一起來學REST（12.2）——Java中使用REST

 原文地址：http://rest.elkstein.org/

Learn REST: A Tutorial

發送HTTP GET請求
主要的類是HttpURLConnection，通過對一個URL調用openConnection可以得到這個類，openConnection方法的簽名指向一個超類URLConnection，我們還需要對其進行類類型向下轉型。

下面的方法發送一個請求，並且返回一個長字串：

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();}

(這段代碼有些粗糙，需要加上適當的try/catch/finally來保證reader可以關閉，等等。記住，如果URL包含參數，必須進行適當的編碼（例如空格是%20，等等）。類URLEncoder用來進行這樣的編碼。

發送HTTP POST請求
在POST請求中的URL也需要編碼，如下面的方法所示：

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 site (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 ascaching.

By Dr. M. Elkstein