I didn't know about a Guice prohibition on doing things in constructors,
unless it was to avoid problems with circular dependencies. (So don't have
circular dependencies!)
I don't see a problem with Proposals A or D. I have yet to see it
documented this way, but the Guice folks have assured me over the years
that there is a happens-before edge between writes in @Inject-ed methods
and reads of an injected object (unless you try very hard and do evil
things during provision). Proposal A looks fine to me; Proposal D looks
safe as long as the non-final Map is always accessed read-only after
construction/injection (which seems to be the case).
Proposals B and C seem unnecessarily verbose; the use of @Named to inject a
Map<String, String> is overkill. If you want to use a @Provides method,
isn't this sufficient?
@LazySingleton @Provides CachedThings cachedThings(DB db) {
Map<String, String> cache = readCache(db);
return new CachedThings(cache);
}
This last seems like the nicest approach because you get to keep the cache
field final, and you keep the reading separate from the access. "readCache"
could be a static method of some other class in the same package, so that
the Module's only role would be to say "To make the CachedThings from the
DB, first read the cache from the DB into a map and construct the
CachedThings from that map."
--tim
On Fri, Jul 20, 2012 at 6:37 PM, Leigh Klotz, Jr. <[email protected]>wrote:
> I'd like to get some guidance on correct, clear, and concise
> initialization of singleton, read-only access objects
> wrapping cached run-once computations, with either lazy or eager
> initialization.
>
> I brought this up peripherally in
>
> http://groups.google.com/forum/?fromgroups#!topic/google-guice/u0V97-FZBTQ
> but I wanted to avoid taking my attempted threadjacking any further and am
> starting a new topic.
>
> Here's a sample use case:
>
> A Database provides a table of String keys and values. I'd like to
> provide a read-only get(String)->String access
> object, and I'd like to initialize it safely, so that the resulting object
> is thread-safe. I'd like to express this
> concisely, in a way that is clear to code readers so they will be incented
> to copy the pattern.
>
> Lazy init can be done in Java using the static hack, as descrbied by Bob
> here
> http://blog.crazybob.org/2007/01/lazy-loading-singletons.html
> and in Wikipedia here
> http://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom
>
> but I'd like to do it using Guice, for all the usual reasons, such as so I
> can integrate these providers in with normal injection.
>
> I hope my problem statement is clear. I'm looking for guidance on the best
> solution.
>
> Here are three unsatisfactory ideas and one maybe OK one.
>
> Proposal A:
> Proposal A does the work of reading the database in the constructor.
>
> This code is clear, and it's certainly concise, and as near as I can
> understand, it's thread-safe.
>
> Unfortunately, I belive it isn't correct in Guice because of the problems
> associated with doing work in constructors
> (proxy objects, for example).
>
> @LazySingleton
> class CachedThings {
> private final Map<String,String> cache;
>
> @Inject CachedThings(DB db) {
> this.cache = readCache(db);
> }
>
> public String get(String x) {
> return cache.get(x);
> }
>
> private Map<String,String> readCache(DB db) {
> Map<String,String> result = new HashMap<String,String>();
> for (Row row : db) { map.add(row.a, row.b); }
> return result;
> }
>
> }
>
> Proposal B:
>
> Proposal B splits the data access object from the database reader
> operation, moving the database read into its own
> Provider, where it can operate safely. This appears to be just as
> thread-safe as the previous version, but is
> considerably less concise. It also exposes a @Named TypeLiteral that is
> not only ugly, but also opens the
> possibility of someone directly injecting that Map and causing unwanted
> expensive database reads.
>
> @LazySingleton
> class CachedThings {
> private final Map<String,String> cache;
>
> @Inject CachedThings(@Named("hack") Map<String,String> cache) {
> this.cache = cache;
> }
>
> public String get(String x) {
> return cache.get(x);
> }
>
> static class MyModule extends AbstractModule {
> protected void configure() {
> bind(new
> TypeLiteral<Map<String,String>>(){/**/}.annotatedWith(Names.named("hack")).toProvider(ThingProvider.class).in(LazySingleton.class);
>
> }
> }
>
> @LazySingleton
> static class ThingProvider implements Provider<Map<String,String>> {
> private final DB db;
>
> @Inject ThingProvider(DB db) {
> this.db = db;
> }
>
> private Map<String,String> get() {
> Map<String,String> result = new HashMap<String,String>();
> for (Row row : db) { map.add(row.a, row.b); }
> return result;
> }
> }
>
>
> Proposal C:
> Proposal C uses @Provides methods so avoid to the problematic verbosity of
> TypeLiteral, and indeed the code is cleaner, but we still have the @Named
> hack and
> internal data exposure. (I've written the MyModule as a static class of
> CachedThings, which is questionable, so we might need to deduct a few points
> for the additional verbosity needed to move the module out.)
>
> @LazySingleton
> class CachedThings {
> private final Map<String,String> cache;
>
> @Inject CachedThings(@Named("hack") Provider<Map<String,String>>
> thingProvider) {
> this.cache = thingProvider.get();
> }
>
> public String get(String x) {
> return cache.get(x);
> }
>
> static class MyModule extends AbstractModule {
> protected void configure() { }
>
> @LazySingleton @Provides @Named("hack")
> Provider<Map<String,String>> getThing(DB db) {
> Map<String,String> result = new HashMap<String,String>();
> for (Row row : db) { map.add(row.a, row.b); }
> return result;
> }
> }
> }
>
> Proposal D:
> I don't know much about injected methods other than that they run after
> constructors and the results can't be final.
> Is this correct with regard to Guice initialization sequence? Is it safe
> for multi-threaded readonly access of the resulting HashMap?
>
> Maybe this is the right solution is to use an @Inject setCache(DB) method?
>
> @LazySingleton
> class CachedThings {
> private Map<String,String> cache;
>
> @Inject CachedThings(...) {
> ... other stuff here if necessary ...
> }
>
> @Inject void setCache(DB db) {
> Map<String,String> result = new HashMap<String,String>();
> for (Row row : db) { map.add(row.a, row.b); }
> cache = result;
> }
>
> public String get(String x) {
> return cache.get(x);
> }
> }
>
> Thank you,
> Leigh.
>
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