Java GET Using

Here’s a simple code snippet showing how to fetch the contents of a URL. The URL to fetch is in the string url_string , and html stores the contents of the fetched file.

URL url = new URL(url_string);
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream()));
String html = "";
String line = "";
while ((line = reader.readLine()) != null) {
    html += line;

This code may throw a if an error is encountered while fetching the URL.

This code requires the below imports:


A Simple Backends.XML File For Java Deployments

Here’s a simple example of the XML required to set up a backend:

    <backend name="worker">

This sets up a B2 class backend named worker which can be addressed at the URL worker . APPLICATION-ID . appspot . com . Save this as backends.xml in the /war/WEB-INF/ directory.

Access-Control-Allow-Origin In JSONP Responses

A quick note for today: JSONP responses need to include an Access-Control-Allow-Origin header set to a wildcard. If this header isn’t included, browsers won’t allow JSONP responses to be read in by Javascript.

The header key and value should look like this:

Access-Control-Allow-Origin: *

Here’s an example of this header in Java:

resp.setHeader("Access-Control-Allow-Origin", "*");

The variable resp represents a HttpServletResponse reference. Make sure to add this line before writing the body of the response to the client.

Communicating With Sockets On Compute Engine

Writing custom applications on Compute Engine requires the use of sockets to communicate with clients. Here’s a code example demonstrating how to read and write to sockets in Java.

Assume that client_socket is the socket communicating with the client:

 * Contains the socket we're communicating with.
Socket client_socket;

Create a socket by using a ServerSocket (represented by server_socket ) to listen to and accept incoming connections:

Socket client_socket = server_socket.accept();

Then extract a PrintWriter and a BufferedReader from the socket. The PrintWriter out object sends data to the client, while the BufferedReader in object lets the application read in data sent by the client:

 * Handles sending communications to the client.
PrintWriter out;
 * Handles receiving communications from the client.
BufferedReader in;
try {
    //We can send information to the client by writing to out.
    out = new PrintWriter(client_socket.getOutputStream(), true);
    //We receive information from the client by reading in.
    in = new BufferedReader(new InputStreamReader(client_socket.getInputStream()));
     * Start talking to the other server.
    //Read in data sent to us.
    String in_line = in.readLine();
    //Send back information to the client.
}//end try
catch (IOException e) {
    //A general problem was encountered while handling client communications.

A line of text sent by the client can be read in by calling in.readLine() demonstrated by the in_line string. To send a line of text to the client, call out.println(send_info) where send_info represents a string. If an error occurs during communication, an IOException will be thrown and caught by the above catchstatement.

Remember to add the following imports:


App Engine Default Time & Date

The default time zone for the Java runtime on App Engine is UTC (frequently referred to as GMT). The following code will record the current time and date in UTC:

Date current_date = new Date();
String date = new java.text.SimpleDateFormat("MM-dd-yyyy").format(current_date);
String time = new java.text.SimpleDateFormat("hh:mm:ss aa").format(current_date);

For instance, date would be set to 09-21-2013 (September 21, 2013) and time would be set to 01:40:42 AM . If your application needs to record time in a different time zone, use the setTimeZone method of SimpleDateFormat .

Don’t forget to import the Date class:

import java.util.Date;

HTTP POST URLFetch Using The Low Level Java API

Here’s a code snippet demonstrating how to generate a POST request using the low level urlfetch API.

The variables post_url and post_data represent the request URL and the content of the POST as strings. The response from the urlfetch is stored in response_content . If the request was unsuccessful (the target server returned a non-200 HTTP status code) a RuntimeException will be thrown.

HTTPRequest request = new HTTPRequest(new URL(post_url), HTTPMethod.POST);
request.setHeader(new HTTPHeader("User-Agent", "Example Application"));
HTTPResponse response = URLFetchServiceFactory.getURLFetchService().fetch(request);
//If the response wasn't successful, throw an exception.
if (response.getResponseCode() != 200) {
    throw new RuntimeException("Response code was not 200: " + response.getResponseCode());
String response_content = new String(response.getContent());

Remember to import the URLFetch library:


CRLF In HTTP Response Headers

A quick note: App Engine will convert CRLF characters in headers (new line characters, such as /r and /n) into underscore characters.

For example, setting the following header should cause Line One and Line Two to appear on two different lines:

resp.setHeader("Example-Header-Name", "Line One \r\n Line Two");

App Engine will run this code, but it will convert the newline characters into two underscore characters before sending the header to the client browser.

While the HTTP 1.1 specification allows header fields to be split across multiple lines (RFC 2616 Section 4.2), App Engine apparently doesn’t allow this. It isn’t too much of an inconvenience, since response headers can be grouped up into one line, but it’s a good idea to keep this limitation in mind especially when communicating with APIs.

Sleeping An Application In Java

Some applications need to pause execution for a short amount of time. In App Engine, there are two ways to do that.

For short pauses, most programming languages have the concept of sleep operations. Here’s how to sleep for 2,000 milliseconds (2 seconds) in Java:

try {
catch (InterruptedException e) {

If you want to pause the application for a few seconds, using the sleep function is a good idea. But if you need a longer pause – more than a minute or so – it’s a better idea to use the Task Queue’s countdownMillis function.

Here’s how to create a task that will wait for 5 minutes, then launch off another request:

TaskOptions task = TaskOptions.Builder.withCountdownMillis(5 * 60 * 1000);

In short, whenever you need a long pause in execution, queue up a task with a countdownMillis() delay. Then the task can call a request handler which can continue any application logic needed.