DigitalJoel

As I’m sure you can tell from previous entries, I’ve been playing with JSF 2 in Google App Engine.  I decided to give Google’s Guice a try for dependency injection instead of Spring.  Why?  This is really just a learning app and why not try something new.

So how do you get Guice to play nicely with JSF?  One solution I saw online was to write a custom ELResolver for JSF that will do the injection.  I found this resource http://snippets.dzone.com/posts/show/7171 that had a full ELResolver implementation.  I had to modify it slightly to get it to work.  Namely, I had to move the Guice Injector creation out of getValue and into a private static variable.  I suspect I could make it not static, but I never gave it a try.  Here’s the source just in case that link dies at some point.  Again, I take no credit for this code other than the small modification mentioned.

/**
 * Bulk of the code taken from
 * http://snippets.dzone.com/posts/show/7171
 **/
package jota.soc.guice;

import com.google.inject.Guice;
import com.google.inject.Injector;
import com.google.inject.servlet.ServletModule;
import java.beans.FeatureDescriptor;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import javax.el.ELContext;
import javax.el.ELResolver;

/**
 */
public class GuiceResolver extends ELResolver
{

    Injector injector = Guice.createInjector(
            new ServletModule(),
            new SocGuiceModule() );

    //We only need to see the currently processed Objects in our Thread, that
    //prevents multithread issues without synchronization
    private static ThreadLocal currentlyProcessedThreadLocal = new ThreadLocal()
    {
        @Override
        protected Object initialValue()
        {
            return new LinkedList();
        }
    };

    //Im not sure if the synchronized lists seriously slow down the whole EL
    //resolving process
    private static List<WeakReference> alreadyInjectedObjects = Collections.
            synchronizedList( new LinkedList() );

        @Override
    public Object getValue( ELContext context, Object base, Object property )
    {

        //if the list of currently processed property objects doesnt exist for this
        //thread, create it
        List<Object> currentlyProcessedPropertyObjects = (List<Object>) currentlyProcessedThreadLocal.
                get();

        //Handle only root inquiries, we wont handle property resolving
        if ( base != null )
        {
            return null;
        }

        //checking if this property is currently processed, if so ignore it -> prevent
        //endless loop
        if ( checkIfObjectIsContained( property,
                currentlyProcessedPropertyObjects ) )
        {
            return null;
        }

        //add the to-be-resolved object to the currently processed list
        currentlyProcessedPropertyObjects.add( property );

        //now we can savely invoke the getValue() Method of the composite EL
        //resolver, we wont process it again
        Object resolvedObj = context.getELResolver().getValue( context, base,
                property );

        //ok, we got our result, remove the object from the currently processed list
        removeObject( property, currentlyProcessedPropertyObjects );

        if ( resolvedObj == null )
        {
            return null;
        }

        //ok we got an object
        context.setPropertyResolved( true );

        //check if the object was already injected
        if ( !checkIfObjectIsContainedWeak( resolvedObj, alreadyInjectedObjects ) )
        {
            injector.injectMembers( resolvedObj );
            //prevent a second injection by adding it as weakreference to our list
            alreadyInjectedObjects.add( new WeakReference( resolvedObj ) );
        }

        return resolvedObj;
    }

    /**
     * This method will search for an object in a Weak List. If there are any
     * WeakReferences on the way that were removed by the garbage collection
     * we will remove them from this list
     * @param object
     * @param list
     * @return
     */
    private boolean checkIfObjectIsContainedWeak( Object object,
                                                  List<WeakReference> list )
    {
        for ( int i = 0; i < list.size(); i++ )
        {
            WeakReference curReference = list.get( i );
            Object curObject = curReference.get();
            if ( curObject == null )
            {
                //ok, there is are slight chance that could go wrong, if you
                //have to prevent a double injection by all means, you might
                //want to add a synchronized block here
                list.remove( i );
                i--;
            }
            else
            {
                if ( curObject == object )
                {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * checks if an object is contained in a collection (really the same object '==' not equals)
     * @param object
     * @param collection
     * @return
     */
    private boolean checkIfObjectIsContained( Object object,
                                              Collection collection )
    {
        for ( Object curObject : collection )
        {
            if ( object == curObject )
            {
                return true;
            }
        }
        return false;
    }

    /**
     * removes an object from a list. really removes the given instance, not an other
     * object that fits equals
     * @param object
     * @param list
     */
    private void removeObject( Object object, List list )
    {
        for ( int i = 0; i < list.size(); i++ )
        {
            if ( list.get( i ) == object )
            {
                list.remove( i );
            }
        }
    }

    @Override
    public Class<?> getType( ELContext context, Object base, Object property )
    {
        return null;
    }

    @Override
    public void setValue( ELContext context, Object base, Object property,
                          Object value )
    {
    }

    @Override
    public boolean isReadOnly( ELContext context, Object base, Object property )
    {
        return false;
    }

    @Override
    public Iterator<FeatureDescriptor> getFeatureDescriptors( ELContext context,
                                                              Object base )
    {
        return null;
    }

    @Override
    public Class<?> getCommonPropertyType( ELContext context, Object base )
    {
        return null;
    }
}

That sure looks like a lot of code to me. I know Spring also provides a custom ELResolver for dependency injection. I found two links talking about resource injection here and here and thought I would try writing my own resource injector. Here’s how it ended up:

package jota.soc.guice;

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

import com.sun.faces.spi.InjectionProvider;
import com.sun.faces.spi.InjectionProviderException;
import com.sun.faces.vendor.WebContainerInjectionProvider;

/**
 * JSF injection provider for Guice.
 * @author Joel Weight
 */
public class GuiceInjectionProvider implements InjectionProvider
{
    public GuiceInjectionProvider()
    {
        System.out.println( "creating guice injection provider" );
    }

    /**
     * default injector provided by the web container.
     */
    private static final WebContainerInjectionProvider con =
                                                       new WebContainerInjectionProvider();

    /**
     * Custom guice injector that will load our modules.
     */
    private static final Injector injector = Guice.createInjector(
            new ServletModule(),
            new SocGuiceModule() );

    @Override
    public void inject( Object managedBean ) throws InjectionProviderException
    {
        // allow the default injector to inject the bean.
        con.inject( managedBean );
        // then inject with the google injector.
        injector.injectMembers( managedBean );
    }

    @Override
    public void invokePostConstruct( Object managedBean )
            throws InjectionProviderException
    {
        // don't do anything here for guice, just let the default do its thing
        con.invokePostConstruct( managedBean );
    }

    @Override
    public void invokePreDestroy( Object managedBean ) throws
            InjectionProviderException
    {
        con.invokePreDestroy( managedBean );
    }
}

It’s a very simple implementation. It delegates everything to the default container InjectionProvider, and then adds a touch of Guice. I then added this to web.xml:

    <context-param>
        <param-name>com.sun.faces.injectionProvider</param-name>
        <param-value>jota.soc.guice.GuiceInjectionProvider</param-value>
    </context-param>

And all my guice injections are occurring as expected. I am no longer using the custom ELResolver implementation. Obviously it’s not a general solution since I am instantiating my guice modules right in the provider rather than configuring it somehow, but it was enough for me right now.

If I run into any problems with this solution I’ll let you know, otherwise I would love to hear from any gurus as to how this could be problematic.