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  • Tutorial for Camel on Google App Engine

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Camel on Google App Engine Tutorial


titleWork in progress



Goal of this tutorial is to get a non-trivial Camel application running on Google App Engine (GAE



Goal of this tutorial is to get a Camel application running on Google App Engine that can send weather reports to user-defined email accounts. The user enters the name of a city and the email address of the report receiver into a form and submits it to the tutorial application. Retrieving weather data for that city and sending the weather report is done in the background.

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The packaged tutorial application can be downloaded from here. It is an Eclipse project that requires the Google Eclipse Plugin and the App Engine Java SDK 1.2.6 for running. A Maven-based version of the project should be available soon. The following sections describe how to build that application from scratch.

Create a GAE application

For setting up the Camel-independent parts of a GAE application refer to the Java Getting Started Guide in the GAE documentation. This tutorial uses the Google Plugin for Eclipse and the App Engine Java SDK 1.2.6. After installing the SDK and the plugin, create a new GAE project via File->New->Web Application Project.

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After pressing Finish the new GAE project appears in the package explorer.

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Add required libraries

The created project is not a Maven project so all required dependencies must be added manually to the war/WEB_INF/lib directory. These are

Then right-click on the Tutorial project and add these libraries to the classpath via Properties->Java Build Path->Libraries->Add JARs.... After adding they should now appear in the Referenced Libraries container.

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Setup deployment descriptors

The deployment descriptors web.xml and appengine-web.xml are located in the war/WEB-INF directory.


). For developing that application the Camel Components for Google App Engine are used. The example web application generates a weather report for a city that is entered by the user into a form and sends the weather report via email to either the currently logged-in user or a user-defined receiver. The following figure gives an overview. Users of this application need to login with their Google account.

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POSTed form data are dispatched to the Camel application via the ghttp component. The application transforms the report request and enqueues it with the gtask component for further background processing. It then generates an immediate HTML response containing information about the city and the receiver of the report. The response also contains a link logging out from the application. In the background, the application retrieves weather data from the Google Weather Service, transforms the data to generate a simple weather report and sends the report by email via the gmail component.

titleJava clients

Clients other than web browsers may also interact with this application by POSTing the form data and an authentication cookie. At the moment the example application doesn't include a standalone Java client that demonstrates how to do that. This is work in progress. In the meantime, refer to the glogin component documentation for instructions how to access security-enabled GAE services.

titleTutorial online

The next two sections describe how to checkout, build and deploy the example application. If you want to skip these steps, go directly to the #Usage section and use the pre-deployed example application at


  • Sign up for a Google App Engine account if you don't have one.
  • Create a new application via the admin console or reuse an existing one for uploading the example.
  • Install the Google App Engine SDK for Java. This tutorial has been tested with version 1.3.6.


First, checkout the sources from the Camel code repository.

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svn co camel 

Navigate to the camel-example-gae and install from here.

Open the camel-example-gae/src/main/webapp/WEB-INF/application-web.xml file and replace the template application name replaceme with the name of the application that you created in the previous section. Optionally, adjust the version number if needed.

Code Block
<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="" version="2.5"1.0">

       <servlet-name>CamelServlet</servlet-name> Set your application name and version here
 <url-pattern>/camel/*</url-pattern>     </servlet-mapping>
    <welcome-file-list> <static-files>


The <application> element in appengine-web.xml requires a GAE application name. In the following example, replace the value replaceme with a valid GAE application name.

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<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns=""<exclude path="/index.html" />
    <!-- Configure java.util.logging -->

        <property name="java.util.logging.config.file" value="WEB-INF/"/>

Setup the form

Users provide input data to the application by submitting form data. The following form definition should be stored in war/WEB-INF/index.html.


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>Weather Report</title>

<h1>Weather Report</h1>
<form action="camel/weather" method="post">
        <td><input type="text" name="city"></td>
        <td><input type="text" name="mailto"></td>
        <td align="right"><input type="submit" value="Submit" /></td>

When the user presses the Submit button the form data are POSTed to http://<appname> The input field values will be available as in message headers city and mailto.

Setup the routes

Add the following TutorialRouteBuilder class to the org.apache.camel.example.gae package. The previously generated TutorialServlet in that package can be deletedThen go to the camel-example-gae directory and enter

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mvn install

This will create the application war file in the target directory. Finally use the appcfg command-line tool of the App Engine SDK to deploy the application.

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appcfg update target/camel-example-gae-<version>

where version needs to be replaced with the version of Camel you're using. You will be prompted for the email address and password of your Google App Engine account. After deployment the example application is ready to use.


In the following, the application name camelcloud will be used as an example. If you deployed the example application somewhere else, use your application name instead. Go to The application will redirect you to a login page (see also Security for Camel GAE Applications).

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After login, the application displays the tutorial's main page where you can enter the name of a city and optionally enter an email address where to send the waether report. If you check Send report to me the report will be send to the email address that you used for login.

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After pressing Submit the response is

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The home link brings you back to the main page, the logout link is to logout from the application (which brings you back to the login page). In the background, the current weather conditions for the user-entered city will be retrieved from the Google weather service and a formatted weather report will be send by email. Submitting the form the first time initializes the application on Google App Engine which can take several seconds. Subsequent submissions (within a certain time frame) are served much faster. Check your emails and you should now see a new email with subject Wheather report and content similar to this one:

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Weather report for:  London, England
Current condition:   Klar
Current temperature: 12 (Celsius) 

The report is partly internationalized, depending on the language settings of your browser.

