So i have Something like
class A {
#Inject // there are providers for things elsewhere
A(Thing1 o, Thing2t){
}
}
class B {
#Provides
A provideA(A a){
return A
}
interface component {
A getA();
}
}
So my thought was that since we are using #Inject in A, dagger should know how to build A. Then in B we define a provider and we already know how to build A so it should inject A as argument and then B should be able to provide A. But i get a circular dependency error.
Why is this a circular dependency?
Related
I have a subcomponent that I need to pull something out of:
#Subcomponent(modules = {SubModule.class})
#SubScope
public interface SubComp {
// ...
Thing getThing();
}
Each time #getThing is called, I need a new instance of Thing.
Thing also has its own orbit of items that need to be created with it. Naturally, my instinct is to create another subcomponent for it:
#Subcomponent(modules = {ModuleThing.class})
#ThingScope
public interface SubCompThing {
// ...
}
But here is my dilemma: Which piece of code should actually be creating Thing?
If I put a provider into SubModule, I then need to bind that instance into SubCompThing, but I get a warning about binding multiple instances. The warning is fatal in my build, and actually states that the warning will become an error in the future:
#Module(subcomponents = {SubCompThing.class})
interface SubModule {
#Provides
static providesThing(SubCompThing.Factory thingCompFactory) {
Thing thing = new Thing();
thingComp = thingCompFactory.create(thing); // Warning about duplicate binding.
// Do some stuff with thingComp
return thing;
}
}
If I have SubCompThing create Thing directly itself, my warning turns into an error with the same problem:
#Module
interface ModuleThing {
#Provides
#ThingScope
static Thing providesThing() {
return new Thing();
}
}
#Module(subcomponents = {SubCompThing.class})
interface SubModule {
#Provides
static Thing providesThing(SubCompThing.Factory thingCompFactory) {
thingComp = thingCompFactory.create();
// Do some stuff with thingComp
return thingComp.getThing();
}
}
(Dagger compiles that Thing is bound twice, since there are two providers for it.)
How can I have my top level SubComp return new Things on demand, and have each of those Things have their own subcomponent instance associated with them?
You're going to need to use a qualifier annotation.
The root of the problem here is that subcomponents inherit bindings from their parent components, so within your deepest subcomponent SubCompThing your exposed Thing binding on your component might very well be injecting the Thing binding that you're installing in SubComp's SubModule...because Dagger doesn't know that your #Provides method in SubModule is itself about to call SubCompThing's getThing() method!
As we discussed in the comments, what you're describing is very much like Dagger's official documentation on subcomponents for encapsulation, which sneakily depicts but does not describe its reliance on a qualifier annotation:
#Subcomponent(modules = DatabaseImplModule.class)
interface DatabaseComponent {
#PrivateToDatabase Database database();
/* ^^^^^^^^^^^^ */
}
That's the disambiguating trick: Your ModuleThing should bind #Provides static #PrivateThing Thing, and your SubCompThing should expose #PrivateThing Thing getThing() such that the only bound unqualified Thing is bound in your SubModule. (Incidentally, this would allow you to inject a Thing in SubCompThing, delegating to SubModule's implementation to create a brand new SubCompThing instance from within ModuleThing's call stack.)
I've defined a cheap qualifier annotation here, but you're welcome to use #Named to prototype it.
#Qualifier #Retention(RetentionPolicy.RUNTIME) public #interface PrivateThing {}
#Module
interface ModuleThing {
#Provides
#ThingScope
#PrivateThing
static Thing providesThing() {
return new Thing();
}
}
#Subcomponent(modules = {ModuleThing.class})
#ThingScope
public interface SubCompThing {
#PrivateThing getThing();
}
At that point, your SubModule will work with no additional modifications, abstracting away the creation of Thing to a black-box implementation of SubCompThing (which I prefer slightly, such that you don't have to bring Thing's instantiation details into SubModule). It also means you can keep Thing scopeless so you can instantiate more than one of them: When you call the provider, you'll see something new. That way you can make both thing1 and thing2.
I was reading about the dependency inversion principle and as far as I understand the relation is inverted because package A (high level) defines the interface and package B (low level) implements the interface, instead of directly invoking the class in package B.
