Yet another way to get the Guice injector

I previously wrote about a way to get ahold of the Guice injector in a JSP by putting it in the ServletContext; however, I have another case where I need it in a class that doesn’t have access to the ServletContext, either. The class happens to be a background task that implements the Deferrable interface, which for sake of interface simplicity does not provide access to the ServletContext even though the task does in fact run in a servlet.

The solution is simply to create the Guice injector in a static factory:

package com.roa.server.guice;

import com.google.inject.Guice;
import com.google.inject.Injector;

public class GuiceFactory
{
	private static final Injector inj = Guice.createInjector(...);
	
	public static Injector getInjector()
	{
		return inj;
	}
}

Now any class that needs access to a Guice-managed object can get it easily, including the ultra-simplified Guice context listener that’s registered in web.xml:

package com.roa.server.guice;

import com.google.inject.Injector;
import com.google.inject.servlet.GuiceServletContextListener;

public class GuiceConfig extends GuiceServletContextListener
{

	@Override
	protected Injector getInjector()
	{
		return GuiceFactory.getInjector(); 
	}
	
}

WARNING: this works fine in AppEngine because there’s only one WAR per JVM in that environment. If you’re not running in AppEngine and have multiple WARs in a single JVM, as is commonly the case, you may or may not want a single Guice injector for the whole JVM. I have not tested it, but I would think you wouldn’t be able to use Guice ServletModules in that case.