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Building SOAP services on Google App Engine in Java: Part 1 – The Servlet


Google App Engine (GAE) is an interesting platform that aims to allow for the development and hosting of Java, Python, and Go web applications.  The pricing scheme is particularly nice:  low-usage applications are hosted for free, pricing is solely dependent on resource usage (with the ability to set budgets and limits), and the minimum charge is only $2.10 per week.  No, I’m not affiliated with or being compensated by Google, but I do think this platform has some potential.

Google supplies a nice Eclipse plugin for use with the Google Web Toolkit and the App Engine, which allows for pretty easy construction of your standard web applications.  I’ve heard that it’s fairly easy to construct user interfaces, JSP pages, etc., but that’s not what we’re talking about here.

The Task

How can we build an application that uses the Google App Engine which uses SOAP for an API?  The primary problem (for me) is that one cannot use the Axis2 libraries (on the server side) due to permission issues, so things must be done a little more manually.

Google has an article covering the topic, but I wanted to do things slightly differently.  Specifically, I wanted to be able to create my service class in Java, generate the WSDL (with appropriately named fields in the XML types – not Parameter1,2,…), generate XML types which will be shared between the server and client projects, and to do it in such a way that I do not have to worry about keeping track of QNAMEs or other low-level details.


For this project, I use:

• Eclipse for development, with the Eclipse plugin for GAE

• Axis2 for the client class (also for the java2wsdl and wsdl2java tools)

• JAXB for XML types (auto-generated by Axis2)

The Server

Overview of the Process

Here’s the logic flow for the server:

1. SOAP request arrives via Post into a HttpServlet.

2. HttpServlet builds a SOAPMessage for the request, and passes it to the SoapHandler class.

3. SoapHandler unmarshalls the XML message object, detects its type, and passes a specific application request to the ServiceAdapter class.

4. ServiceAdapter translates from the XML request message (in a safe fashion) to the Service method call.

5. Service method is run, business logic is performed, all the magic happens.

6. ServiceAdapter translates Service method response to XML response message.

7. SoapHandler builds SOAPMessage response (including catching thrown exceptions and turning them into SOAP Fault messages).

8. HttpServlet writes response to client.

All of that seems like a lot of work, but it’s pretty straightforward.  The best part?  I’ll give you the skeleton for all of this; there’s not much application-specific content in all of this.  The process for adding a method to your service class is: implement service class method, run Axis2 generators, modify/fix Axis2 client (the same every time, some auto-gen code must be changed), add 2 lines to SoapHandler, add an adaption method to SoapAdapter, and add a method to the client class.  It’s easier than it sounds.

Part 1 – The HttpServlet

Good news!  There’s no service specific code here.  I’ll just post the relevant parts of what I use:

public class SOAPServerServlet extends HttpServlet {
	public static final String URL = "";

	private static final Logger log = Logger
	private static MessageFactory messageFactory;
	private static ServiceSOAPHandler soapHandler;

	static {
		try {
			messageFactory = MessageFactory.newInstance();
			soapHandler = new ServiceSOAPHandler();
		} catch (Exception ex) {
			throw new RuntimeException(ex);

	public void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws IOException {"SOAPAction: " + req.getHeader("SOAPAction"));
		log.entering("SOAPServerServlet", "doPost", new Object[] {
				req, resp });
		try {
			// Get all the headers from the HTTP request
			MimeHeaders headers = getHeaders(req);

			// Construct a SOAPMessage from the XML in the request body
			InputStream is = req.getInputStream();
			SOAPMessage soapRequest = messageFactory.createMessage(headers, is);

			ByteArrayOutputStream out = new ByteArrayOutputStream();
			String strMsg = new String(out.toByteArray());

			log.finer("SOAP request: " + strMsg);

			// Handle soapReqest
			SOAPMessage soapResponse = soapHandler

			// Write to HttpServeltResponse
			OutputStream os = resp.getOutputStream();
		} catch (SOAPException e) {
			throw new IOException("Exception while creating SOAP message.", e);
		log.exiting("SOAPServerServlet", "doPost");

	static MimeHeaders getHeaders(HttpServletRequest req) {
		Enumeration headerNames = req.getHeaderNames();
		MimeHeaders headers = new MimeHeaders();
		while (headerNames.hasMoreElements()) {
			String headerName = (String) headerNames.nextElement();
			String headerValue = req.getHeader(headerName);
			StringTokenizer values = new StringTokenizer(headerValue, ",");
			while (values.hasMoreTokens()) {
				headers.addHeader(headerName, values.nextToken().trim());
		return headers;

Leave a comment if you have a question about what’s going on, I’ll not explain it for brevity’s sake (also: because I am lazy).