But how can I apply the dependency inversion principle when I do not own package B? I'm using PHP and through the Composer package manager I import some third party libraries. Since I'm not in control of that code, I cannot make the classes in that library implement my high level interface.
I’ve searched on Google and Stackoverflow but i can’t seem to find questions or articles that mention this use-case.
The standard solution is to write an Adapter over the third-party library. In C#, it would look like the following:
public interface IFoo
{
void Bar(int baz);
string Qux(bool quux);
}
This interface would be defined in your A pakcage.
In another package, you'd reference the third-party library and write the Adapter:
public class ThirdPartyFoo : IFoo
{
private ThirdPartyObject thirdPartyObject;
// Class initialisation goes here...
public void Bar(int baz)
{
var corge = // perhaps translate or modify baz first...
thirdPartyObject.SomeMethod(corge);
}
public string Qux(bool quux)
{
var grault = // perhaps translate or modify quux first...
var res = thirdPartyObject.SomeOtherMethod(grault);
var garply = // perhaps translate res
return garply;
}
}
I am newbie for Guice and seeking help for the following use case :
I have developed one package say (PCKG) where the entry class of that package depends on other class like:
A : Entry point class --> #Inject A(B b) {}
B in turn is dependent on C and D like --> #Inject B(C c, D d) {}
In my binding module I am doing :
bind(BInterface).to(Bimpl);
bind(CInterface).to(CImpl);
...
Note I am not providing binding information for A as i want to provide its binding by its consumer class. (this is how the design is so my request is to keep the discussion on main problem rather than design).
Now my consumer class is doing like:
AModule extends PrivateModule {
protected void configure() {
bind(AInterface.class).annotatedWith(AImpl.class);
}
}
Also in my consumer package:
.(new PCKGModule(), new AModule())
Q1. Am i doing the bindings correctly in consumer class. I am confused because when i am doing some internal testing as below in my consumer package:
class testModule {
bind(BInterface).to(Bimpl);
bind(CInterface).to(CImpl)...
}
class TestApp {
public static void main(..) {
Guice.createInstance(new testModule());
Injector inj = Guice.createInstance(new AModule());
A obj = inj.getInstance(A.class);
}
}
It is throwing Guice creation exception.Please help me get rid of this situation.
Also one of my friend who is also naive to Guice was suggesting that I need to create B's instance in AModule using Provides annotation. But i really didn't get his point.
Your main method should look like this:
class TestApp {
public static void main(..) {
Injector injector = Guice.createInjector(new TestModule(), new AModule());
A obj = injector.getInstance(A.class);
}
Note that the Java convention is for class names to have the first letter capitalised.
I'm pretty sure your implementation of AModule isn't doing what you think it's doing either, but it's hard to be certain based on the information you've provided. Most likely, you mean to do this:
bind(AInterface.class).to(AImpl.class)`
There's no need to do anything "special" with A's binding. Guice resolves all the recursion for you. That's part of its "magic".
annotatedWith() is used together with to() or toInstance(), like this:
bind(AInterface.class).to(AImpl.class).annotatedWIth(Foo.class);
bind(AInterface.class).to(ZImpl.class).annotatedWIth(Bar.class);
Then you can inject different implementations by annotating your injection points, e.g.:
#Inject
MyInjectionPoint(#Foo AInterface getsAImpl, #Bar AInterface getsZImpl) {
....
}
It's worth also pointing out that you can potentially save yourself some boilerplate by not bothering with the binding modules (depending how your code is arranged) and using JIT bindings:
#ImplementedBy(AImpl.class)
public interface AInterface {
....
}
These effectively act as "defaults" which are overridden by explicit bindings, if they exist.
I am trying to understand Components in Dagger 2. Here is an example:
#Component(modules = { MyModule.class })
public interface MyComponent {
void inject(InjectionSite injectionSite);
Foo foo();
Bar bar();
}
I understand what the void inject() methods do. But I don't understand what the other Foo foo() getter methods do. What is the purpose of these other methods?
Usage in dependent components
In the context of a hierarchy of dependent components, such as in this example, provision methods such as Foo foo() are for exposing bindings to a dependent component. "Expose" means "make available" or even "publish". Note that the name of the method itself is actually irrelevant. Some programmers choose to name these methods Foo exposeFoo() to make the method name reflect its purpose.
