How does the Built-in Bindings of Google Guice work? - dependency-injection

I tried Google Guice the first time and find it very nice.
But, when I reached the part of Built-in Bindings I do not understand the examples.
For me it looks like I can use it for logging like an interceptor, but I don't know how.
Could someone of you explain this type of Binding and how I can use it? And maybe (if it's possible) use it for logging?

What the documentation indicates is that the bind(Logger.class).to(...) call has already been made for you. Thus, you don't need to call the bind method. Instead, you can inject it directly as if you had already called bind:
class DoSomething {
private final Logger logger;
#Inject public DoSomething(Logger logger) {
this.logger = logger;
}
}
Alternatively, you can get it from the injector:
/* even without specifying modules, logger is present */
Injector injector = Guice.createInjector();
Logger logger = injector.getInstance(Logger.class);
If you need to override the logger, then you'll have to use this method.

All the example you point to is showing is that you don't have to provide a binding for Logger.class in code such as the example. Since the 99% case is
Logger logger = Logger.getLogger(ConsoleTransactionLog.class);
Guice will provide that logger for you as a convenience. If you need a different loggers (i.e., one not based on the class being injected into), you can provide your own binding.

Related

Inject vs Injectable vs Custom Providers

I’m having difficulty trying to figure out when to use what. I understand DI and have done this for years, but within NestJS it seems like there are things missing from the docs that would help me understand why some assigned properties go missing from an injected item.
We have a logger that gets injected via a custom provider.
From looking at the docs, they'll add #Inject during the constructor area, but if I'm injecting via a custom provider, why use #Inject?
Equally, why use #Injectable if we have custom providers handling all the injection setup for the module?
Where the question comes up is that filename is getting lost among other injected services where sometimes when the logger gets executed in one service, the filename is the same, while other times the service being different, it will retain the filename from the ctor name of a different service.
Anyway, I don't understand why to use one set up over another.
Update: I found we follow the LifeCycle on some Services, but I find the logger doesn't retain at the time of execution.
//custom-provider
export ClientProvider = {
provide: Client,
userFactory: (logger: Logger): Client => {
return new Client(logger);
},
inject: [Logger.getToken()]
};
//client
export class Client{
private readonly logger;
constructor(logger:Logger){
this.logger = logger;
if (this.logger) this.logger.fileName = this.constructor.name;
}
}
//logger
export class Logger {
static getToken(): string { return 'the-token-key'; }
private filename: string;
constructor(filename: string){
this.filename = filename;
}
}
//cat-service
export class CatService implements OnModuleInit {
private readonly logger;
constructor(#Inject(Logger.getToken()) private logger:Logger){
this.logger = logger;
}
onModuleInit() {
if (this.logger) this.logger.fileName = this.constructor.name;
}
}
So, high level, Nest naturally uses metadata reflected from Typescript to know what to bind and where. Normally, this metadata is things like Class Names that exist at runtime, which is why you cannot use interfaces.
#Injectable() tells Nest that it should look for the metadata provided in this class's constructor and find the correct providers. This is how Nest does most of the DI, as mentioned before.
#Inject() tells Nest, "Hey, I know this says it is a Logger class, but really inject the provider with the injection token I tell you", which is useful for things like using specific instances of, say, a Logger class.
With a custom provider, #Injectable() is not necessary if you use a factory, as you have shown, but it would be if you were to use a class instead, because factories return the instance of the value to be provided, whereas with useClass you are returning a class definition that Nest will need to instantiate.
The only reason I could see your filename being overwritten is that providers are singleton scope between modules, but if the same provider is in the providers array then it should be a new instance each time, so maybe something with that is acting a little wonky

