Implementing a -noserver flag for GWTTestCase

May 14th, 2008 Alex Moffat Posted in GWT | 14 Comments »

GWTTestCase is great for testing the command and model parts of a GWT UI. However, it has one big drawback, there’s no way to write a test case that makes an RPC call to an external server, something you can do with GWT hosted mode using the -noserver flag. There are two things that contribute to this situation.

First, GWTTestCase is actually in two pieces. One executes in the hosted mode browser and calls the actual code under test, either as JavaScript or bytecode. The other executes in an embedded tomcat server which both serves the JavaScript/bytecode to test and communicates with the JUnit test framework to log results etc.

Second, the same-origin policy prevents JavaScript loaded from site A making XmlHttpRequest calls to site B. This means you can’t just load the JavaScript for the test case from the embedded server and then make the RPC call to a different server.

Our solution to this problem turns out to be pretty simple. The code and configuration for the embedded server is in the gwt-dev-<platform>.jar file where <platform> is windows, mac or linux. This includes a web.xml file com/google/gwt/dev/etc/tomcat/webapps/ROOT/WEB-INF/web.xml that maps GWTShellServlet to the url-pattern /* so all requests to the embedded server are directed to this servlet. The solution then is to somehow replace GWTShellServlet so that we can handle the requests the way we’d like to.

To do this we override web.xml by placing a modified version earlier in the classpath used for executing the test cases than the gwt-dev-<platform>.jar file. This web.xml has a different mapping for the /* url-pattern so that all requests to the embedded server are directed to a new servlet.

The new servlet is a subclass of GWTShellServlet and does one of two things with each request. If it’s a request for code related to a test case then it handles it in the same way as GWTShellServlet, otherwise it proxies the request to a different server. In this way all requests from the client are made to the embedded server but when they get there some are actually served by the external server.

We override two methods, service(ServletRequest, ServletResponse) and init(ServletConfig) in the GWTShellServlet subclass. In the service method the requested uri is examined and if it is for a test case then it is passed to super.service so that the GWTShellServlet code can handle it. If it’s not for a test case then it is proxied to the external server. We need configuration to define what is considered a test case and where the external server is. This is read from web.xml in the init method. <init-param> values define the host and port for the external server and the fully qualified class names for all the test classes. If a request uri starts with a test class FQN then it’s served locally, otherwise it’s proxied. This does mean that we have to add a new entry to web.xml for each new test class. An alternative would be to use a particular package for all test classes and examine the uri for that prefix instead.

Overall this has proved to work very well. We use maven for our build and use the cargo-maven2-plugin to start and stop the external server around the execution of the test cases.

Here is the source for the
Proxy Servlet for GWTTestCases and the web.xml file. The web.xml must be placed in a directory corresponding to com.google.gwt.dev.etc.tomcat.webapps.ROOT.WEB-INF. When you run the test case it will be copied into tomcat/webapps/ROOT/WEB-INF/web.xml.