package com.atlassian.plugin.osgi.bridge.external;
   
   import com.atlassian.plugin.PluginException;
   import org.osgi.framework.BundleContext;
   import org.osgi.framework.InvalidSyntaxException;
   import org.osgi.framework.ServiceEvent;
   import org.osgi.framework.ServiceListener;
   import org.osgi.framework.ServiceReference;
   import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  import org.springframework.beans.factory.FactoryBean;
  import org.springframework.beans.factory.InitializingBean;
  import org.springframework.osgi.context.BundleContextAware;
  
  import java.lang.reflect.InvocationHandler;
  import java.lang.reflect.InvocationTargetException;
  import java.lang.reflect.Method;
  import java.lang.reflect.Proxy;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  import static org.osgi.framework.ServiceEvent.REGISTERED;
  
  /**
   * Simple factory bean to resolve host components.  Since we know host components won't change during the bundle's
   * lifetime, we can use a direct reference instead of the fancy proxy stuff from Spring DM.
   *
   * @since 2.2.0
   */
  public class HostComponentFactoryBean implements FactoryBean, BundleContextAware, InitializingBean
  {
      private BundleContext bundleContext;
      private String filter;
      private Object service;
      private Class<?>[] interfaces;
  
      public Object getObject() throws Exception
      {
          return findService();
      }
  
      public Class getObjectType()
      {
          return (findService() != null ? findService().getClass() : null);
      }
  
      public boolean isSingleton()
      {
          return true;
      }
  
      public void setBundleContext(BundleContext bundleContext)
      {
          this.bundleContext = bundleContext;
      }
  
      /**
       * Sets the OSGi service filter.
       *
       * @param filter OSGi filter describing the importing OSGi service
       */
      public void setFilter(String filter)
      {
          this.filter = filter;
      }
  
      public void setInterfaces(Class<?>[] interfaces)
      {
          this.interfaces = interfaces;
      }
  
      /**
       * Finds a service, if the bundle context is available.
       *
       * @return The service, null if not found or the bundle context isn't available yet
       * @throws com.atlassian.plugin.PluginException
       *          If either 0 or more than 1 service reference is found
       */
      private Object findService() throws PluginException
      {
          return service;
      }
  
      /**
       * Wraps the service in a dynamic proxy that ensures the service reference is still valid
       *
       * @return A proxy that wraps the service
       */
      private Object createHostComponentProxy()
      {
          // we use the bundleContext's classloader since it was loaded from the main webapp
          return Proxy.newProxyInstance(bundleContext.getClass().getClassLoader(), interfaces, new DynamicServiceInvocationHandler(
                  bundleContext, filter));
      }
  
      public void afterPropertiesSet() throws Exception
      {
          checkNotNull(bundleContext);
          checkNotNull(interfaces);
          service = createHostComponentProxy();
     }
 
     /**
      * InvocationHandler for a dynamic proxy that ensures all methods are executed with the
      * object class's class loader as the context class loader.
      */
     static class DynamicServiceInvocationHandler implements InvocationHandler
     {
         private static final Logger log = LoggerFactory.getLogger(DynamicServiceInvocationHandler.class);
         private volatile Object service;
         private final String filter;
 
 
         DynamicServiceInvocationHandler(final BundleContext bundleContext, final String filter)
         {
             this.filter = filter;
             try
             {
                 ServiceReference[] refs = bundleContext.getServiceReferences((String) null, filter);
                 if (refs != null && refs.length > 0)
                 {
                     service = bundleContext.getService(refs[0]);
                 }
 
                 bundleContext.addServiceListener(new ServiceListener()
                 {
                     public void serviceChanged(ServiceEvent serviceEvent)
                     {
                         // We ignore unregistered services as we want to continue to use the old one during a transition
                         if (REGISTERED == serviceEvent.getType())
                         {
                             if (log.isDebugEnabled())
                             {
                                 log.debug("Updating the host component matching filter: " + filter);
                             }
                             service = bundleContext.getService(serviceEvent.getServiceReference());
                         }
                     }
                 }, filter);
             }
             catch (InvalidSyntaxException e)
             {
                 throw new IllegalArgumentException("Invalid filter string: " + filter, e);
             }
         }
 
         public Object invoke(final Object o, final Method method, final Object[] objects) throws Throwable
         {
             if (service == null)
             {
                 throw new IllegalStateException("Unable to locate host component with filter: " + filter);
             }
             try
             {
                 return method.invoke(service, objects);
             }
             catch (final InvocationTargetException e)
             {
                 throw e.getTargetException();
             }
         }
     }
 }