12.2. Using REST in Java

Issuing HTTP GET Requests
The key class here is HttpURLConnection, obtained by invoking openConnection on a URL object. Sadly, openConnection method's signature specifies that it returns the superclass type, URLConnection, and we have to downcast the result.

The following method issues a request and returns the entire response as one long string:

public static String httpGet(String urlStr) throws IOException {
  URL url = new URL(urlStr);
  HttpURLConnection conn =
      (HttpURLConnection) url.openConnection();

  if (conn.getResponseCode() != 200) {
    throw new IOException(conn.getResponseMessage());
  }

  // Buffer the result into a string
  BufferedReader rd = new BufferedReader(
      new InputStreamReader(conn.getInputStream()));
  StringBuilder sb = new StringBuilder();
  String line;
  while ((line = rd.readLine()) != null) {
    sb.append(line);
  }
  rd.close();

  conn.disconnect();
  return sb.toString();
}

(This code is very raw; it should be properly padded with try/catch/finally blocks to ensure the reader is closed, etc.)

Remember that if the request URL includes parameters, they must be properly encoded (e.g., a space is %20, etc.). The class URLEncoder can be used to perform this encoding.

Issuing HTTP POST Requests
URL encoding is also required for POST requests, as shown in the following method:

public static String httpPost(String urlStr, String[] paramName,
String[] paramVal) throws Exception {
  URL url = new URL(urlStr);
  HttpURLConnection conn =
      (HttpURLConnection) url.openConnection();
  conn.setRequestMethod("POST");
  conn.setDoOutput(true);
  conn.setDoInput(true);
  conn.setUseCaches(false);
  conn.setAllowUserInteraction(false);
  conn.setRequestProperty("Content-Type",
      "application/x-www-form-urlencoded");

  // Create the form content
  OutputStream out = conn.getOutputStream();
  Writer writer = new OutputStreamWriter(out, "UTF-8");
  for (int i = 0; i < paramName.length; i++) {
    writer.write(paramName[i]);
    writer.write("=");
    writer.write(URLEncoder.encode(paramVal[i], "UTF-8"));
    writer.write("&");
  }
  writer.close();
  out.close();

  if (conn.getResponseCode() != 200) {
    throw new IOException(conn.getResponseMessage());
  }

  // Buffer the result into a string
  BufferedReader rd = new BufferedReader(
      new InputStreamReader(conn.getInputStream()));
  StringBuilder sb = new StringBuilder();
  String line;
  while ((line = rd.readLine()) != null) {
    sb.append(line);
  }
  rd.close();

  conn.disconnect();
  return sb.toString();
}

As you can see, it's not a pretty sight (and that's before adding proper try/catch/finally structures). The problem is that, out of the box, Java's support for handling web connections is pretty low-level.

A good solution can be found in the popular Apache Commons library, and in particular the httpclient set of packages. See Yahoo! guide to REST with Java for details and examples. The documentation covers several interesting extras, such as caching.