
-
int status = httpConnection.getResponseCode();
-
for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
System.out.println(header.getKey() + "=" + header.getValue());
}
-
When the Content-Type contains a charset parameter, then the response body is likely text based and we'd like to process the response body with the server-side specified character encoding then.
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.
}
Maintaining the session
The server side session is usually backed by a cookie. Some web forms require that you're logged in and/or are tracked by a session. You can use the
CookieHandler API to maintain cookies. You need to prepare a
CookieManager with a
CookiePolicy of
ACCEPT_ALL before sending all HTTP requests.
// 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();
// ...
Note that this is known to not always work properly in all circumstances. If it fails for you, then best is to manually gather and set the cookie headers. You basically need to grab all Set-Cookie headers from the response of the login or the first GET request and then pass this through the subsequent requests.
// 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]);
}
// ...
The split(";", 2)[0] is there to get rid of cookie attributes which are irrelevant for the server side like expires, path, etc. Alternatively, you could also use cookie.substring(0, cookie.indexOf(';')) instead of split().
Streaming mode
The
HttpURLConnection will by default buffer the
entire request body before actually sending it, regardless of whether you've set a fixed content length yourself using
connection.setRequestProperty("Content-Length", contentLength);. This may cause
OutOfMemoryExceptions whenever you concurrently send large POST requests (e.g. uploading files). To avoid this, you would like to set the
HttpURLConnection#setFixedLengthStreamingMode().
httpConnection.setFixedLengthStreamingMode(contentLength);
But if the content length is really not known beforehand, then you can make use of chunked streaming mode by setting the
HttpURLConnection#setChunkedStreamingMode() accordingly. This will set the HTTP
Transfer-Encoding header to
chunked which will force the request body being sent in chunks. The below example will send the body in chunks of 1KB.
httpConnection.setChunkedStreamingMode(1024);
User-Agent
It can happen that a request returns an unexpected response, while it works fine with a real web browser. The server side is probably blocking requests based on the
User-Agent request header. The
URLConnection will by default set it to
Java/1.6.0_19 where the last part is obviously the JRE version. You can override this as follows:
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.
Error handling
If the HTTP response code is 4nn (Client Error) or 5nn (Server Error), then you may want to read the HttpURLConnection#getErrorStream() to see if the server has sent any useful error information.
InputStream error = ((HttpURLConnection) connection).getErrorStream();
If the HTTP response code is -1, then something went wrong with connection and response handling. The HttpURLConnection implementation is in older JREs somewhat buggy with keeping connections alive. You may want to turn it off by setting the http.keepAlive system property to false. You can do this programmatically in the beginning of your application by:
System.setProperty("http.keepAlive", "false");
Uploading files
You'd normally use
multipart/form-data encoding for mixed POST content (binary and character data). The encoding is in more detail described in
RFC2388.
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();
}
If the other side is a
HttpServlet, then its
doPost() method will be called and the parts will be available by
HttpServletRequest#getPart() (note, thus
not getParameter() and so on!). The
getPart() method is however relatively new, it's introduced in Servlet 3.0 (Glassfish 3, Tomcat 7, etc). Prior to Servlet 3.0, your best choice is using
Apache Commons FileUpload to parse a
multipart/form-data request. Also see
this answer for examples of both the FileUpload and the Servelt 3.0 approaches.
Dealing with untrusted or misconfigured HTTPS sites
Sometimes you need to connect a HTTPS URL, perhaps because you're writing a web scraper. In that case, you may likely face a javax.net.ssl.SSLException: Not trusted server certificate on some HTTPS sites who doesn't keep their SSL certificates up to date, or a java.security.cert.CertificateException: No subject alternative DNS name matching [hostname] found or javax.net.ssl.SSLProtocolException: handshake alert: unrecognized_nameon some misconfigured HTTPS sites.
The following one-time-run static initializer in your web scraper class should make HttpsURLConnection more lenient as to those HTTPS sites and thus not throw those exceptions anymore.
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);
}
}
Last words
Parsing and extracting HTML
If all you want is parsing and extracting data from HTML, then better use a HTML parser like
Jsoup