Guice multiple implementations, parameterized constructor with dependencies

I'm struggling with a particular dependency injection problem and I just can't seem to figure it out. FYI: I'm new to guice, but I have experience with other DI frameworks - that's why I believe this shouldn't be to complicated to achieve.
What am I doing:
I'm working on Lagom multi module project and using Guice as DI.
What I would like to achieve:
Inject multiple named instances of some interface implementation (lets' call it publisher, since it will publishing messages to kafka topic) to my service.
This 'publisher' has injected some Lagom and Akka related services (ServiceLocator, ActorSystem, Materializer, etc..).
Now I would like to have two instances of such publisher and each will publish messages to different topic (So one publisher instance per topic).
How would I achieve that?
I have no problem with one instance or multiple instances for the same topic, but if I want to inject different topic name for each instance I have a problem.
So my publisher implementation constructor looks like that:
#Inject
public PublisherImpl(
#Named("topicName") String topic,
ServiceLocator serviceLocator,
ActorSystem actorSystem,
Materializer materializer,
ApplicationLifecycle applicationLifecycle) {
...
}
If I want to create one instance I would do it like this in my ServiceModule:
public class FeedListenerServiceModule extends AbstractModule implements ServiceGuiceSupport {
#Override
protected void configure() {
bindService(MyService.class, MyServiceImpl.class);
bindConstant().annotatedWith(Names.named("topicName")).to("topicOne");
bind(Publisher.class).annotatedWith(Names.named("publisherOne")).to(PublisherImpl.class);
}
}
How would I bind multiple publishers each for it's own topic?
I was playing around with implementing another private module:
public class PublisherModule extends PrivateModule {
private String publisherName;
private String topicName;
public PublisherModule(String publisherName, String topicName) {
this.publisherName = publisherName;
this.topicName = topicName;
}
#Override
protected void configure() {
bindConstant().annotatedWith(Names.named("topicName")).to(topicName);
bind(Publisher.class).annotatedWith(Names.named(publisherName)).to(PublisherImpl.class);
}
}
but this led me nowhere since you can't get injector in you module configuration method:
Injector injector = Guice.createInjector(this); // This will throw IllegalStateException : Re-entry is not allowed
injector.createChildInjector(
new PublisherModule("publisherOne", "topicOne"),
new PublisherModule("publisherTwo", "topicTwo"));
The only solution which is easy and it works is that I change my PublisherImpl to abstract, add him abstract 'getTopic()' method and add two more implementations with topic override.
But this solution is lame. Adding additional inheritance for code reuse is not exactly the best practice. Also I believe that Guice for sure must support such feature.
Any advises are welcome.
KR, Nejc
Don't create a new Injector within a configure method. Instead, install the new modules you create. No child injectors needed—as in the PrivateModule documentation, "Private modules are implemented using parent injectors", so there's a child injector involved anyway.
install(new PublisherModule("publisherOne", "topicOne"));
install(new PublisherModule("publisherTwo", "topicTwo"));
Your technique of using PrivateModule is the one I'd go with in this situation, particularly given the desire to make the bindings available through binding annotations as you have it, and particularly if the full set of topics is known at runtime. You could even put the call to install in a loop.
However, if you need an arbitrary number of implementations, you may want to create an injectable factory or provider to which you can pass a String set at runtime.
public class PublisherProvider {
// You can inject Provider<T> for all T bindings in Guice, automatically, which
// lets you configure in your Module whether or not instances are shared.
#Inject private final Provider<ServiceLocator> serviceLocatorProvider;
// ...
private final Map<String, Publisher> publisherMap = new HashMap<>();
public Publisher publisherFor(String topicName) {
if (publisherMap.containsKey(topicName)) {
return publisherMap.get(topicName);
} else {
PublisherImpl publisherImpl = new PublisherImpl(
topicName, serviceLocatorProvider.get(), actorSystemProvider.get(),
materializerProvider.get(), applicationLifecycleProvider.get());
publisherMap.put(topicName, publisherImpl);
return publisherImpl;
}
}
}
You'd probably want to make the above thread-safe; in addition, you can avoid the explicit constructor call by using assisted injection (FactoryModuleBuilder) or AutoFactory, which will automatically pass through explicit parameters like topicName while injecting DI providers like ServiceLocator (which hopefully has a specific purpose, because you may not need much service-locating within a DI framework anyway!).
(Side note: Don't forget to expose your annotated binding for your PrivateModule. If you don't find yourself injecting your topicName anywhere else, you might also consider using individual #Provides methods with the assisted injection or AutoFactory approach above, but if you expect each Publisher to need a differing object graph you might choose the PrivateModule approach anyway.)
Guice's approach to dependency injection is that the DI framework complements your instantiation logic, it doesn't replace it. Where it can, it will instantiate things for you, but it doesn't try to be too clever about it. It also doesn't confuse configuration (topic names) with dependency injection - it does one thing, DI, and does that one thing well. So you can't use it to configure things, the way you can with Spring for example.
So if you want to instantiate an object with two different parameters, then you instantiate that object with two different parameters - ie, you invoke new twice. This can be done by using provider methods, which are documented here:
https://github.com/google/guice/wiki/ProvidesMethods
In your case, it might look something like adding the following method to your module:
#Provides
#Named("publisherOne")
#Singleton
Publisher providePublisherOne(ServiceLocator serviceLocator,
ActorSystem actorSystem,
Materializer materializer,
ApplicationLifecycle applicationLifecycle) {
return new PublisherImpl("topicOne", serviceLocator,
actorSystem, materializer, applicationLifecycle);
}
Also, you probably want it to be a singleton if you're adding a lifecycle hook, otherwise you could run into memory leaks each time you add a new hook every time it's instantiated.

