Guice proxy to support circular dependency

Question

In my code, the following error appears on startup:

Tried proxying com.bar.Foo to support circular dependency, but this is not the interface.

How exactly does this proxy work? If I just throw enough classes for interfaces, will everything be okay?

(I know that circular dependencies are usually the smell of code, but I think that is normal in this case.)

Answer 1

While the "introduce interface" approach is completely valid and may even be the best solution in some cases, in general, you can use a simpler solution: Providers.

For each class, "A" can be managed, and also offers a " Provider<A> ". This is an internal implementation of the javax.inject.Provider interface whose get() message will be " return injector.getInstance(A.class) ". You do not need to implement the interface yourself, its part of "magic magic".

Thus, you can shorten the example A-> B, BA to:

 public class CircularDepTest { static class A { private final Provider<B> b; private String name = "A"; @Inject public A(Provider<B> b) { this.b = b; } } static class B { private final Provider<A> a; private String name = "B"; @Inject public B(Provider<A> a) { this.a = a; } } @Inject A a; @Inject B b; @Before public void setUp() { Guice.createInjector().injectMembers(this); } @Test public void testCircularInjection() throws Exception { assertEquals("A", a.name); assertEquals("B", abget().name); assertEquals("B", b.name); assertEquals("A", baget().name); }} 

I prefer this because its more readable (you are not being fooled to keep in mind that the constructor already has an instance of “B”), and since you could implement the Providers yourself, it would still work “manually”, outside the guice context (for testing, for example).

Answer 2

I am new to this concept, but here is my understanding.

Say you have interfaces A and B , as well as implementations of Ai and Bi .

If Ai has a dependency on B , and Bi has a dependency on A , then Guice can create an implementation of the proxy server A (let's call it Ap ), which will be provided at some point in the future by Ai for delegation. Guice gives Ap to Bi for his dependence on A , allowing Bi to complete the instance. Then, since Bi was created, Guice can create an instance of Ai using Bi . Then, since Ai now good at doing, Guice tells Ap delegate Ai .

If A and B were not interfaces (and you only had Ai and Bi ), this would simply not be possible, because creating Ap would require the extension Ai , which Bi already requires.

Here's what it might look like with code:

 public interface A { void doA(); } public interface B { void doB(); } public class Ai implements A { private final B b; @Inject public Ai(B b) { this.b = b; } public void doA() { b.doB(); } } public class Bi implements B { private final A a; @Inject public Bi(A a) { this.a = a; } public void doB() { } } 

The proxy class that Guice does will look like this:

 public class Ap implements A { private A delegate; void setDelegate(A a) { delegate = a; } public void doA() { delegate.doA(); } } 

And all this will be connected with this basic idea:

 Ap proxyA = new Ap(); B b = new B(proxyA); A a = new A(b); proxyA.setDelegate(a); 

And this is how it would be if you had only Ai and Bi , without interfaces A and B

 public class Ap extends Ai { private Ai delegate; public Ap() { super(_); //a B is required here, but we can't give one! } } 

If I just throw enough classes for interfaces, will everything be okay?

I would suggest that there are strict restrictions on how a proxy server can interact with a constructor. In other words, if B tries to call A before Guice has the opportunity to populate the proxy with real A, then I would expect a RuntimeException.

Answer 3

Here is @ jan-galinski's answer redone in Scala:

 import javax.inject.Inject import com.google.inject.{Guice, Injector, Provider} import net.codingwell.scalaguice.InjectorExtensions._ /** Demonstrates the problem by failing with `Tried proxying CircularDep1$A to support a circular dependency, but it is not an interface. while locating CircularDep1$A for parameter 0 at CircularDep1$B.<init>(CircularDep.scala:10) while locating CircularDep1$B for parameter 0 at CircularDep1$A.<init>(CircularDep.scala:6) while locating CircularDep1$A` */ object CircularDep1 extends App { class A @Inject() (val b: B) { val name = "A" } class B @Inject() (val a: A) { val name = "B" } val injector: Injector = Guice.createInjector() val a: A = injector.instance[A] val b: B = injector.instance[B] assert("A" == a.name) assert("B" == abname) assert("B" == b.name) assert("A" == baname) println("This program won't run!") } /** This version solves the problem by using `Provider`s */ object CircularDep2 extends App { class A @Inject() (val b: Provider[B]) { val name = "A" } class B @Inject() (val a: Provider[A]) { val name = "B" } val injector: Injector = Guice.createInjector() val a: A = injector.instance[A] val b: B = injector.instance[B] assert("A" == a.name) assert("B" == abget.name) assert("B" == b.name) assert("A" == baget.name) println("Yes, this program works!") }