Code walkthrough

The TutorialRouteBuilder implements the message processing routes shown in the #Overview section. Input form data are received via the ghttp component. After receiving the request a RequestProcessor adds the form data and information about the current user to a ReportData POJO. The ReportData object is then serialized and queued for background processing. Queueing messages on GAE is done with the gtask component. After adding the ReportData object to the queue an HTML response is generated with the ResponseProcessor.

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package org.apache.camel.example.gae;

import org.w3c.dom.Document;

import org.apache.camel.Exchange;
import org.apache.camel.builder.RouteBuilder;
import org.apache.camel.component.gae.mail.GMailBinding;
import org.apache.camel.processor.aggregate.AggregationStrategy;

public class TutorialRouteBuilder extends RouteBuilder {

    public void configure() throws Exception {
            .to("gtask://default")process(new RequestProcessor())
            .setHeadermarshal(Exchange.CONTENT_TYPE, constant("text/plain"))).serialization()
            .transformunmarshal(constant("Weather report will be sent to ").append(header("mailto")))).serialization()
            .process(new ResponseProcessor());
            .setHeader(Exchange.HTTP_QUERY, constant("weather=").append(header("city"
            .process(new WeatherProcessor(, reportDataAggregator())
                    .setHeader(GMailBinding.GMAIL_SUBJECT, constant("Weather report"))
            .setHeader(GMailBinding.GMAIL_TOSENDER, header("mailto"ReportData.requestor())


Form data are received via the ghttp component. After receiving the request it is added to the default queue for background processing. Queueing messages on GAE is done with the gtask component. After queueing the request a response is generated for being displayed in the browser. The value of the mailto header is the email address the user entered in the form.

Background processing of the queued messages starts from("gtask://default"). The first step is the construction of the Google weather service URL followed by the weather service invocation using the ghttp component. For example, if the user entered London in the city field of the form the resulting URL is The resulting XML data are processed by the WeatherProcessor:

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package org.apache.camel.example.gae;

import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathFactory;

import org.apache.camel.Exchange;
import org.apache.camel.Processor;
import org.w3c.dom.Document;

public class WeatherProcessor implements Processor {

    public void process(Exchange exchange) throws Exception {
        // convert XML body to DOM tree
        Document doc = exchange.getIn().getBody(Document.class);setHeader(GMailBinding.GMAIL_TO, ReportData.recipient())
            .process(new ReportGenerator())        
 XPathFactory xpfactory = XPathFactory.newInstance();         XPath xpath = xpfactory.newXPath();

        // Extract result values via XPath
        String city = xpath.evaluate("//forecast_information/city/@data","gmail://default");

  String cond = xpath.evaluate("//current_conditions/condition/@data", doc);
        String temp = xpath.evaluate("//current_conditions/temp_c/@data", doc);
private static AggregationStrategy reportDataAggregator() {
        Stringreturn msg = null;
        if (city != null && city.length() > 0) new AggregationStrategy() {
            msgpublic =Exchange new StringBufferaggregate()Exchange reportExchange,                .append("\n").append("Weather report for:  ").append(city)Exchange weatherExchange) {
                ReportData reportData = reportExchange.appendgetIn("\n").append("Current condition:   ").append(cond)getBody(ReportData.class);
                reportData.append("\n").append("Current temperature: ").append(temp).append(" (Celsius)").toString()setWeather(weatherExchange.getIn().getBody(Document.class));
        } else {      return reportExchange;
     // create an error message   }
         msg = "Error getting weather report for " + exchange.getIn().getHeader("city", String.class)};


This processor extracts data from the XML result with XPath expressions and creates a simple text-based report for being sent by email. Add this class to the org.apache.camel.example.gae package. For sending the email the gmail component is used. The sender is given by the endpoint URI where <replaceme> must be replaced with the username of the admin account of the GAE application (usually the account to deploy the GAE application). The recipient is derived from the user-defined mailto header.

titleLocal development server

Please note that on the local development server sending emails does not work and queued tasks must be executed manually from the developer console.

Setup the application context

Add the following application context XML file to the project's src directory (needs to be on the classpath).


<beans xmlns=""
    <bean id="camelContext" 
        <property name="routeBuilder" ref="tutorialRouteBuilder" />
    <bean id="tutorialRouteBuilder"

Deploy the application

To deploy the application right-click on the Tutorial project and select Google->Deploy to App Engine. In the dialog enter the project name, your email address and password for logging into App Engine and press Deploy.

Use the application

Go to http://<appname> where <appname> must be replaced with a valid GAE application name. The following form should now appear.

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Enter the name of a city and your email address, for example:

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Then press Submit. Submitting the form the first time will initialize the application on Google App Engine which can take more than 10 seconds. Subsequent submissions are served much faster. Check your emails and you should now see a new email with subject Wheather report and content similar to this one:

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Weather report for:  London, England
Current condition:   Klar
Current temperature: 12 (Celsius) 


Background processing of the queued messages starts from("gtask://default"). The first step is to enrich the previously generated ReportData object with data from the Google weather service. The service URL is dynamically constructed by setting the Exchange.HTTP_QUERY header to weather=<city>. For example, if the user entered London in the city field of the form the resulting URL is g The weather service is accessed with the ghttp component and the weather service response is aggregated into the ReportData object using a custom aggregator returned by the reportDataAggregator() method. Generating a simple weather report from ReportData is done by the ReportGenerator. The report is then sent by email with the gmail component.