Explanation:
When you write a component in Dagger 2, you group together modules containing #Provides methods. These #Provides methods can be thought of as "bindings" in that they associate an abstraction (e.g., a type) with a concrete way of resolving that type. With that in mind, the Foo foo() methods make the Component able to expose its binding for Foo to dependent components.
Example:
Let's say Foo is an application Singleton and we want to use it as a dependency for instances of DependsOnFoo but inside a component with narrower scope. If we write a naive #Provides method inside one of the modules of MyDependentComponent then we will get a new instance. Instead, we can write this:
#PerFragment
#Component(dependencies = {MyComponent.class }
modules = { MyDependentModule.class })
public class MyDependentComponent {
void inject(MyFragment frag);
}
And the module:
#Module
public class MyDepedentModule {
#Provides
#PerFragment
DependsOnFoo dependsOnFoo(Foo foo) {
return new DependsOnFoo(foo);
}
}
Assume also that the injection site for DependentComponent contains DependsOnFoo:
public class MyFragment extends Fragment {
#Inject DependsOnFoo dependsOnFoo
}
Note that MyDependentComponent only knows about the module MyDependentModule. Through that module, it knows it can provide DependsOnFoo using an instance of Foo, but it doesn't know how to provide Foo by itself. This happens despite MyDependentComponent being a dependent component of MyComponent. The Foo foo() method in MyComponent allows the dependent component MyDependentComponent to use MyComponent's binding for Foo to inject DependsOnFoo. Without this Foo foo() method, the compilation will fail.
Usage to resolve a binding
Let's say we would like to obtain instances of Foo without having to call inject(this). The Foo foo() method inside the component will allow this much the same way you can call getInstance() with Guice's Injector or Castle Windsor's Resolve. The illustration is as below:
public void fooConsumer() {
DaggerMyComponent component = DaggerMyComponent.builder.build();
Foo foo = component.foo();
}
Dagger is a way of wiring up graphs of objects and their dependencies. As an alternative to calling constructors directly, you obtain instances by requesting them from Dagger, or by supplying an object that you'd like to have injected with Dagger-created instances.
Let's make a coffee shop, that depends on a Provider<Coffee> and a CashRegister. Assume that you have those wired up within a module (maybe to LightRoastCoffee and DefaultCashRegister implementations).
public class CoffeeShop {
private final Provider<Coffee> coffeeProvider;
private final CashRegister register;
#Inject
public CoffeeShop(Provider<Coffee> coffeeProvider, CashRegister register) {
this.coffeeProvider = coffeeProvider;
this.register = register;
}
public void serve(Person person) {
cashRegister.takeMoneyFrom(person);
person.accept(coffeeProvider.get());
}
}
Now you need to get an instance of that CoffeeShop, but it only has a two-parameter constructor with its dependencies. So how do you do that? Simple: You tell Dagger to make a factory method available on the Component instance it generates.
#Component(modules = {/* ... */})
public interface CoffeeShopComponent {
CoffeeShop getCoffeeShop();
void inject(CoffeeService serviceToInject); // to be discussed below
}
When you call getCoffeeShop, Dagger creates the Provider<Coffee> to supply LightRoastCoffee, creates the DefaultCashRegister, supplies them to the Coffeeshop constructor, and returns you the result. Congratulations, you are the proud owner of a fully-wired-up coffeeshop.
Now, all of this is an alternative to void injection methods, which take an already-created instance and inject into it:
public class CoffeeService extends SomeFrameworkService {
#Inject CoffeeShop coffeeShop;
#Override public void initialize() {
// Before injection, your coffeeShop field is null.
DaggerCoffeeShopComponent.create().inject(this);
// Dagger inspects CoffeeService at compile time, so at runtime it can reach
// in and set the fields.
}
#Override public void alternativeInitialize() {
// The above is equivalent to this, though:
coffeeShop = DaggerCoffeeShopComponent.create().getCoffeeShop();
}
}
So, there you have it: Two different styles, both of which give you access to fully-injected graphs of objects without listing or caring about exactly which dependencies they need. You can prefer one or the other, or prefer factory methods for the top-level and members injection for Android or Service use-cases, or any other sort of mix and match.