Coming Soon – Part 2


Eclipse RCP: Setting P2 repositories (update sites) programmatically (for when p2.inf fails).

When deploying a p2 enabled RCP product, you often want to set the update sites that can be used so that they include yours.  There are many resources that explain about using p2.inf to do this.  However, there is an issue when trying to do this with an application that will be installed into Program Files (or any non-user-writable location) on the user’s machine.  p2.inf operates by writing the repository to disk the first time that the RCP application is run.  If the application is not user-writable, this will silently fail.

To set the repositories manually, you can use the same technique as the P2 property page:

import org.eclipse.equinox.internal.p2.ui.model.MetadataRepositoryElement;
import org.eclipse.equinox.internal.p2.ui.model.ElementUtils;

public class P2Util {
  private static String UPDATE_SITE = "";

  public static void setRepositories() throws InvocationTargetException {
    try {
      final MetadataRepositoryElement element = new MetadataRepositoryElement(null, new URI(UPDATE_SITE), true);
      ElementUtils.updateRepositoryUsingElements(new MetadataRepositoryElement[] {element}, null);
    } catch (URISyntaxException e) {
      throw new InvocationTargetException(e);

Note that you need all of the repositories to be present in the


call; repositories not in that array will be removed.


Easily load Xtext files and objects in Eclipse plugin or RCP projects using adapters


I’m currently working on a fairly involved Eclipse RCP project which makes heavy use of Xtext for grammar parsing.  One task that’s come up fairly often is that we need to be able to perform some action on a model object contained in an Xtext file.  For example, we want to provide context menu commands to open up a view to display the structure of a model; to do this we need to load the file, parse it with Xtext, and pass the model object off to the view.

The Xtext FAQ provides instructions for doing this in a standalone Java here, but it involves calling the StandaloneSetup method to create and obtain an injector, which isn’t the best for performance.  If we’re already in a RCP application that includes our Xtext language’s UI plugin, we can efficiently obtain the injector from the activator.  In addition to this, we can wrap the whole thing in a class that ties into Eclipse’s adapter manager framework so that other plugins can load our Xtext models without needing a direct dependency on the UI plugin (though of course they still need to depend on the plugin that  defines the model classes).

The Eclipse adapter framework: IAdapterFactory, IAdaptable, and IAdapterManager

In a nutshell, the Eclipse adapter framework works by letting plugins register ‘adapter factory’ classes (that implement IAdapterFactory).  These classes are designed to attempt the singular task of taking and object of some source class and figuring out how to transform it into an object of another target class.  If the transformation is possible, the AdapterFactory must return an object that can be cast as an instance of the target class.  If it is not possible, the AdapterFactory must return null.  In addition, AdapterFactory classes implement a method that returns a list of all target classes supported by the factory, and their defining plugin must provide an extension to org.eclipse.core.runtime.adapters specifying what transformations can be performed.  Traditionally, objects that support being adapted implement the IAdaptable interface, but this is not strictly required.

Once an adapter has been registered that can perform a transformation, any plugin can use this with a simple call:

final TargetObjType target = (TargetObjType)Platform.getAdapterManager().getAdapter(sourceObject, TargetObjType.class);
if (target==null) { /* Adaptation failed */ }

The caller is guaranteed that getAdapter will either return null or an object that can be cast as TargetObjType.

The ModelLoadingAdapter

To create a ModelLoadingAdapter, we must implement the code to load domain models from files and register this in plugin.xml.  As an easy extension we will also let the same adapter handle selections and structured selections (allowing us to directly adapt a list selection to a domain model, if the item selected is a file).  After this is done, any plugin can easily load Xtext DSL models from files with a minimum performance hit.

For this example we’re going to assume the projects are called “org.xtext.example.mydsl” and “org.xtext.example.mydsl.ui”, the language is called “org.xtext.example.mydsl.MyDsl”,  it uses files with extension “mydsl”, and the root model of a document is a DslModel.

The IAdapterFactory

Our actual adapter must do three things:  Obtain the Injector for our DSL, use the injector to obtain a properly initialized XtextResourceSet, and use the resource set to load the file.

