Tuesday, August 25, 2009

JRuby Embed API Update: Servlet Examples

I added Servlet Examples section in JRuby Embed API Wiki. Right now, just three examples are in that section. (I'll add more examples later.) Those are:

  • HelloWorldServlet
    Simple "Hello World" example, but helpful to get started.

  • GreetingServlet
    Two methods written in Ruby are called from Servlet.

  • SortableServlet
    Java interface is implemented in two ways in Ruby.

I tested these Servlets on Google App Engine and felt relieved since all three Servlets worked well. I've wanted to verify that Embed API works on GAE, which has some restrictions in programming on it. Embed API doesn't use any unsupported API, so there should not be any problem. However, I realized that I had to specify a classpath explicitly, and the classpath to be specified should not include appengine-tools-api.jar. If no classpath is given, Embed API sees java.class.path system property that has a path to appengine-tools-api.jar. This means, JRuby tries to load appengine-tools-api.jar onto Ruby runtime. The result is ... simply getting an exception. Thus, when Embed API is used with Servlet, especially, with Google App Engine, setting classpath is really important.

Embed API has two ways of setting a classpath. One is to use org.jruby.embed.class.path system property. This is easy, but not a preferred way in a web application. Since system property is common on Java VM, so the value is shared by every Servlet in more than one war archives and mutliple web applications on a single web application server. Some servlet might set classpath "A" using org.jruby.embed.class.path. At the same time another servlet might try to set classpath "B" using org.jruby.embed.class.path. We don't know what classpath is actually used.

Another way of setting classpath is to use setLoadPaths method of API. For example,

public class HelloWorldServlet extends HttpServlet {
private ScriptingContainer container;

public void init() {
String classpath = getServletContext().getRealPath("/WEB-INF/classes");
List loadPaths = Arrays.asList(classpath.split(File.pathSeparator));
container =
new ScriptingContainer(LocalContextScope.SINGLETHREAD);

Technically, we don't need to set the classpath to /WEB-INF/classes, since it has been already set by a server. But, some path with no further trouble is needed, so I chose that.

JRuby Embed API has a public method to set classpath, but JSR 223 implementation is unable to have such method. The specification doesn't define such method. The only way to set classpath for JSR 223 implementation is to use system property. It is true also in RedBridge. So, be careful to choose a harmless classpath to all Servlets on a web application server.

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