How do I pass parameters into a Provider with Guice?

Question

Let's say I have this interface:

public interface DbMapper{
}

And then this implementation:

public interface NameDbMapper extends DbMapper {

    @SqlUpdate("insert into names (name) values (:name)")
    void insert(@Bind("name") String name);
}

This implementation exists in a module, so I don't know all DbMappers at compile time. I discover DbMappers through reflection:

public class GuiceModule extends AbstractModule{

    @Override
    protected void configure() {
        Reflections reflections = new Reflections("com.company");
        Set<Class<? extends DbMapper>> dbMappers = reflections.getSubTypesOf(DbMapper.class);

        for (Class<? extends DbMapper> dbMapper : dbMappers) {
            Class<DbMapper> db = (Class<DbMapper>) dbMapper;
            binder().bind(db).toProvider(DbMapperProvider.class);
        }
    }

Then I instansiate mappers in my provider:

public class DbMapperProvider implements Provider<DbMapper> {

    private final User user;

    @Inject
    public DbMapperProvider(User user) {
        this.user = user;
    }

    @Override
    public DbMapper get() {
        String jdbc = user.getJdbc();

        DBI userSpecificDatabase = new DBI(jdbc, "user", "password");
        //How to replace NameDbMapper.class here with the db variable in GuiceModule?
        DbMapper dbMapper = userSpecificDatabase.onDemand(NameDbMapper.class);
        return dbMapper;
    }
}

User is a @RequestScoped instance, so I can't create providers regularly in GuiceModule. Injecting User works but how do I pass which Class DBI should use instead of hardcode NameDbMapper in DbMapperProvider here?

I've tried the approach suggested in http://google-guice.googlecode.com/git/javadoc/com/google/inject/assistedinject/FactoryModuleBuilder.html but couldn't get it to work.

The goal here is that modules shouldn't have to write their own providers, is this achievable?

Answer 1

You can bind to a provider instance, like

for (Class<? extends DbMapper> dbMapper : dbMappers) {
    bind(dbMapper).toProvider(new DbMapperProvider<>(db));
}

Then modify your provider like this:

public class DbMapperProvider<T extends DbMapper> implements Provider<T> {
    // Use field or method injection
    @Inject
    private Provider<User> user;

    private final Class<T> type;

    public DbMapperProvider(Class<T> type) {
        this.type = type;
    }

    @Override
    public T get() {
        String jdbc = user.get().getJdbc();

        DBI userSpecificDatabase = new DBI(jdbc, "user", "password");
        DbMapper dbMapper = userSpecificDatabase.onDemand(type);
        return dbMapper;
    }
}