Obtaining the domain-specific language injector

Update: From the comments below, the easy/correct way to use injection in Eclipse RCP/plugin projects is to make use of the ExecutableExtensionFactory class. If you specify, in plugin.xml, the name of your AdapterFactory class as “ExecutableExtensionFactory:AdapterFactory” then you do not need to manually get the injector. You can just specify a private class variable using “@Inject
private XtextResourceSetProvider resourceSetProvider;”

When Xtext generates the .ui project, it creates an activator for this plugin that is responsible for creating and storing the Injector for this language.  We can retrieve the injector from this class using the getInjector method, which takes the name of the language as an argument and either returns the injector or null.  The code for this would be

final private static Injector injector = MyDslActivator.getInstance().getInjector("org.xtext.example.mydsl.MyDsl");

Loading the model

This is very similar to the FAQ’s instructions, we just omit the creation of the injector and use the one that we retrieve from the activator.

XtextResourceSet resourceSet = (XtextResourceSet) injector
resourceSet.addLoadOption(XtextResource.OPTION_RESOLVE_ALL, Boolean.TRUE);
Resource resource = resourceSet.getResource(URI.createURI(file.getLocationURI().toString()),true);
DslModel model = (DslModel) resource.getContents().get(0);

Putting it all together

The only details we’re missing are relatively minor: error checking, transforming selections to files, and the format required by IAdapterFactory.  Our complete class ends up looking like this:

package org.xtext.example.mydsl.util;

import org.eclipse.core.resources.IFile;
import org.eclipse.core.runtime.IAdapterFactory;
import org.eclipse.emf.common.util.URI;
import org.eclipse.emf.ecore.resource.Resource;
import org.eclipse.jface.viewers.ISelection;
import org.eclipse.jface.viewers.IStructuredSelection;
import org.eclipse.xtext.resource.XtextResource;
import org.eclipse.xtext.resource.XtextResourceSet;
import org.xtext.example.mydsl.myDsl.DslModel;
import org.xtext.example.mydsl.ui.internal.MyDslActivator;
/* Author: Robert Coop
* See:
public class ModelLoadingAdapter implements IAdapterFactory {
	private static org.apache.log4j.Logger log = org.apache.log4j.Logger
	final private static Injector injector = MyDslActivator

	public Object getAdapter(Object adaptableObject, Class adapterType) {
		if (adapterType == DslModel.class) {
			if (injector==null) {
				log.error("Could not obtain injector for MyDsl");
				return null;

			if (adaptableObject instanceof ISelection) {
				final ISelection sel = (ISelection)adaptableObject;
				if (!(sel instanceof IStructuredSelection)) return null;
				final IStructuredSelection selection = (IStructuredSelection) sel;
				if (!(selection.getFirstElement() instanceof IFile))
					return null;
				adaptableObject = (IFile)selection.getFirstElement();
			if (adaptableObject instanceof IFile) {
				final IFile file = (IFile)adaptableObject;
				if (!file.getFileExtension().toLowerCase().equals("mydsl")) return null;

				XtextResourceSet resourceSet = (XtextResourceSet) injector
				resourceSet.addLoadOption(XtextResource.OPTION_RESOLVE_ALL, Boolean.TRUE);
				Resource resource = resourceSet.getResource(URI.createURI(file.getLocationURI().toString()),true);
				DslModel model = (DslModel) resource.getContents().get(0);
				return model;

		return null;

	public Class[] getAdapterList() {
		return new Class[] { DslModel.class };

Finishing Up

To tie everything together, we just need to modify plugin.xml to tell Eclipse about the adapter, then we can use it to load models easily.  Add the following to plugin.xml

<extension point="org.eclipse.core.runtime.adapters">
      <factory adaptableType="org.eclipse.core.resources.IFile" class="org.xtext.example.mydsl.util.ModelLoadingAdapter">
         <adapter type="org.xtext.example.mydsl.myDsl.DslModel" />
      <factory adaptableType="org.eclipse.jface.viewers.ISelection" class="org.xtext.example.mydsl.util.ModelLoadingAdapter">
         <adapter type="org.xtext.example.mydsl.myDsl.DslModel" />

Using our new adapter

Using the adapter from any plugin is very simple.  Given any IFile or ISelection object representing a mydsl file, we can obtain a DslModel using this code

DslModel target = (DslModel)Platform.getAdapterManager().getAdapter(sourceObject, DslModel.class);
if (target==null) { /* Adaptation failed */ }

Final Thoughts

This technique can be applied to any adapter with the advantage of not needing direct dependencies on the plugin providing the adapter factory.  Hopefully this will prove useful to you either because it demonstrates a good way to load Xtext files or it demonstrates some of the adapter framework uses.  Enjoy!

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