Is it "bad practice" to pass argument to guice module

Checking out Guice and I love it. I currently have problem where guice solved it by injecting all the required dependencies I need. But I wonder if I am using Guice in the wrong way. What I require though is define bindings depending on specific instance. And to achieve this I passed the instance in the module.
For instance, consider the following (somewhat similar to my problem):
public class CustomerModule extends AbstractModule {
private Customer customer;
public CustomerModule(Customer customer){
this.customer = customer;
}
#Override
public void configure() {
bind(ReportGenerator.class).to(HtmlReportGenerator.class);
}
#Provides
Account providePurchasingAccount() {
return customer.getPurchasingAccount();
}
}
I use this module to get Account dependency injected to the report generator class that needs the account of a specific customer. For example, a user chooses a specific customer and say, wants to show a generated report. I have method like
public void printReport (Customer customer){
Injector injector = Guice.createInjector(new CustomerModule(customer));
ReportGenerator reportGenerator = injector.getInstance(ReportGenerator.class);
showReport(reportGenerator.generate())
}
Once the work is done, I am done with this module.
Is this a ok use of guice?
It is appropriate and useful to accept a constructor argument for a Module. This is an especially common pattern when making bindings for similar objects. Example:
// Installs #Named("accounts") Db to the given impl, backed with the given cache.
install(new DbModule("accounts", AccountDb.class, InMemoryCache.class));
// Same as above.
install(new DbModule("users", UserDb.class, DiskCache.class));
install(new DbModule("products", ProductDb.class, CustomProductCache.class));
That said, it is not common to create a new root Injector per action (such as printReport). Injector creation can take a long time as Guice reflectively queries classes and their dependencies. Instead, it is much more common to create the root Injector at application startup, and then create a child injector when you need to bind specific objects the way you have them.
Though it may make sense for you to temporarily create a brand new root Injector for each action, the way you have it, bear in mind that future development may make warrant singleton or application-level scope that persists beyond a single action, or your object graph may grow such that mid-action root Injector creation is no longer performant enough for your uses. If/when that happens, you may want to shift most of your Injector creation and configuration to a predictable startup flow, and only bind your Customer (and nothing else) into a child injector.

Castle Windsor Property Injection with paramters

I am new to this so I appreciate the help. I am using Castle Windsor as my container with XML config. For several of my core services this works perfectly, I simply declare a public property named the same thing and it injects it as expected. Most of my core services have no-arg constructors, those ones work out of the box. But what about if I have a core service that itself has it's own dependencies, how best to deal with that? For example, say I have a configurable message queuing service:
public interface IQueueService {
void SendMessage(string msg);
}
public class SQSService : IQueueService {
private ServiceConfig _config;
public SQSService(ServiceConfig config) {
_config = config;
}
public void SendMessage(string msg){
//do message stuff
}
}
SQSService itself requires a config to initialize properly (endpoint, port, etc). Is there an easy way to configure the DI to resolve this config? Or am I better off re-factoring the SQSService to not have this dependency?
When injecting something that is dependent on a config, session, request, etc., you have the flexibility to break away from making your dependency object have the same structure as the underlying source (in fact, you should as much as possible, otherwise you may end up accidentally writing code that implicitly depends on that structure). Inject the properties you need, rather than the config itself. I am not sure how to accomplish this using the XML config, but I know the fluent API allows for injection using .FromAppSettings(), or you can use the web/app.config to house your config, using .FromAppConfig() as explained here:
How to instantiate a class based on web.config file with Castle Windsor?

Ninject.Extensions.Logging.nlog2 - How to?

Browsing the nuget library, i came across Ninject.Extensions.Logging.nlog2. Some googling and trying to figure things out, I can't seem to find how or why you would use this extension.
Is it advisable to use with MVC 3?
What exactly is the point?
How do you use it?
It's really very simple; both NLog and log4net expect you to use singleton/static references to obtain logger instances:
private static Logger logger = LogManager.GetCurrentClassLogger();
This is widely considered to be an anti-pattern, but even if you have no problem with it, it's still going against the grain if you're trying to implement dependency injection. In the case of NLog it's not even an ILog or ILogger interface like log4net, it's an actual class. That carries certain disadvantages such as the inability to create proxies, deferred loading, caching, etc.
What the Ninject.Extensions.Logging project does is first provide an abstract ILogger class with simple methods like Info, Error, etc., so you can inject it as a dependency and switch the logging framework if you want:
public class WidgetProvider
{
private readonly ILogger log;
public WidgetProvider(ILogger log)
{
this.log = log;
}
}
This is how DI is supposed to work - a class never goes out to grab its own dependencies, instead they're supplied by the constructor or caller as above. Assuming you've already integrated Ninject itself into your project, that's really all you have to do, there is no additional work.
As for what Ninject.Extensions.Logging.NLog2 does specifically - it just provides an implementation for Ninject.Extensions.Logging based on NLog2. The base Logging library doesn't actually contain any implementations of ILogger, you have to plug in one of the specific libraries (NLog, NLog2, or log4net) in order to get it to work.
If you switch your DI library more often than you switch loggers then don't bother with it. But if you're like me and use Ninject in almost every project, then it's a nice way to decouple your code from any specific logging library.
If you want to use dependency injection via the constructor you can pass the ILoggerFactory interface.
This is how I did it.
using Ninject.Extensions.Logging;
public class MyClass
{
private readonly ILogger _log;
public MyClass(ILoggerFactory logFactory)
{
_log = logFactory.GetCurrentClassLogger();
}
public void DoWork()
{
_log.Info("Doing work!");
}
}
Problem solved!
Hope this helps someone.

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