Spring Java Config with Multiple Dispatchers

Question

I've some experience Spring now and also have some pure java config web-apps in use. However, these are usually based on a quiet simple setup:

• application config for services / repositories
• dispatcher config for one dispatcher (and some controllers)
• (optional) spring security to secure the access

For my current project I need to have separate dispatcher contexts with different configuration. That's not a problem with the XML based configuration as we have a dedicated ContextLoaderListener that's independent from Dispatcher Configuration. But with java config I'm not sure if what I'm doing is fine so far ;)

Here's a common DispatcherConfig:

public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

  @Override
  protected Class<?>[] getRootConfigClasses() {
    return new class[]{MyAppConfig.class};
  }

  @Override
  protected Class<?>[] getServletConfigClasses() {
    return new Class[]{MyDispatcherConfig.class};
  }

  @Override
  protected String[] getServletMappings() {
    return new String[]{"/mymapping/*"};
  }

  @Override
  protected String getServletName() {
    return "myservlet";
  }
}

As said, I need a second (third, ...) dispatcher with another mapping (and view resolvers). So, I copied the config and added for both getServletName() (otherwise both will be named as 'dispatcher' which will cause errors). The second config was looking like that:

public class AnotherWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

  @Override
  protected Class<?>[] getRootConfigClasses() {
    return new class[]{MyAppConfig.class};
  }

  @Override
  protected Class<?>[] getServletConfigClasses() {
    return new Class[]{AnotherDispatcherConfig.class};
  }

  @Override
  protected String[] getServletMappings() {
    return new String[]{"/another_mapping/*"};
  }

  @Override
  protected String getServletName() {
    return "anotherservlet";
  }
}

When I use it like this, starting application results in a problem with ContextLoaderListener:

java.lang.IllegalStateException: Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!
    at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:277)
...

So I removed the second MyAppConfig.class return from one of the AbstractAnnotationConfigDispatcherServletInitializer and it works fine. However, that doesn't feel to be the right way ;)

For my understanding: should all DispatcherConfig be handled within one AbstractAnnotationConfigDispatcherServletInitializer or should I separate them as I did? I tried to configure them in one class but then my config was totally mixed (so I believe that's not the desired way).

How do you implement such a case? Is it possible to set the ContextLoaderListener in java config outside of the AbstractAnnotationConfigDispatcherServletInitializer? Or should I create a DefaultServlet which has only the root config? What about implementing the base interface of that configuration WebApplicationInitializer?

Answer 1

Mahesh C. showed the right path, but his implementation is too limited. He is right on one point : you cannot use directly AbstractAnnotationConfigDispatcherServletInitializer for multiple dispatcher servlet. But the implementation should :

• create a root application context
• gives it an initial configuration and say what packages it should scan
• add a ContextListener for it to the servlet context
• then for each dispatcher servlet
  • create a child application context
  • gives it the same an initial configuration and packages to scan
  • create a DispatcherServlet using the context
  • add it to the servlet context

Here is a more complete implementation :

@Override
public void onStartup(ServletContext servletContext) throws ServletException {
    // root context
    AnnotationConfigWebApplicationContext rootContext =
            new AnnotationConfigWebApplicationContext();
    rootContext.register(RootConfig.class); // configuration class for root context
    rootContext.scan("...service", "...dao"); // scan only some packages
    servletContext.addListener(new ContextLoaderListener(rootContext));

    // dispatcher servlet 1
    AnnotationConfigWebApplicationContext webContext1 = 
            new AnnotationConfigWebApplicationContext();
    webContext1.setParent(rootContext);
    webContext1.register(WebConfig1.class); // configuration class for servlet 1
    webContext1.scan("...web1");            // scan some other packages
    ServletRegistration.Dynamic dispatcher1 =
    servletContext.addServlet("dispatcher1", new DispatcherServlet(webContext1));
    dispatcher1.setLoadOnStartup(1);
    dispatcher1.addMapping("/subcontext1");

    // dispatcher servlet 2
    ...
}

That way, you have full control on which beans will end in which context, exactly as you would have with XML configuration.

Answer 2

I think you can work it out if you use generic WebApplicationInitializer interface rather than using abstract implementation provided by spring - AbstractAnnotationConfigDispatcherServletInitializer.

That way, you could create two separate initializers, so you would get different ServletContext on startUp() method and register different AppConfig & dispatcher servlets for each of them.

One of such implementing class may look like this:

public class FirstAppInitializer implements WebApplicationInitializer {

    public void onStartup(ServletContext container) throws ServletException {

        AnnotationConfigWebApplicationContext ctx = new AnnotationConfigWebApplicationContext();
        ctx.register(AppConfig.class);
        ctx.setServletContext(container);

        ServletRegistration.Dynamic servlet = container.addServlet(
                "dispatcher", new DispatcherServlet(ctx));

        servlet.setLoadOnStartup(1);
        servlet.addMapping("/control");

    }

}