(Note: Beyond their use as entry points into your object graph, no-arg getters known as provision methods are also useful for exposing bindings for component dependencies, as David Rawson describes in the other answer.)
i'm looking for a larger example of dependency injection and how it can be implemented. If class A depends on class B and passes a reference of class C to B's constructor, must not class A also take a reference to class C in it's constructor? This means that the main method in the application should create all classes really, which sounds wierd?
I understand that using DI frameworks we can have it in XML files somehow, but that sounds like it could be hard to quickly see what type that really is instanciated? Especially if it a very large application.
You are correct and each DI framework has a different way of managing it.
Some use attributes on the properties etc to denote dependency and then "automagically" supply an instance of the correct type, while others (say castle windsor for .net) allow xml configuration, fluent or other methods for "wiring up" the dependency graph.
Also no, class A takes a built reference to an instance of B which was built using an instance of C. A needs to know nothing about C unless exposed via B.
public class C { }
public class B { public B(C c) { ... }}
public class A { public A(B b) { ... }}
// manual wireup
C c = new C();
B b = new B(c);
A a = new A(b);
// DI framework
InversionOfControlContainer container = new InversionOfControlContainer(... some configuration);
A a = container.ResolveInstanceOf<A>();
// container dynamically resolves the dependencies of A.
// it doesnt matter if the dependency chain on A is 100 classes long or 3.
// you need an instance of A and it will give you one.
Hope that helps.
to answer your question about classes A,B,and C, A only needs a reference to B.
Most DI frameworks do not require you to use XML for configuration. In fact, many people prefer not to use it. You can explicitly set things up in code, or use some kind of conventions or attributes for the container to infer what objects should fulfil dependencies.
Most DI frameworks have a facility for "lazy loading" to avoid the creation of every single class up front. Or you could inject your own "factory or builder" objects to create things closer to the time when they will be used
You've not said what language you are using. My example below is in C# using the Unity container. (obviously normally you would use interfaces rather than concrete types):
container = new UnityContainer();
container.RegisterType<C>();
container.RegisterType<B>();
A a = container.Resolve<A>();
here's a few examples from the PHP Language, hope this helps you understand
class Users
{
var $Database;
public function __construct(Database $DB)
{
$this->Database = $DB;
}
}
$Database = Database::getInstance();
$Users = new Users($Database);
From this example the new keyword is used in the method getInstance(), you can also do
$Users = new Users(Database::getInstance());
Or another way to tackle this is
class Users
{
/*Dependencies*/
private $database,$fileWriter;
public function addDependency($Name,$Object)
{
$this->$Name = $Object;
return $this;
}
}
$Users = new Users();
$Users->addDependency('database',new Database)->addDependency('fileWriter',new FileWriter);
Update:
to be honest, I never use Dependency Injection as all its doing is passing objects into classes to create a local scope.
I tend to create a global entity, and store objects within that so there only ever stored in 1 variable.
Let me show you a small example:
abstract class Registry
{
static $objects = array();
public function get($name)
{
return isset(self::$objects[$name]) ? self::$objects[$name] : null;
}
public function set($name,$object)
{
self::$objects[$name] = $object;
}
}
Ok the beauty of this type of class is
its very lightweight
it has a global scope
you can store anything such as resources
When your system loads up and your including and initializing all your objects you can just store them in here like so:
Registry::add('Database',new Database());
Registry::add('Reporter',new Reporter());
Where ever you are within your runtime you can just use this like a global variable:
class Users
{
public function getUserById($id)
{
$query = "SELECT * FROM users WHERE user_id = :id";
$resource = Registry::get("Database")->prepare($query);
$resource->bindParam(':id',$id,PDO::PARAM_INT);
if($resource->execute())
{
//etc
}else
{
Registry::get('Reporter')->Add("Unable to select getUserById");
}
}
}
i see this way of object passing is much cleaner
If anybody is still looking for a good example which shows DI without IoC Containers (poor man's DI) and also with IoC Container (Unity in this example) and registering the types in code and also in XML you can check this out: https://dannyvanderkraan.wordpress.com/2015/06/15/real-world-example-of-dependeny-injection/