Best way to implement factory with Guice

[Barak Yaish]

Hi,

Doing my first steps with Guice, I thought the fastest way to understand it would migrate parts of existing application to Guice style. The application is kind of web crawler, and I have factory creating crawlers based on the input url:

public static Crawler getCrawler( String url )

{

try

{

if( url.contains( "www.site1.com" ) )

return new Site1( AppConfig.getInstance() );

else if( url.contains( "www.site2.com" ) )

return new Site2( AppConfig.getInstance() );

}

catch ( Exception e )

{

logger.error( "failure", e );

}

return null;

}

Is the Guice version is only take the crawler instances from the injector and injecting the AppConfig?

public static Crawler getCrawler( String url )

{

try

{

if( url.contains( "www.site1.com" ) )

return GuiceInjector.getInstance( Site1.class );

else if( url.contains( "www.site2.com" ) )

return GuiceInjector.getInstance( Site2.class );

}

catch ( Exception e )

{

logger.error( "failure", e );

}

return null;

}

Is there a better and/or more elegant way? 

Thanks!

[Laszlo Ferenczi]

Hi,

There are many ways to solve this problem, one easy enough to understand:

@Inject

Provider<CrawlerOne> crawlerOneProvider;

@Inject

Provider<CrawlerTwo> crawlerTwo;

public Crawler getCrawler(String url) {

    if (url...) {

        return crawlerOneProvider.get();

    }

    else {

        return crawlerTwoProvider.get();

    }

}

As an alternative you can inject the injector itself and get the instance from it.

Also a slightly more advanced, but much more elgant way is to use AssistedInject (it's exectly for this use case) See the guice docs for more info.

PS: no need to make everything static, the whole point of the DI framework that you don't need static anchors in your code.

--

L

--
L

--
You received this message because you are subscribed to the Google Groups "google-guice" group.
To unsubscribe from this group and stop receiving emails from it, send an email to google-guice...@googlegroups.com.
To post to this group, send email to google...@googlegroups.com.
Visit this group at http://groups.google.com/group/google-guice.
To view this discussion on the web visit https://groups.google.com/d/msgid/google-guice/8911e587-58bd-4631-80a2-7a5d91007431%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[Stephan Classen]

Don't inject the injector. Its almost always a code smell.

[Barak Yaish]

Thanks. Is there a way to overcome the exhausting if-else if- else if structure, some magical way to support new crawlers in the system without "register" them in that factory?

[Sam Berlin]

Take a look at MapBinder or Multibinder: https://github.com/google/guice/wiki/Multibindings

sam

To view this discussion on the web visit https://groups.google.com/d/msgid/google-guice/c809db73-6456-493c-8047-95543ee382ef%40googlegroups.com.