Do a Simple HTTP Request in Java

1. Overview

In this quick tutorial, we present a way of performing HTTP requests in Java — by using the built-in Java class HttpUrlConnection. 

Note that starting with JDK 11, Java provides a new API for performing HTTP requests, which is meant as a replacement for the HttpUrlConnection, the HttpClient API.

2. HttpUrlConnection

The HttpUrlConnection class allows us to perform basic HTTP requests without the use of any additional libraries. All the classes that we need are part of the java.net package.

The disadvantages of using this method are that the code can be more cumbersome than other HTTP libraries and that it does not provide more advanced functionalities such as dedicated methods for adding headers or authentication.

3. Creating a Request

We can create an HttpUrlConnection instance using the openConnection() method of the URL class. Note that this method only creates a connection object but doesn’t establish the connection yet.

The HttpUrlConnection class is used for all types of requests by setting the requestMethod attribute to one of the values: GET, POST, HEAD, OPTIONS, PUT, DELETE, TRACE.

Let’s create a connection to a given URL using GET method:

URL url = new URL("http://example.com");
HttpURLConnection con = (HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");

4. Adding Request Parameters

If we want to add parameters to a request, we have to set the doOutput property to true, then write a String of the form param1=value&param2=value to the OutputStream of the HttpUrlConnection instance:

Map<String, String> parameters = new HashMap<>();
parameters.put("param1", "val");

con.setDoOutput(true);
DataOutputStream out = new DataOutputStream(con.getOutputStream());
out.writeBytes(ParameterStringBuilder.getParamsString(parameters));
out.flush();
out.close();

To facilitate the transformation of the parameter Map, we have written a utility class called ParameterStringBuilder containing a static method, getParamsString(), that transforms a Map into a String of the required format:

public class ParameterStringBuilder {
    public static String getParamsString(Map<String, String> params) 
      throws UnsupportedEncodingException{
        StringBuilder result = new StringBuilder();

        for (Map.Entry<String, String> entry : params.entrySet()) {
          result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
          result.append("=");
          result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
          result.append("&");
        }

        String resultString = result.toString();
        return resultString.length() > 0
          ? resultString.substring(0, resultString.length() - 1)
          : resultString;
    }
}

5. Setting Request Headers

Adding headers to a request can be achieved by using the setRequestProperty() method:

con.setRequestProperty("Content-Type", "application/json");

To read the value of a header from a connection, we can use the getHeaderField() method:

String contentType = con.getHeaderField("Content-Type");

6. Configuring Timeouts

HttpUrlConnection class allows setting the connect and read timeouts. These values define the interval of time to wait for the connection to the server to be established or data to be available for reading.

To set the timeout values, we can use the setConnectTimeout() and setReadTimeout() methods:

con.setConnectTimeout(5000);
con.setReadTimeout(5000);

In the example, we set both timeout values to five seconds.

7. Handling Cookies

The java.net package contains classes that ease working with cookies such as CookieManager and HttpCookie.

First, to read the cookies from a response, we can retrieve the value of the Set-Cookie header and parse it to a list of HttpCookie objects:

String cookiesHeader = con.getHeaderField("Set-Cookie");
List<HttpCookie> cookies = HttpCookie.parse(cookiesHeader);

Next, we will add the cookies to the cookie store:

cookies.forEach(cookie -> cookieManager.getCookieStore().add(null, cookie));

Let’s check if a cookie called username is present, and if not, we will add it to the cookie store with a value of “john”:

Optional<HttpCookie> usernameCookie = cookies.stream()
  .findAny().filter(cookie -> cookie.getName().equals("username"));
if (usernameCookie == null) {
    cookieManager.getCookieStore().add(null, new HttpCookie("username", "john"));
}

Finally, to add the cookies to the request, we need to set the Cookie header, after closing and reopening the connection:

con.disconnect();
con = (HttpURLConnection) url.openConnection();

con.setRequestProperty("Cookie", 
  StringUtils.join(cookieManager.getCookieStore().getCookies(), ";"));

8. Handling Redirects

We can enable or disable automatically following redirects for a specific connection by using the setInstanceFollowRedirects() method with true or false parameter:

con.setInstanceFollowRedirects(false);

It is also possible to enable or disable automatic redirect for all connections:

HttpUrlConnection.setFollowRedirects(false);

By default, the behavior is enabled.

When a request returns a status code 301 or 302, indicating a redirect, we can retrieve the Location header and create a new request to the new URL:

if (status == HttpURLConnection.HTTP_MOVED_TEMP
  || status == HttpURLConnection.HTTP_MOVED_PERM) {
    String location = con.getHeaderField("Location");
    URL newUrl = new URL(location);
    con = (HttpURLConnection) newUrl.openConnection();
}

9. Reading the Response

Reading the response of the request can be done by parsing the InputStream of the HttpUrlConnection instance.

To execute the request, we can use the getResponseCode(), connect(), getInputStream() or getOutputStream() methods:

int status = con.getResponseCode();

Finally, let’s read the response of the request and place it in a content String:

BufferedReader in = new BufferedReader(
  new InputStreamReader(con.getInputStream()));
String inputLine;
StringBuffer content = new StringBuffer();
while ((inputLine = in.readLine()) != null) {
    content.append(inputLine);
}
in.close();

To close the connection, we can use the disconnect() method:

con.disconnect();

10. Reading the Response on Failed Requests

If the request fails, trying to read the InputStream of the HttpUrlConnection instance won’t work. Instead, we can consume the stream provided by HttpUrlConnection.getErrorStream().

We can decide which InputStream to use by comparing the HTTP status code:

int status = con.getResponseCode();

Reader streamReader = null;

if (status > 299) {
    streamReader = new InputStreamReader(con.getErrorStream());
} else {
    streamReader = new InputStreamReader(con.getInputStream());
}

And finally, we can read the streamReader in the same way as the previous section.

11. Building the Full Response

It’s not possible to get the full response representation using the HttpUrlConnection instance.

However, we can build it using some of the methods that the HttpUrlConnection instance offers:

public class FullResponseBuilder {
    public static String getFullResponse(HttpURLConnection con) throws IOException {
        StringBuilder fullResponseBuilder = new StringBuilder();

        // read status and message

        // read headers

        // read response content

        return fullResponseBuilder.toString();
    }
}

Here, we’re reading the parts of the responses, including the status code, status message and headers, and adding these to a StringBuilder instance.

First, let’s add the response status information:

fullResponseBuilder.append(con.getResponseCode())
  .append(" ")
  .append(con.getResponseMessage())
  .append("\n");

Next, we’ll get the headers using getHeaderFields() and add each of them to our StringBuilder in the format HeaderName: HeaderValues:

con.getHeaderFields().entrySet().stream()
  .filter(entry -> entry.getKey() != null)
  .forEach(entry -> {
      fullResponseBuilder.append(entry.getKey()).append(": ");
      List headerValues = entry.getValue();
      Iterator it = headerValues.iterator();
      if (it.hasNext()) {
          fullResponseBuilder.append(it.next());
          while (it.hasNext()) {
              fullResponseBuilder.append(", ").append(it.next());
          }
      }
      fullResponseBuilder.append("\n");
});

Finally, we’ll read the response content as we did previously and append it.

Note that the getFullResponse method will validate whether the request was successful or not in order to decide if it needs to use con.getInputStream() or con.getErrorStream() to retrieve the request’s content.

12. Conclusion

In this article, we showed how we can perform HTTP requests using the HttpUrlConnection class.